Home        The Boat        Photo Essay        Contact

Traction Control and Suspension: From preventing wheel spin to active stability control systems.

Open differentials have long been problematic on uneven or slippery surfaces because the wheel with the least amount of traction determines the amount of driving force that can be applied. Traction aids in differentials reduce or eliminate these problems, but each type of mechanical traction aide also comes with it's own unique problems. Modern electronic traction control systems have the potential to do better than the traditional systems, but require high levels of sophistication to function smoothly and seamlessly.

Open Differentials
Limited Slip
The Locker
Spools and Welded Differentials
Electronic Traction Control
Adjustable Suspension
Active Suspension
Suspension Geometry


Open Differentials - Standard functionality with poor traction.

The purpose of the differential is to allow the inside wheel to spin more slowly than the outside wheel when going around a turn. Without a differential a rear wheel drive vehicle skids it's way around turns chewing up tires and also chewing up any dirt or gravel driveways that it encounters. On a front wheel drive vehicle a differential is absolutely essential, as the vehicle strongly resists turning with both front wheels driving at the same speed. Open differentials are necessary for most types of driving, but become a huge problem on uneven terrain, or in patchy ice where small areas are extremely slippery. Even on flat and even pavement open differentials in solid rear axles limit drive traction in high power vehicles because the torque in the drive shaft tends to lift one wheel. More reduction at the ring and pinion reduces this problem by putting more power to the wheels with less drive shaft torque. Independent rear suspension totally eliminates this problem, and front wheel drive cars never have problems with torque lifting of a wheel because they always have independent front suspension. On a four wheel drive vehicle the same open differential problems exist, but it can be particularly frustrating when a four wheel drive that is supposed to handle rough terrain instead easily gets stuck when wheels at opposite corners of the vehicle come off the ground crossing a ditch or large rut. Picking lines that minimize lifting of two wheels is essential, and often the only way to get through areas of very uneven terrain is to carry momentum and bounce across the worst obstacles. This high speed open differential driving technique requires a good suspension system, and still tends to be hard on the vehicle and it's occupants. The classic example of four wheel drive vehicles breaking U-joints and axle shafts is when two wheels lose traction and accelerate up to high speed before slamming down on a high traction area putting huge shock loads on the drive train.

Limited Slip - The least intrusive traditional traction aid.

Because traction problems with open differentials were so severe there was a large demand for some form of traction aid. The first solution for low speed vehicles was the use of individual steering brakes. If one wheel began to spin the brake for that wheel was applied. This worked well for tractors and some rear wheel drive vehicles, but was only really any good once the vehicle was stuck. Trying to apply the correct brake to maintain traction while traveling at speed was problematic in that it distracted from other driving tasks like picking lines, shifting and smoothly applying power. In the 1960's the limited slip differential, or posi-track, came on the scene as a highly functional alternative to open differentials for a wide range of applications. The basic idea of a limited slip differential is that the spider gears have an angled shape to them that tends to push the output gears away from each other, and this outward force can be used to "lock" the differential with clutch packs. A limited slip differential has a clutch pack after each output gear that tend to lock the driveshafts to the ring gear carrier. Even more outward force can be generated with a wedge shaped drive outside of the spider gears, and most limited slip units do use some sort of auxiliary force generating wedge other than the spider gears themselves.

The way that a limited slip differential works is that it has a certain amount of spring pressure on the clutches so that it is never a totally open differential. With no torque being put through the differential each wheel is free to move independently with only a small amount of clutch force tending to hold the wheels at the same speed. When drive power is applied though both clutches are subjected to much higher clamping forces, and the two wheels are strongly constrained to turn at the same speed. The reason that limited slip differentials work so well over such a wide range of applications is that they can be tuned for different levels of traction response by changing the amount of spring pressure on the clutches as well as by changing the angle of the wedge that strongly applies the clutches. For a high horsepower rear wheel drive street machine essentially no spring pressure is used on the clutches, and the angle of the force generating wedges is steeper so that less clutch clamping force is generated at low torque input levels. This makes for a car that drives essentially the same as one with an open differential in all maneuvering situations, but under heavy acceleration in low gears the differential locks up sufficiently to prevent the driverís side wheel from breaking loose.

For a off-road vehicle, or any vehicle that needs improved drive traction in compromised traction situations, the limited slip differential is setup with more spring pressure and shallower angles on the clamping force generating wedges. The shallower angle wedges mean that more locking force will be applied with smaller levels of applied driving torque for use on slipperier surfaces. More spring pressure means that more driving torque can be applied even when one wheel is totally off the ground or on the slipperiest of ice. The disadvantage of the off-road setup for limited slip differentials is that the inside wheel spins going around turns in compromised traction. The only real problem with this is that it is harder on dirt roads, driveways and trails with tight turns. An off-road setup limited slip is usually no kind of a problem on pavement, even though it does cause a bit of inside wheel spin when accelerating out of tight turns in the rain. This small amount of inside wheel slip is usually not much of a handling problem as a limited slip equipped vehicle will be more predictable and easier to control in challenging situations on any type of road surface. For high speed cornering on dry pavement there is however a very slight problem of the rear end breaking free more easily when powering out of turns with a tightly setup limited slip. Most road vehicles, especially four wheel drive trucks and Jeeps, are not expected to be particularly fast on a high speed race track anyway, so this ends up being essentially totally insignificant.

The best compromise for good all around performance and a minimum of inside wheel spin on dirt roads and trails is a limited slip setup where shallow wedge angles are able to apply large amounts of clamping force with small drive torque inputs, but rather small amounts of spring pressure are used. With small amounts of spring pressure the limited slip differential may be reluctant to lock up in the most challenging off-road situations, but a bit of brake pressure can be used in place of the spring pressure to assure a solid lock-up of the differential even when one wheel is totally off the ground.

Generally limited slip differentials can very easily be made to work nearly perfectly in the rear differential, but front limited slips are much more problematic and tricky. Limited slip differentials are almost never used in front wheel drive cars, because any small amount of locking force causes the vehicle to resist turning. On four wheel drive vehicles limited slip units are sometimes used in the front, but they are not the nearly universal advantage of a rear limited slip. Generally front limited slip units must be setup with essentially no spring pressure so that the vehicle will turn easily. With a no spring pressure front limited slip a vehicle will turn normally on flat ground as long as little or no driving force is applied during the turn. If driving force is applied though the locking action causes the vehicle to resist turning. What this generally means is leaving the vehicle in two wheel drive until a challenging obstacle or climb is encountered. With no spring pressure on the front limited slip unit simply shifting the transfer case into two wheel drive removes all turning problems.

The Locker - Full traction, but large drivability problems.

Lockers eliminate the differential altogether, but still provide a modicum of resistance to inside wheel spin on pavement. The lockers are essentially just the clutches and angled wedges out of a limited slip unit with no differential between them. Because the locker must lock up solid with very small amounts of input torque the clutches are not clutches at all, but rather teeth that engage and disengage. Some lockers have smoother tooth designs that operate more quietly and smoothly, but the basic principle of operation remains the same. When no driving torque is applied the locker is fully disengaged, with each wheel spinning free. As soon as torque is applied the locker locks up solid and both wheels are driven at the same speed.

Aside from some noise and clunkiness this works quite well for the most challenging off-road situations where one or more wheels is fully off the ground. With no need to feather the brake pedal to get the limited slip units to lock up the driver can focus on smooth application of power and picking the best lines to quickly and easily get over or through the toughest obstacles. For on road use though lockers are generally horrible. The only way to get around a turn without inside wheel spin is to push in the clutch and coast all the way around the turn until the front wheels are pointed straight again. Not very convenient, and not very fast either. Any time that there is an uphill turn where driving power is required the inside wheel will spin. Lockers are also very hard on dirt roads and trails, again because any applied driving power in a turn causes the inside wheel to spin. When used in the front differential of a four wheel drive vehicle a locker requires similar concessions be made as when using a front limited slip unit. The advantage of a front locker is that when the transfer case is shifted to two wheel drive it always disappears, allowing the vehicle to negotiate turns as a two wheel drive.

Spools and Welded Differentials - Simple, cheap and of limited application.

The simplest way to eliminate problems with open differentials is just to eliminate the differential with nothing in it's place. Spools are available to replace most common differentials, and are just solid pieces that the ring gear bolts to and the bearings mount to. The same thing can be accomplished by welding the spider gears of a differential, although it takes a large amount of welding to produce a sufficiently strong finished product. Spools are relatively cheap, and also allow reinstalling the open differential when the problems of a fully locked front or rear end become onerous.

Spools work great for the toughest off-road situations because they just smoothly stay locked up all the time with no noise, harshness or unpredictability. Spools are also extremely strong, never giving any trouble in the harshest abuse. A rear spool also works just fine for most types normal driving, and even on dry pavement a solidly locked rear end does not cause much in the way of performance problems. Again though the rear end does tend to break free a bit more easily during high speed cornering, making spools inappropriate for most types of racing. Usually the biggest problem with a rear spool is that it severely limits the amount of weight that can be carried on dry pavement. A pickup truck with a rear spool can extremely easily break axles if a heavy load is carried on dry pavement. In the front differential spools are more problematic, as disengaging the fully locked front end requires not just shifting the transfer case out of four wheel drive, but also getting out and unlocking at least one of the locking hubs. Front spools absolutely cannot be used without locking hubs or some system for disengaging one of the front axles.

Selectable Lockers - Highest level of functionality for mechanical traction aids.

Because each type of traction aid has it's own pitfalls the best solution is a system that can be turned off. The selectable locker is a standard open differential with a cable, electric or compressed air operated locking mechanism that turns the differential into a spool. The oldest type of selectable lockers were cable lockers offered stock on certain four wheel drive vehicles. For many decades the only type of aftermarket selectable locker was the air locker, which used vulnerable plastic compressed air lines to lock the differential. Horror stories abound of having to dig under a vehicle stuck in the mud to install a spare plastic line.

The advantages of selectable lockers are however immense, and recently more models of aftermarket electric and cable operated lockers have become available for popular housings. Both the front and the rear benefit from selectable lockers, but especially in the front a selectable locker is by far the best type of traction aid that can be installed. With open differentials the vehicle has no disadvantages or drawbacks, and being able to use four wheel drive with an open front differential means that the vehicle can easily be driven a whole lot faster in compromised traction situations. Usually selectable lockers are installed in both ends of four wheel drive vehicles. This allows four wheel drive with open differentials for easy smooth operation on slippery roads with a minimum of wear and tear on the dirt road or trail itself. When the going gets a bit tougher the rear locker can be engaged as needed without having to get out of the vehicle. Because the vehicle does need to be stopped to lock the differential though the rear differential does end up being left locked for substantial amounts of off-road use. This is a bit harder on trails, but overall selectable lockers are an advantage from every perspective. Vehicles with just open differentials tend to be very hard on more challenging sections of trails because they have to go faster with lots of wheel spin to have any chance. When the going gets really tough being able to lock the front end is an enormous advantage because most vehicles are a lot heavier on the front than in the back. Particularly if the vehicle has to be backed up out of a hole that it has gotten stuck in a locked front differential can provide literally several times more driving force than an open front differential.

Electronic Traction Control - Higher functionality for nearly all applications.

The basic idea of electronic traction control is to use individual electronically controlled actuators on the brakes for each wheel to limit wheel spin. The actuators and wheel speed sensors for electronic traction control are the same as are used for antilock braking systems, meaning that any vehicle with antilock brakes can very easily also have an electronic traction control system. When one wheel spins faster than the other one the brakes on that wheel are applied to slow it. On two wheel drive vehicles a system for cutting engine power to limit total wheel spin has also usually been used. On the earliest carbureted models cutting engine power was accomplished by cutting out the ignition, which was not perfectly smooth and caused emissions control system problems (melted catalytic converters). On four wheel drive vehicles there was no way for the control system to know how much faster the wheels were spinning than the vehicle was moving, so there was no good way to automatically limit total wheel spin. Ultimately the total wheel spin limiting systems are not a good idea, because only the driver is really able to judge just how much total wheel spin might be required for any particular situation.

The main problem with early traction control systems was that they were not fast enough, and they actually caused much more wheel spin than limited slip differentials or selectable lockers. With a delay between the time of initial wheel spin and the application of the brakes there was inevitably a certain amount of wheel spin. The longer the delay the more wheel spin would be generated. Because this choppy application of the brakes was not smooth it meant that usually the only way that a traction control system could be effectively used in the most challenging of situations was to get all the wheels spinning together. With all of the wheels spinning the handling of the vehicle was then smooth and predictable, and the traction control system helped out immensely by preventing any one wheel (or two wheels on a four wheel drive) from accelerating up to much higher speeds than the other wheel(s).

Much more problematic for most drivers has been the fact that early antilock braking systems were also too slow. The longer stopping distances required by antilock brake equipped cars on dry pavement has long been widely discussed, but it was compromised traction situations where the early antilock breaking systems really fell flat on their faces. In snow, ice or mud early antilock brake equipped vehicles seemed to require nearly twice as long of a distance to come to a stop.

Electronic traction control systems can (and have) been made to work quite well. Faster reaction times and less choppy application of the breaks makes for a smooth and seamless system that really does keep some wheels from spinning faster than others. The level of sophistication required to get traction control to work really well though is substantial. One of the key additional levels of sophistication is integration of the traction control system with a steering angle sensor. When the front wheels are turned, the inside wheels have to spin more slowly. When this is implemented well it creates a whole new type of vehicle control system that has come to be known as active stability control. With the inside wheels being forced to spin more slowly the vehicle has a form of brake steering, and as it turns out computer controlled brake steering can do all kinds of amazing things for getting vehicles to go faster and stay better in control on all types of surfaces, including dry pavement.

Not all electronic stability control systems have however been competent. The classic example of this is cars with electronic stability control systems that attempt to emulate the understeer of a poorly setup independent front suspension system. One way that this has manifest is with the three strikes and youíre out rule for active stability control systems. If the active stability control system has to disengage in three consecutive corners to prevent a crash then it just shuts down and will not restart. What happens is that the stability control system induces understeer by applying the outside brakes, and the vehicle simply will not stick to the road as well as it otherwise would. If the driver is familiar with that particular suspension system, and knows that it will in fact stick to the road better, he can engage in a scary (and often risky) maneuver where a corner is entered not at the speed indicated by the induced understeer but rather at a higher speed that the vehicle would normally be able to handle. No matter how sophisticated the active stability control system, it has no choice but to shutdown in the interest of self preservation. This is a very jerky maneuver that is harder to control than just about any kind of real situation, but if the car is inherently well balanced with good handling characteristics it can be pulled off. The three strikes and you are out rule comes into play because such a difficult maneuver is not likely to be successfully pulled off time after time, turn after turn. If the active stability control system proves to be a hindrance to safe driving three times it shuts down and pretends that it was never there in the first place.

Functional active stability control systems also exist, mostly on high end supper cars with so much power that they otherwise would not work. The basic idea of a functional active stability control system is that electronically controlled breaking can be used to induce a rotational force on the car even in a skid. If the driver induces oversteer then the active stability control system can limit the amount of oversteer so that optimal traction can be maintained. The driver still has to minimize skidding to keep the tires from overheating, but oversteer can be used to get that little bit of extra grip when a corner is entered slightly too fast. Essentially the electronic stability control systems can get a front engine sedan to reliably and safely negotiate a high speed track as well as a mid engined supper car of the past.

The application of good functioning active stability control systems on high power cars comes into play during heavy acceleration from a stop or out of a tight turn. With so much power that the rear wheels (or even all four wheels) will spin freely on dry pavement something has to be done to make the car safe for normal road use. Traditionally this was done with racing mode keys, and launch control options that had to be selected while the car was stationary. Active stability control systems can allow full power to be used to destroy the tires as fast as the owner chooses whenever he chooses without the car becoming unmanageable. With all four wheels smoking wildly accelerating up to 50mph a car would normally be difficult to reliably control. A fast acting electronic stability control system though can keep the car pointed straight ahead as long as the steering wheel is pointed straight ahead, or even cause the car to pivot when the steering wheel is turned.

Functional active stability control systems could also potentially be useful on more mundane vehicles that don't have enough power to light the tires up on dry pavement. One example of this would be a front wheel drive vehicle with a lightweight engine and transmission. With a bit of a load in the trunk such a vehicle can become difficult to control with just 100hp on wet pavement if heavy acceleration is attempted out of a tight turn. Normally such a small amount of power as 100hp causes little in the way of handling problems even on very slippery surfaces, but an active stability control system that could apply a bit of inside wheel breaking when the steering wheel was turned would make the vehicle faster and easier to manage in some conditions. When it comes to skidding in corners a functional active stability control system could be extremely useful on just about any type of vehicle. Obviously having extra cornering grip available can be a significant safety advantage when something unexpectedly goes wrong, and most people would also like their cars to be as fast as possible if it does not cause other problems.

Another thing that active stability control systems can do is actually provide stability, that is prevent vehicles from tipping over. It is only very tall and top heavy vehicles that can tip over on flat pavement, and even these do not generally tip over simply because they stick to the pavement so well that they roll over. The way that a tall and top heavy vehicle tips over is that the suspension is loaded first in one direction in a skid, and then overcorrection causes the vehicle to immediately go into a skid in the other direction. When the vehicle flops over onto the new skid direction the unloading of the suspension and the momentum of the vehicle coming upright brings a huge additional tipping impulse into play. The suspension system has a lot to do with how easily a tall top heavy vehicle will tip over even if there is no active stability control system used. The main suspension parameter that has to with tip over prevention is the rebound damping. More rebound damping, particularly volume dependant rebound valving, can do a whole lot to prevent a tall and top heavy vehicle from tipping over when it skids out and then immediately overcorrects into a skid in the other direction. Of course stiffer spring rates and stiffer sway bars can also contribute to less body roll, and therefore more stability.

Vehicles that understeer are often considered to be more resistant to tipping over, but this is generally just wishful thinking. In the kind of really tough conditions where vehicles sometimes tip over even pushy understeer prone suspension systems can easily end up in a skid, and can actually be more prone to overcorrection than better functioning suspension systems. Where active stability control systems come into play in the actual stability of tall and top heavy vehicles is that they can be used to prevent the overcorrection situation so prone to cause tipping over. If the oversteer of a vehicle is limited to some reasonable amount then overcorrection is less likely, and of course the active stability control systems can also continue to provide braking based corrections to slow the rotation of the vehicle as it comes close to straight ahead and act to prevent a skid in the other direction. It is not that vehicles with active stability control systems are totally immune from overcorrection, it just adds a large additional means of reducing the likelihood of going into a skid in the other direction.

About the largest potential problem with universal application of functional active stability control systems would be that people who otherwise would not have been able to drive fast might become accustomed to pushing cars to their limits. The point here is that the ability to control a skid manually in a good handling car or the ability break heavily while also maintaining steering control go hand in hand with all of the other measures of diligence that are required to push a car to it's limits on public roads without causing huge safety problems.

It is not that active stability control systems themselves are a problem, but rather that they change the nature of driving. And this is where the induced understeer comes into play. If people are not used to cars going fast around corners, then they are less likely to drive fast. The problem with inducing handling problems electronically though is that it is likely to infuriate traditional motoring enthusiasts who want the best performance possible from their cars. Inducing understeer on a car that otherwise would be able to bend turns well is in the same category of problems as the early antilock breaking systems that caused radically increased stopping distances on snow, ice and mud. At best it will make certain people angry, jaded and desirous of revenge. At worst it will flat out cause crashes and wrecks because the car is not capable of doing what it otherwise would be capable of doing.

A smoother transition from induced understeer to controlled managed oversteer might be considered one solution, but it is the dishonesty of this type of programming that causes the problems. People who grow up driving cars with electronic stability control systems that try to pretend to understeer, but then switch to managed oversteer will naturally tend to see the initial induced understeer as simply a quirky delay that has to be compensated for before the car switches to "normal" mode. Ultimately any electronically induced understeer is problematic because it makes the car slow to respond to steering inputs. A better solution would simply be a dash light indicator that comes on when excessive cornering forces are encountered. Cornering so hard that oversteer is likely on dry pavement is very hard on tires, and is avoided by most drivers most of the time anyway. A further sophistication might be an indicator of excessive cornering forces that are likely to induce oversteer on a wet road. This light might be used only when the road is actually wet, or might be left on all the time as a training reminder.

Far from the concerns of ultimate handling capability on dry pavement the traction control system itself is very useful, and would be considered indispensable by anyone who routinely drives in snowy conditions. One concern that is often raise about traction control systems is that more drive traction will cause people to drive in slipperier conditions where accidents due to lack of breaking and cornering ability might occur. This argument does not however hold water. What traction control systems allow is driving in deeper snow, and the deeper the snow the better the breaking and cornering conditions tend to be. The danger of driving in snow and ice is when one small part of the road becomes extremely slippery due to a sheet of smooth ice. These extremely slippery sections of ice can occur anytime that freezing conditions exist, and are actually more likely with just a few inches of snow that any vehicle can easily drive through. The classic example of a traction aid leading to increased accidents is the use of snow chains on rear wheel drive vehicles. Particularly two wheel drive pickup trucks with no weight in the bed have very poor drive traction in snow and ice. By adding snow chains a rear wheel drive vehicle can much better develop enough drive traction to head out in the snow, but the chains on the rear wheels do not contribute much to increased stopping ability. Compared to front wheel drive vehicles with chains the rear wheel drive vehicles with chains are a huge hazard. Traction control could be considered the same type of hazard, but the reality is that it is only two wheel drive pickups that are made more dangerous with the addition of snow chains or traction control. And the only reason that snow chains or traction control makes two wheel drive pickups more dangerous is that without the traction aids more of them stay parked when it snows. Ultimately it is the responsibility of individual drivers to not become a hazard, and for most people who routinely drive in the snow the most likely way that they are going to become a hazard is if their vehicle gets stuck on a upgrade. People who do drive in the snow can do it more safely with a good traction control system.

When traction control systems are used for extended driving in deep snow there is another problem that crops up, and that is excessive use of the brakes. For most conditions the application of the brakes for traction control is no kind of a problem because it is used infrequently for very short periods of time. When one wheel is off the ground or on such a slippery surface that it cannot contribute at all to driving the vehicle then the traction control system is very inefficient, requiring up to twice as much driving power and dissipating half of that power as friction at brakes. This double power requirement could be a significant concern for heavy off road use, but there is a solution for both instances of excessive brake use. Adding a rear limited slip unit to an electronic traction control system eliminates nearly all of the extra driving power requirement. The limited slip unit can be setup with no spring pressure on the clutches, so there is no resistance to the wheels freely turning independently when coasting around a turn. Only a small amount of braking force is required to increase the torque to the differential to get the limited slip clutches to engage. Once the clutches are engaged more driving force can be applied, which more strongly applies the clutches. For heavy off road driving this means that only a small bit of extra power has to be put into the brakes when a wheel is off the ground, and a traction control system then is only limited by the speed and effectiveness of the control system. For driving in the snow there is normally only a slight difference in traction between one wheel and the other. If the traction is right on the limit of what is required to drive the vehicle though, the brakes on the compromised traction wheel can end up dragging for long periods of time. A rear limited slip would eliminate this long term dragging of brakes. These advantages of adding a limited slip unit to a vehicle with traction control would however only apply to rear wheel drive or four wheel drive vehicles, as it would never be a good idea to use a limited slip unit on a front wheel drive car.

Adjustable Suspension - More broadly competent vehicles.

On the fly adjustable suspension systems are not new, adjustable air springs and remotely adjustable rebound damping circuits on shock absorbers have been available for many decades. Electronic controls for adjustable suspension systems however allow broader applicability and additional advantages when integrated with active stability control systems. As in the past one of the biggest things that an adjustable suspension system can do is provide more rebound damping for high speed driving and less rebound damping for better comfort and control over bumps at lower speeds. A good damping system can work amazingly well over a wide range of conditions (for more on hydraulic damping see the "Truck and Trailer" section of  Failure Fiasco), but adding on the fly adjustability of rebound damping can allow the suspension system to be just that much more flexible and competent. The key to adjustable rebound damping is making the adjustment fast and easy. Turning dials to try to set the correct rebound damping for changing conditions can be distracting. With electronic controls just a single button can be pushed to activate high speed stability mode with more rebound damping. Automated systems can activate certain levels of rebound damping based on vehicle speed, but some kind of override is also necessary because the size and shapes of the bumps has a great deal to do with just how high the vehicle speed needs to be before insufficient rebound damping can cause loss of control.

Adjustable ride height and adjustable spring rate systems have always been a bit more tricky to make work well, but there are a few things that a simple air bag system can do very well. Air bags that inflate to support the suspension can be put to good use on vehicles that sometimes operate with much heavier loads than normal. The additional spring support of the air bags allows the vehicle to be set to the normal ride height even with much more weight in it, which is good for safety and control with a heavy load as well as making the ride more comfortable when carrying a heavy load. Adjustable rebound damping goes very well with air bags for carrying heavy loads, as stiffer spring rates normally require stiffer rebound damping to keep the suspension system working well.

The main problem with using air bags for adjustable ride height is that the spring rates are then softer when the vehicle is low and stiffer when the vehicle is lifted up higher. This is somewhat backwards, because normally a vehicle would need to be lower and stiffer for high speed driving on pavement and higher and softer for off-road use. Just being able to increase the ground clearance can however be an enormous benefit in some instances, even if the suspension system does lose some of it's comfort and functionality. One example of this would be to get over an isolated obstacle without scraping the underside too terribly. Loss of suppleness in the suspension system can be acceptable for a few minutes to negotiate the toughest of obstacles, and the stiffer spring rate with the suspension lifted all the way up can also help to keep the vehicle as high as possible over the obstacle. Where raising the suspension ride height with air bags is even more advantageous is in deep snow. A four wheel drive vehicle with traction control can go through quite deep snow, and the depth of snow that it can make it through is limited by the ground clearance under the differentials. Adding a lift kit to a truck or Jeep with solid axles does not do much for increasing the amount of snow that it can go through, but adding bigger tires does. A four wheel drive vehicle with four wheel independent suspension and an adjustable ride height system can actually get the differentials substantially higher above the ground for dealing with deeper snow. Little or no negative travel in the suspension when the vehicle is lifted all the way up would make for a rough ride and poor handling at speed, but for driving through deep snow the stiff suspension and lack of negative travel are only rather minor problems.

To get a vehicle to be able to be stiff and low for high speed driving and tall and supple for off-road use would require two sets of air springs acting against each other. This is certainly possible, but some problems do crop up. Two sets of air springs tends to be much easier to attain with piston type air springs instead of air bags. The reason for this is that air bags tend to be a highly progressive type of spring. Piston type air springs on the other hand can be made to have any level of progressiveness that is required. Making a piston type air spring less progressive is easily acomplished by adding an external reservoir of compressed air. The larger the reservoir the more linear the spring rate becomes. The problem with the piston type air springs is that they inherently have a certain amount of friction in the seals, which is not good for a supple ride. One set of air springs is bad enough, but two sets makes the friction problem twice as bad. Keeping friction low means the use of the correct diameter piston, too large of a piston unnecessarily increases the friction. Smaller piston diameters mean higher operating pressures which require more robust sealing systems. If the friction can be kept to a minimum a dual air spring suspension system can be made to work quite well, and offers a number of significant advantages. Of course the biggest advantage is that holy grail of adjustable suspension systems, the low stiff ride height for the highest speed operation and a lifted and very soft suspension for smoothly negotiating rough terrain. The dual air spring suspension system also has the ability to vary the stiffness of the spring rate at some fixed intermediate ride height where the vehicle would normally be operated. The vehicle could then be set to be softer for bumpy dirt roads where extra ground clearance is not required, but also could be set to be stiffer at that same standard ride height for more surefooted handling at higher speeds over moderately rough terrain.

Another way that a variable ride height system could be made to work without the use of harsh sticky piston type air springs would be an electric motor driven variable position spring mount. This would do nothing to change the spring rates, but rather would simply raise or lower the vehicle. An electric motor driven actuator might also be used to change the spring pre-load, which would actually make the suspension stiffer or softer. The combination of both a variable upper spring mount position and an adjust on the fly spring pre-load system would allow the suspension to be either tall and soft or low and firm, but there is a bit of a hitch. Although increasing the pre-load on a spring does make it stiffer it also makes the spring less progressive. Likewise decreasing the pre-load does make a spring softer, but it also makes the spring more progressive. What this would mean would be that the high and soft suspension setting would have a more progressive spring rate, where the low and stiff setting would have a more linear spring response. This would not necessarily be unusable, but the highly progressive nature of the lower pre-load setting would mean that the spring rate would get very stiff towards the bottom of the stroke when the vehicle was in the raised and soft position.

To get the spring rates to remain more linear over the entire range of ride heights the only option would be a reverse acting auxiliary spring so that the two springs would act against each other similar to the way that the dual air spring suspension system works. If the only adjustment on the dual spring system was the engagement of the auxiliary reverse acting spring then the suspension could only be set to be tall and soft or low and firm. Of course any ride height in-between could also be used, but there would be only one corresponding spring rate available for each ride height setting. Because most vehicles would tend to work better with soft suspension when in the higher ride height settings and work better with stiffer spring rates in the lower ride height settings this system could be made to work quite well. The way this would work would be that the tallest and softest suspension setting would be with the auxiliary spring backed off so far that it did nothing. To lower (and stiffen) the suspension the auxiliary spring would be forced into position with an electric motor driven actuator. The farther the auxiliary spring was driven into position the lower and stiffer the suspension would become. If an additional measure of spring rate adjustment was required then an additional electric motor driven actuator could also be used to adjust the upper mounting position of the main load supporting spring. With adjustments on both springs stiffer or softer spring rates could be selected at any ride height, which for many applications would be considered highly desirable. To simplify the suspension system another option would be to use the adjustable reverse acting auxiliary spring in conjunction with traditional air bags. This would eliminate the need for a large capacity actuator for adjusting the mounting position of the main load carrying spring, while also providing the ability to carry heavier loads at higher speeds with the suspension in it's higher ground clearance position.

Any adjustable ride height system would of course work best with a long travel suspension system. In general all vehicles benefit from quite long suspension travel for all applications. As long as the hydraulic damping is competent extra suspension travel really does not interfere with any type of driving, even extremely high speed race track type driving. The only time that shorter suspension travel is desirable is in the absence of good and sufficient rebound damping. In the absence of rebound damping cars certainly can get around a track faster with less suspension travel, but what also goes along with insufficient rebound damping are extremely stiff spring rates that make for the harshest of rides. Vehicles normally work best with about one half or a bit less of the suspension travel being negative travel. That is a 10 inch travel suspension system will compress another five or six inches from normal ride height and can extend another four or five inches from normal ride height. Very conveniently higher speed driving on smooth roads tends to require less compressive travel and negotiating rough terrain at moderate speeds tends to require larger amounts of compressive travel. That ten inch travel suspension system could therefore quite easily switch from four inches of compressive travel for high speed driving to seven inches of compressive travel for negotiating rougher terrain.

The main concern for how much travel can be provided for has to do with the length of the drive shafts. This is because of limitations on how much of an angle the CV joints can handle without either becoming inefficient or wearing out quickly. A drive shaft that is eighteen inches from CV joint to CV joint normally works quite well on ten inch travel suspension systems, but there is a bit of a trick here. These ten inch travel suspension systems with eighteen inch drive shafts sit at normal ride height with the drive shafts down just an inch or two so that the suspension is already compressed an inch or two when the drive shafts are parallel. In this way it is the fully extended position of the suspension that puts the largest angles on the CV joints, since little or no power is put to a wheel when the suspension is fully extended the CV joints do not become loaded when they are at their most extreme angles. For a variable ride height system to work at it's best the normal driving height still needs to be very close to the parallel position of the drive shafts. At the lowest ride height the drive shafts might angle up slightly, but this would need to be just the same inch or two on an eighteen inch drive shaft. So an eighteen inch drive shaft really is only good for a maximum of about a three or four inch range of normal ride heights. Maximum ride height could of course still be higher than the two inches above parallel, with the assumption that this maximum ride height would be used a whole lot less than the standard ride heights. Longer drive shafts would allow both longer maximum suspension travel as well as a wider range of standard ride heights that would work well for long term efficient cruising. The key to longer drive shafts is of course narrower differentials. The standard for a independent front suspension system is an IFS Pickup truck or JEEP with the differential offset to one side for use with standard transfer cases. This offset position of the differential makes the drives shafts much shorter than they otherwise could be. Locating the differential in the center means that the drive shafts can easily be made much longer both for longer suspension travel and more efficient longer lasting CV joints. A 70 inch wide car with center differentials easily has room for drive shafts that are two feet between the CV joints. These two foot drive shafts would allow for 13 inches of suspension travel and a five inch range of normal ride heights. This extra five inches of normal ride height would allow for a car that was very low to the highway for best handling and lowest fuel consumption while also being able to come up to a full nine inches of ground clearance for easy negotiating of all normal types of rough terrain. The thirteen inches of total suspension travel would also allow for an additional three or four inches more ground clearance for negotiating extreme obstacles or deep snow.

With just air bags for raising the ride height of a vehicle there would be no way to go lower than some standard ride height. This standard ride height would probably be about four to six inches of ground clearance, and the air bags could be used to temporarily raise the vehicle up much higher. With just ten inches of suspension travel the maximum ride height of a foot of ground clearance would be all the way topped out with no negative suspension travel. Going to the longer two foot drive shafts and thirteen inches of total suspension travel would allow the same foot of ground clearance or even a bit more with several inches of negative travel remaining. Having a bit of negative travel is always a huge advantage for providing a smooth ride, so even with just air bags there is good reason for the drive shafts to be made as long as possible on a certain size vehicle.

The integration of an adjustable suspension system with the active stability control system could potentially allow for a large number of advantages in adapting to radically changing driving conditions. Paramount among them though tends to be the ability of an active stability control system to keep the vehicle under control even when a high ride height and soft spring rate are selected. The main way that this would be accomplished would be through integration of a rapid rebound damping adjustment system with the active stability control system. If the suspension was heavily compressed at the same time that an abrupt turn was being initiated the system would rapidly dial in more rebound damping to slow the rise of the suspension. This would help to remove the largest handling problem associated with vehicles setup for smoothest possible ride quality over bumps. That is the bouncing up from a bump coming at just the time that a turn is being initiated. In these conditions a skid can be induced even at moderate speeds in fairly good traction conditions. By preventing the rapid rebounding from a big bump at the beginning of or during a turn much softer rebound damping could safely be used at higher speeds over rough terrain.

Active Suspension - Robotic suspension for leaping vehicles.

An active suspension system is a suspension system that reacts to changing conditions in less than the time of one compression and rebound cycle. A system that increases the rebound damping to prevent bouncing up from a big bump at a particular time is in fact a type of active suspension system. This mid stroke rebound adjustment is the most rudimentary type of active suspension system, but is also the most likely to actually be widely used. A similar type of active suspension adjustment would be an increase in compression damping once a vehicle was thrown up in the air. The way this would work would be that if the suspension remained topped out for more than the shortest period of time it would be assumed that it was about to come crashing down from a high altitude. In this instance a larger amount of compression damping could go a long way to making the landing soft and comfortable as opposed to a harsh bottoming out of the suspension.

More sophisticated forms of active suspension would involve scanning the terrain ahead to anticipate what suspension settings would be best. The ultimate sophistication in active suspension might be a system that was able to lift a wheel with an electric motor driven actuator to get it over a particularly sharp and nasty obstacle. This last sophistication would be the most difficult to implement, and is also the least likely to actually be used on real vehicles. An optical or laser scanning system that was capable of identifying and measuring obstacles could actually do a huge amount of ride smoothing simply by adjusting the damping circuits to match individual obstacles. This would normally mean backing all the way off on the compression damping to absorb a sharp bump small enough as to not require the vehicle to move up to get over it. Similarly a large hole in a paved road could be much more smoothly traversed if the rebound damping was turned all the way up just before the wheel would otherwise drop into the hole. With a radically increased amount of rebound damping the wheel could be prevented from moving down into the hole as far as it otherwise would, making the impact on the far side of the hole much less severe.

Real robotic suspension with electric motor driven actuators is more the stuff of comic books and sci-fi movies, but there are some possibilities there as well. The main idea with robotic suspension is that electrical power would be used to lift a wheel or push a wheel down. This would of course require high capacity electric motors driving the suspension either through gear reduction systems or possible as direct acting linear electric motors. With enough high speed driving force a robotic suspension system could not only lift a wheel to get it more smoothly over an obstacle, but could actually hop to get the vehicle up over a taller obstacle than would otherwise be possible. To get an idea of what this would look like think about a motorcycle or bicycle trials competition. Especially the bicycle trials riders are able to leap up onto large obstacles with the wheeled vehicle hardly getting in the way. Motorcycle trials riders use power and acceleration up sheer faces to get the much heavier bike up onto large obstacles, but the acrobatics involved also hints at what a functional robotic suspension system might look like.

Again that proverbial pot hole in a paved road is a more mundane example of what a robotic suspension system might be useful for. Instead of simply adjusting the damping circuits to take the impact as smoothly as possible a robotic suspension system could lift a wheel over the hole in the road, or even give a small upward push just before a large hole in the road to keep the vehicle mostly level during the entire maneuver. To facilitate the lifting of one wheel with small sized electric actuators the wheel on the opposite corner of the vehicle could be retracted at the time of the initial hop. In this way a relatively small amount of power would be used to get the vehicle to tilt to lift a wheel. And the power requirement is significant since the forces involved with lifting a heavy vehicle are substantial. The robotic suspension system is the only type of suspension system that dramatically increases the power requirement of the vehicle. All of the other adjustable suspension systems and damping based active suspension systems involve only a control system and rather small actuators to move the hydraulic damping adjusters.

The last perspective on robotic suspension though is the reality that some of the power expended in forcing the suspension up and down for bump handling maneuvers could be recovered by using the electric motor actuators as generators. With this type of regenerative suspension system even normal bumps that would require just rebound damping could be more efficiently traversed by using the electric actuators as generators instead of using traditional hydraulic damping.

This perspective of robotic suspension being used to recover energy from normal damping operations seems to hint at a situation where softer and more supple suspension systems actually increase the driving power requirement of a vehicle. In extreme cases this is true, but the reality is that a better suspension system that more smoothly absorbs bumps allows wheel bearings to be smaller in size, substantially reducing the driving power requirement of the vehicle. It is also the case that a better suspension system that reduces maximum impact forces also allows the wheels and suspension components to be lighter, reducing unsprung weight and improving both the efficiency and effectiveness of the suspension system. Taken one step further it is also true that a better suspension system allows the overall vehicle weight to be lower for two separate reasons. Smoothing the biggest impacts allows a lighter chassis and body to be used simply because the parts are not stressed so severely. Smoothing all impacts, including the smaller ones that commonly cause a harsh ride, makes a small and light weight vehicle more comfortable to ride in and generally more appealing.

Suspension Geometry - The camber versus suspension compression relationship.

When it comes to suspension geometry there are a number of parameters that are significant for how a vehicle works, ultimately though it is the camber that is of most concern because it needs to change in a certain way as the suspension compresses. The other suspension parameters are caster (rake), trail and toe in. The caster is significant because it determines how responsive the steering feels and how stable the vehicle will be at high speed. On the front wheels there needs to be some small amount of positive caster for a vehicle to be stable at speed, but excessive caster makes turning feel sluggish and imprecise at low speeds around sharp turns. Generally about five or seven degrees of caster works well for all applications, but many vehicles have been built with as little as one degree of caster. Motorcycles generally use more like 25 to 28 degrees of caster, but are still able to easily negotiate very sharp turns.

Camber is significant because it determines maximum road holding ability as well as tire wear characteristics. For tires to wear evenly on straight roads there has to be almost no camber at normal ride height, normally less than one degree. On vehicles with a solid axle, that is vehicles without independent suspension, the camber remains constant under all conditions and this small half degree of negative camber works fairly well for allowing the tires to grip the road as well as they can. On independent suspension equipped vehicles the camber is much trickier because body roll tends to tip the wheels over so that the outside edge of the outside tire is taking the cornering load. To counter this tendency for body roll to prevent the tires from griping it is necessary for the suspension system to have more negative camber as the suspension is compressed.

There are two basic types of independent front suspension types, MacPherson strut type and dual A-arm type. The most significant difference between them is how the camber changes as the front suspension is compressed. On a MacPherson strut front ends it is the angle at which the strut is inclined towards the center of the vehicle that determines how the suspension system will work. The more that the struts are angled in towards each other the more the negative camber will increase as the suspension is compressed. On dual A-arm front ends a wide variety of suspension response characteristics can be generated depending on the relative length and angle of the two A-arms. If the A-arms are equal length and parallel to each other then the camber remains constant at all suspension compression levels. The normal way that dual A-arm front ends achieve the goal of flat tire wear on straight roads and good road holding when bending turns is to use a shorter upper A-arm. The shorter upper A-arm pulls the top of the spindle in towards the center of the car before the longer lower A-arm significantly moves the bottom of the spindle as the suspension is compressed. Even more camber response with smaller suspension compression amounts can be attained by angling the upper A-arm down from the spindle to it's mount on the chassis. With the upper A-arm already at a slight angle when the vehicle sits at it's normal ride height the top of the spindle is pulled in towards the center of the car as soon as the suspension begins to compress. Getting unequal length dual A-arm front ends to work well is very tricky, and requires good matching of both the design and the setup to the application. The same general concepts also apply to independent rear suspension systems, but front engine cars with most of the weight on the front end make the rear camber somewhat less significant.

For variable ride height systems the camber response of the suspension system is of great concern. One obvious solution would be an electric motor driven on the fly camber adjustment system. The best way to use an electric motor driven camber adjustment system would be on MacPherson strut independent suspension front and rear so that suspension compression camber response could be provided in the normal manor at any ride height. If an electric motor driven camber adjustment system was not used then it would be impossible to get perfect camber response at all ride heights. Unequal length A-arms would not work for variable ride height systems because the camber changes so radically as the suspension is compressed or extended more than a small amount. Equal length A-arms could be used for a variable ride height suspension system, but there would be no way to provide camber response. The lack of camber response would mean that the negative camber would remain one fixed value. If the negative camber was set to one degree for even tire wear then the vehicle would not stick to the pavement well. This would be acceptable for some types of utility vehicles that were used off-road much of the time, but the lack of road holding ability on dry pavement is a serious problem. If the negative camber was set up at a higher level then the cornering performance of the vehicle would improve, but tire wear problems then could not be avoided on straight roads.

A MacPherson strut suspension system could be made to work much better, but there would be some severe limitations in the absence of electric motor driven camber adjustment capability. The problem would be that the camber would change with the ride height. At the lowest ride height the negative camber would be extremely excessive, causing uneven tire wear. At the highest ride height the camber would become positive, making for extremely poor road holding. The saving grace of this simple type of MacPherson strut variable ride height system though would be that there would be a range of intermediate ride heights that would work quite well for most normal driving. With the ride height set just a bit high the tires would sit flat on the pavement and tire wear would be as low as possible. With this slightly high ride height there would still be some camber response as the suspension was compressed into a turn, and road holding would be at least a bit better than for the equal length A-arm suspension system. With a slightly low ride height the MacPherson strut suspension system would have more negative camber which would cause slightly uneven tire wear on straight roads, but road holding around turns would be even better. The lowest ride height would have the largest amount of negative camber that could be considered practical for high speed cornering. The vehicle might still corner and handle well at the lowest ride height, but the tires would not wear evenly on straight roads. At the lowest ride height there would also tend to be a problem with little camber response or even backwards camber response as the suspension was compressed further. With the A-arms angling up at the lowest ride height further compression of the suspension would tend to yield less negative camber. The extent to which the camber changed as the ride height was changed would depend on how far the struts were angled in towards each other. A wider range of ride heights where flat tire contact was available would be possible with less angle on the struts. Less angle on the struts would however mean less camber response as the suspension was compressed, which would require that the ride height be set lower for good road holding. Less angle on the struts would however also mean that backwards camber response would be a larger problem at the lowest ride height.

Adding electric motor driven camber adjustment to the MacPherson suspension system would mean that either very small negative camber for best tire wear or more negative camber for best road holding could be set at any ride height. Good camber response would also be available at most ride heights, although there would still tend to be a bit of a problem with insufficient camber response at the lowest ride height setting. To solve the problem of insufficient camber response at the lowest ride height the angle of the A-arms could be made slightly different than the angle of the drive shafts. This would mean that in the lowest ride height setting the drive shafts would angle up towards the wheels, where the A-arms would be level. At the highest ride height setting the A-arms would then angle down dramatically, but this would cause no greater problems than a very large camber response. The very large camber response at the highest ride height would be mitigated by the fact that the camber could be set flat for best tire wear regardless of what load was being carried or what suspension firmness was selected. For best possible off-road cornering ability with the highest ride height and softest suspension setting the camber could also be set to be a bit positive when the vehicle was flat so that hard cornering would not cause excessive amounts of negative camber that would actually compromise traction.

An electric motor driven camber adjustment system for an equal length dual A-arm suspension system would be undesirable for two reasons. The obvious problem would be that with no camber response from the compression of the suspension all camber compensation for body roll would have to be provided by the electric motor driven adjustment system. This would mean that the camber would have to be adjusted for every single turn if both good road holding and even tire wear was to be provided. If the electric motor driven camber adjustment system was made fast enough then integration with the stability control system could allow the equal length A-arm suspension system to work, but it would be more complex, heavier and trickier to get to work perfectly than a MacPherson strut based system. The other problem with a camber adjustment system on equal length dual A-arms would be that the electric motor driven system would be required to work against much larger forces, making the system even heavier and bulkier.

The simplest ride height adjustment system that just uses air bags to raise the vehicle would work fine with either a MacPherson strut suspension system or an unequal length dual A-arm suspension system. The vehicle would be operated at the normal ride height the vast majority of the time. With a MacPherson strut suspension system negative camber would be small at that standard ride height, but would increase considerably with suspension compression for good road holding. When the air bags were used to increase ride height the camber would become positive, but this would be acceptable because the higher ride height would only be used for special situations requiring more ground clearance. An unequal length dual A-arm suspension system might work even better with the simple air bag lifting system. Again the normal ride height would have just a small amount of negative camber, but the negative camber would increase considerably as the suspension was compressed in a turn. When the air bags were used to raise the vehicle there would for the first few inches be a situation with positive camber, but then as the ride height was increased further the camber would flatten out again. The camber response at this raised ride height would be backwards, which would mean it would not work for normal driving. With just air bags to raise the vehicle though the suspension would be extremely stiff at the raised ride height, and the backwards camber response would not be much of an additional problem. Again the raised ride height would only be used to traverse extraordinary obstacles, and all normal driving would be done at the standard ride height.



Back to Index to Technical Articles




©2014 Michael Traum