A dirt bike is a motorcycle intended primarily for use off of paved surfaces, although most dirt bikes are also well able to operate on pavement provided that a few minor setup changes are made.
Motorcycles are Dirt Bikes
Anatomy of a Dirt Bike
Riding a Dirt Bike on Pavement
Traditional Engine Setup
Roller Rod Bearings
Dirt Bike Engine Lubrication
The Power Band
Husqvarna of Sweden and Harley Davidson were the original high volume motorcycle producers, both going nearly all the way back to 1903. Husqvarna has stuck with dirt bikes over the intervening 110 years, where Harley Davidson motorcycles have become ever more street oriented. As late as the 1960's there was not a whole lot of difference between dirt bikes and street bikes, and many models were cross over designs intended to work both on dirt and on pavement. Starting in the late 1960's and into the 1970's though street bikes became lower and heavier while dirt bikes got longer travel suspension, retained the upright seating positions and became lighter in weight. The Harley Davidson motorcycles did not go in either direction, and instead created a new category of motorcycles which have become known as cruisers. The cruisers are heavy with short travel suspension like the street bikes, but they retain the upright seating position and usually have no fairings or windshields.
In the past 50 years or so the defining feature of dirt bikes has been their long travel suspension systems. In the 1970's rear shocks were moved forward on the swingarm and angled forward to allow the rear wheel to move farther up and down. To go along with increased rear wheel travel front forks got larger diameter tubes to allow more telescoping travel while remaining stiff enough for good high speed handling. In the late 1970's linkage type rear suspension took the dirt bike world by storm. The linkage allowed small and short shock absorbers to provide long suspension travel with a more linear rising rate. That is, the long 13 inch rear suspension travel could be provided for with a small and lightweight shock and spring without the spring rate becoming overly progressive.
Light weight has also always been important for dirt bikes. While street bikes went to V-twin, inline twin, horizontally opposed twins as well as inline three and inline four cylinder engines dirt bikes have retained the single cylinder engine. The advantage of the single cylinder engine is not only lighter weight but also a much more compact engine being neither excessively wide as an inline two, three or four cylinder street bike engine or excessively long as a V-twin cruiser engine. It is easy to see that a narrower engine is ideal for a small and maneuverable motorcycle, but the short engine is also an enormous advantage. With just the single slightly forward canted cylinder. there is lots of room for the carburetor, air cleaner and shock absorber behind the engine. The forward canted single cylinder engine also places the weight of the engine far forward, which is ideal for keeping the front wheel down on a short and tall bike with lots of power.
In the early 1980's disk brakes were quickly universally adopted on dirt bikes for two separate reasons. The increased stopping power and better heat dissipation of a disk brake versus a drum brake was a particular advantage for high speed street bikes, but on dirt bikes this advantage was also significant. On dirt bikes though the lighter weight of disk brakes was the really big advantage. Long travel suspension working rapidly over successive bumps just cries out for a low as possible unsprung weight, and the disk brakes dramatically reduce unsprung weight.
Later in the 1980's upside down forks once again revolutionized dirt bike suspension. By securely clamping the larger diameter outer fork legs in the triple clamps instead of clamping the smaller diameter inner fork legs in the triple clamps the rigidity of the suspension system was dramatically increased. It is not only the larger diameter of the outer fork legs that makes them more ridged, but also the fact that the outer legs are made out of lighter and stiffer aluminum where the inner legs are typically made of heavier and more springy steel which wears well riding against the bushings in the outer legs. For many years most street bikes stuck with conventional "right side up" forks both because the shorter travel made for a more ridged suspension system and also because low fenders allowed fork braces to hold the outer legs and prevent twisting. For the long travel dirt bike suspension low fenders were not practical both because they increased unsprung weight and also because they would easily clog with mud in nasty conditions.
Generally dirt bikes have a long enough wheel base and a relaxed enough steering head geometry for good stability at speeds up to about 80 to 90mph, which certainly is fast enough for highway use. The most important concerns with using a dirt bike on pavement have to do with rebound damping and tire pressure. For off road use at speeds generally less than 50mph tire pressure is usually reduced to about 10 to 15psi, with up to 18psi used for racing in very rocky conditions. The biased ply dirt bike tires don't have much trouble with overheating at these low pressures because high speeds over 50mph are generally not held for long periods of time, and reduced traction in the dirt does not stress tires as severely as the high traction of dry asphalt. To head out on the highway it is necessary to increase tire pressure up to at least 20psi, and more like 25 or 30 psi is required for aggressive high speed cornering on dry pavement. It is possible to traverse pavement and even highways with just 15psi, but care must be taken not to overheat the tires.
It is often thought that the long travel suspension systems on dirt bikes are not suitable for highway use, but this is not really true. It is not the amount of suspension travel so much as how that suspension travel is controlled by hydraulic damping that is important for high speed operation. At the 20 to 50mph speeds dirt bikes normally operate at off road less rebound damping can safely be used. The higher traction of dry pavement combined with higher speeds requires much more rebound damping to assure controlled and safe operation. If more rebound damping is used off-road large problems crop up. The biggest problem that large amounts of rebound damping cause off road is a lack of drive traction over small bumps. Excessive rebound damping can also cause bottoming of suspension over repeated bumps because the suspension simply does not have time to extend between the bumps. Luckily nearly all dirt bikes from the last three decades have had adjustable rebound circuits so that much larger amounts of rebound damping can be dialed in if aggressive pavement ridding is required. Pump up the tires and dial in more rebound damping and a dirt bike is ready to hit the highway.
Even once a dirt bike is setup to hit the open road though there are some riding style and equipment concerns that should be addressed. The more upright seating position of a dirt bike exposes the rider to the wind, and long hours at high speed can be quite demanding. The light cotton jersey that worked well for spring time off road ridding becomes woefully inadequate for a 100 mile cruise down the highway on a balmy 60 degree sunny afternoon. Goggles or glasses should probably always be worn when off road ridding to protect eyes from branches and flying rocks, but it is easy to get away with casual off road ridding without eye protection. On the highway though wind protection of the eyes is mandatory for speeds above 50mph.
Small displacement dirt bikes that make less than 40hp are pretty easy to ride on pavement because they are reluctant to get up to such high speeds that handling is a problem and they won't accelerate so hard that lifting the front wheel is much of a problem. Powerful 60hp open class dirt bikes on the other hand will easily exceed 100mph and will also lift the front wheel extremely easily in lower gears on pavement. Even with increased tire pressure and a large amount of rebound damping dialed in a dirt bike is not going to be friendly loosing traction on pavement at any speed. Ripping the tread blocks off of the sides of front tires is a common problem when dirt bikes go on pavement, and in most situations this is simply a matter of ruining a tire. At high speed though the change in traction caused by losing the side tread blocks can lead to handling problems. As far as lofting the front end goes it is just a matter of paying attention to smooth power delivery just like on a motorcross track. A tall dirt bike with 60hp is really only a problem when an inexperienced rider is not paying attention to smoothly delivering the power to the pavement.
The advent of modern computer controlled ignition systems has removed most of the idiosyncrasies of motorcycle tuning, but it is still of interest to address the traditional tuning procedures. The basic concept in traditional motorcycle tuning has to do with fixed advance curves. With just a single spark timing value for each engine speed there are severe limitations on what an engine can be made to do. What it comes down to is that an engine with a fixed advance curve is only going to run perfectly at one power output level at any particular engine speed. This sounds worse than it really is though because there are a number of mitigating factors that have allowed simple motorcycle engines to work rather well over a wide range of engine speeds and loads.
A sophisticated ignition system with a throttle position sensor can deliver the correct spark timing for any engine speed and engine load combination. This means more spark advance at low power output and less spark advance under heavier loads. Because traditional points or CDI ignition systems on motorcycles could not adjust for changing loads other tricks had to be used to get the engines to run well. The main thing that was done to allow motorcycle engines to run well was for the spark timing to advance only slightly from 3,000RPM to 10,000RPM. This meant that the spark was early enough for clean and efficient low to medium power output operation down at 3,000 to 5,000RPM while also being late enough for maximum efficiency and maximum power output up at 6,000 to 10,000RPM. With this only slightly advancing spark timing the engine was then harsh and inefficient under a full load at 3,000 to 5,000RPM which was generally loathed by beginner riders. The saving grace of this type of advance curve is the fact that gasoline engines don't run all that well down at 3,000 to 4,000RPM anyway, so not much was really being lost. For more on the limitations of gasoline engines see Combustion Properties of Fuel.
The other mitigating factor in getting motorcycles to run well over a wide range of engine speeds and engine loads is the use of small bore diameters to allow full flame front travel combustion up to higher engine speeds. This was particularly significant for the small bore multi cylinder street bike engines which often had bore sizes of less than three inches. With less than a three inch bore and fast flame front travel premium gasoline a very usable medium power output could be delivered up to 5,000RPM. By keeping the compression ratio down to 9:1 or 10:1 some significant amount of power could be produced in full flame front travel mode for low speed cruising. With the spark timing at about 20 to 30 degrees before top dead center and the throttle just cracked open a range of low power output levels could be smoothly delivered from about 2,000 to 5,000RPM on a small bore engine. Then when the throttle is opened more at 3,000 to 5,000RPM late compression ignition takes place and much more power can be produced. Depending on how small the bore size is and how fast the flame front travel speed of the fuel small amounts of power may be available in full flame front travel combustion mode all the way up to about 6,000RPM, but generally engine speeds of more than about 5,000RPM would only be used with the throttle opened substantially with late compression ignition taking place.
The big difference between dirt bikes and street bikes came in how the advance curve was shaped down at 2,000 to 4,000RPM. For street bikes it was generally necessary to get the engine to run cleanly and reasonably efficiently down at 2,000 to 3,500RPM. This was accomplished in two different ways. One was a rather low 8:1 or 9:1 compression ratio so that the engine could stay in full flame front travel mode up to moderately high power output levels. The problem with the low compression ratio thought was that then making power in late compression ignition mode up at 6,000 to 10,000RPM required a large amount of spark advance that hurt performance and efficiency while also reducing engine life. The other strategy for street bikes was to use a higher 11:1 or 12:1 compression ratio so that the engine easily entered late compression ignition mode at 2,500 to 4,000RPM under moderate loads. Again though this lead to harsh operation at maximum power output at these low engine speeds. From a performance perspective it was better to use the high compression ratio and simply avoid low engine speed when substantial amounts of power were required.
Dirt bikes often used a different strategy so that they could deliver large amounts of power down to low engine speeds. A rather high 11:1 or 12:1 compression ratio was used with a carefully shaped advance curve with quite late spark timing values. With the spark timing quite late at low engine speeds the engine would only enter late compression ignition mode under heavier loads. This allowed the engine to make as much power as it possibly could all the way down to 2,500RPM, but it also caused extremely high fuel consumption and dirty operation. The problem with the spark timing being late for maximum torque generation is that lower power output levels are supported in full flame front travel mode without enough time for the flame front to reach the "far corners" of the combustion chamber. A 12:1 engine running in full flame front travel mode at 5,000RPM is going to be extremely dirty and inefficient.
This maximally advancing curve on an 11:1 or 12:1 engine also tends to be the most sensitive to small changes in the properties of the fuel or small changes in spark timing. A problem with all fixed advance curves is that they only work at their best for one compression ratio and one particular type of fuel. If the compression ratio is changed during a rebuild or a different fuel is used a compensation can be made by changing the static timing setting, but this is only a partial compensation. Since the late spark timing smooth torque dirt bikes are the most sensitive to timing and fuel changes a mismatch between the advance curve and the fuel being used will show up as a more dramatic problem. If the compression ratio is increased or a lower pressure fuel is used then the direction of change will be towards lower fuel consumption at low power output and harsher operation during maximum power delivery at low engine speed. In this case just backing off on the static timing setting will work fairly well, although the range of engine speeds where big power is produced may become somewhat narrower. The narrower power band can generally be lived with though as peak power would increase with a switch to a higher compression ratio or a lower pressure fuel. Going the other way is where the insurmountable problems crop up. If a switch to a higher pressure fuel is made or if the compression ratio is reduced then the stock advance curve is going to provide too much additional advance as the engine speed is increased. If the static timing setting is advanced so that the engine will run at low engine speeds then there is going to be excessive advance at high engine speeds. Too much advance at high engine speeds will cause harsh operation and maximum power output will suffer. Sometimes both the fuel and compression ratio can be changed at the same time to get a stock advance curve to work reasonably well again, but this is tricky to pull off. The main problem is assuring a constant supply of the new fuel required for the new lower compression ratio. The compression ratio was originally set at 11:1 or 12:1 on these dirt bikes because that is what was required to get the commonly available premium gasoline to run well. There may be specialty fuels that will work similarly in a 9:1 engine, but they have not been commonly available.
It is not that dirt bike engines run at particularly low engine speeds, quite the contrary. Most dirt bikes routinely rev to 8,000RPM or even substantially higher and most experienced riders rarely use engine speeds less than 4,500RPM for anything but the slowest casual cruising. The requirement of a dirt bike to make as much power as possible down to lower engine speeds has to do with getting out of tricky situations. If a shift is missed or things get a bit out of hand at low speeds in rough and rocky terrain having as much torque on tap as possible down to at least 3,000RPM is a valuable asset.
Most traditional dirt bikes with fixed advance curves were setup as a compromise so that large torque could be generated down to 3,000RPM with only a moderate amount of harshness. Since this low engine speed high torque generation was mostly for emergency situations with all normal high power generation done at higher engine speeds it was only necessary for the engines to survive the harshness occasionally as opposed to having to live with it all the time. All gasoline engines are somewhat harsh down at 3,0000 4,000RPM, but the fixed advance curves of traditional dirt bikes made for even more severe harshness. Surviving this inevitable harshness at low engine speed required robust bottom ends with sturdy bearings. The solution has been roller rod and main bearings.
Rod bearings are cylindrical roller bearings, sometimes called needle bearings, and main bearings are typically large ball bearings. Large ball main bearings work fine because there is no limit to how big they can be made to handle the high shock loads of harsh low speed operation. Big ball bearings are a bit expensive, but they get the job done. On the big end of the connecting rod size does matter. The rod bearing needs to be as small in diameter as possible to keep the rod light and keep reciprocating losses at high engine speed down to a low level. Cylindrical roller bearings do this well because small sizes can handle enormous loads.
Single cylinder engines with assembled (pressed together) three piece crankshafts are particularly good for high speed operation because they use one piece rods with no rod bolts. The lack of rod bolts reduces the weight of the rod considerably, and this is in a large part responsible for the good high engine speed performance of most dirt bikes and single cylinder street bikes. Two and three cylinder engines sometimes also have used assembled crankshafts and one piece rods, but the assembly of five or seven piece crankshafts tends to be much more problematic. Single cylinder dirt bikes always use one piece rods as three piece crankshafts are actually quite easy to build.
Roller rod bearings work well for dirt bikes because they can handle very high loads, especially for short periods of time. The high load carrying capability of the roller rod bearing allows the size of the crank pin to be rather small with a correspondingly small rod big end diameter further improving high engine speed performance. A 600cc four stroke dirt bike with a three inch stroke can hold up amazingly well with just a 30mm diameter crank pin (30mm rod journal diameter) even running quite harshly down at 3,000RPM. It is this durability of roller rod bearings that has allowed traditional dirt bikes to work fairly well even with fixed spark advance curves.
Most four stroke dirt bikes have used forced lubrication systems where high pressure oil is forced through passages in the crankshaft and directly into the rod bearing. Some four stroke dirt bikes have however used different lubrication systems. Some older motorcycles relied only on the rod bearing splashing into the oil in the crankcase to lubricate the rod bearing, and this did not work well at all at higher engine speeds. The early Husqvarna four stroke dirt bikes from 1983 to 1992 use a R.A.L. (Reed Activated Lubrication) system where differences in crank case pressure force oil from the clutch chamber to the crankcase. Two stroke dirt bikes relied on the oil and fuel mixture blown through the crankcase to lubricate the rod bearing, and this worked well enough under most conditions. With oil atomized all throughout the crankcase some of it made it's way into the roller rod bearing.
The early R.A.L. Husqvarna four strokes were a strange breed. The pressurized oil was not fed through passages in the crankshaft, but rather was simply squirted in the general direction of the rod bearing. The way this worked was that the pressurized oil was forced out as a jet into the crankcase, and the rod bearing crashed into this jet of oil as it speed past. This actually worked a whole lot better than simple oil dippers on the big end of the rod, and the early Husqvarna four strokes were able to reliably operate up to 10,000RPM. There were however some severe limitations on the functionality of this strange lubricating system. One was that blow by past the rings tended to pressurize the crankcase and stop the R.A.L. system from pumping oil. With the throttle closed the R.A.L. was able to pump oil quite effectively and lubricate the rod bearing, but the flow of oil was slowed or interrupted under heavy acceleration. The other big problem was simply that oil got violently slung out of the rod bearing by centrifical force at high engine speed and the jet of oil was not able to keep the rod bearing lubricated if the engine speed was kept very high for a long period of time.
What both of these limitations of the early Husqvarna R.A.L. oiling system add up to is the fact that these otherwise quite competent engines do often work well in dirt bikes but can very quickly be destroyed by continuous high engine speed operation. If the engine is revved up to 10,000RPM just for a second or two before shifting then the rod bearing stays well lubricated and lasts for a reasonably long period of time. Likewise if maximum power output at 6,000 to 9,000RPM is used only for short periods of time on dirt trails and tracks interspersed with periods of coasting then the fact that the lubricating system only works with the throttle closed is not a problem. If these early Husqvarnas are ridden too fast on the open highway though the rod bearings do not last well at all. Just five minutes wide open at 8,000RPM can easily toast a perfectly good rod bearing.
The two stroke dirt bikes sometimes suffered rod bearing failure from continuous high speed operation for similar reasons, but there are some significant differences in how this plays out. The more power a two stroke makes the more fuel and oil is flowing through the crankcase to lubricate the rod bearing. A large amount of fuel evaporating into the crankcase at high engine speed also tends to keep the rod bearing cool. A 125cc two stroke ridden wide open at 10,000RPM in the sand dunes usually held up reasonably well, but instances of rod bearing failure at high engine speed have occurred.
Forcing the lube oil directly into the rod bearing totally solves these high engine speed failure problems, four stroke dirt bike engines generally don't suffer from rod bearing failure even up to 13,000RPM.
In the last few years KTM has introduced plain rod and main bearings on their 250, 350, 450 and 500cc four stroke dirt bikes. The use of plain bearings allows a large crank pin to be used with a smaller rod, but any real advantages are dubious at best since plain bearings cannot support as high of loads as roller bearings. For high engine speed operation the KTM plain bearing crankshafts seem to hold up quite well. Likely the real reason for the switch to plain bearings though is a matter of cost. The roller and ball bearings are expensive where the plain bearings are extremely cheap. The less competent load handling capability of plain bearings at low engine speeds is offset by the computer controlled KTM engines that never create excessive harshness at low engine speed. Not only can the computer controlled engine management system keep the time of late compression ignition as at the latest possible value at low engine speed to prevent excessive harshness , but fuel delivery can also be limited down at less than 4,000RPM to further protect the vulnerable plain bearings. This has all worked out quite well, and the KTM dirt bikes have a reputation for both longevity and very high performance.
With the clutch ridding in the same oil as the engine the number one determinant in engine life is oil change intervals. All four stroke dirt bikes that run the engine oil through the clutch last a whole lot longer if the oil is changed frequently. Particularly if the clutch is slipped a lot during aggressive ridding ridiculously short oil change intervals are required. Regardless of what type of engine bearings are used they are not going to last nearly as long if large amounts of friction material are running through them. Particularly the early Husqvarnas with their pathetic R.A.L. oiling system have a reputation for only lasting well if the oil is changed extremely frequently. On the early Husqvarnas this means 10 to 15 hour oil change intervals for casual ridding (with little or no clutch slipping) and about 3 hour oil change intervals for racing conditions where the engine speed is nearly always above 6,000RPM.
The most important thing about power delivery on a dirt bike is the shape of the power curve versus engine speed. Of course more power is faster, but in many types of compromised traction conditions it is all about smoothly getting what power is available to the ground. What this comes down to is that the power output of the engine has to increase gradually and mostly linearly between shift points. When the next higher gear is first engaged is when traction is most likely to be broken, so a bit less power at the low end of the operable speed range is desirable. As the bike accelerates in the new higher gear and gains speed more power can be applied to the ground without breaking traction, or at least without spinning the rear tire so fast as to cause undue damage to the trail surface or to compromise handling and control. Ideally the power should continue to increase gradually all the way up to the shift point, and this is where the concepts of a power plateau, width of power band and overrev come into play. On many dirt bikes the shift point comes up on a power plateau where the power output of the engine has leveled off and remains at the peak value for 1,000RPM or more. This works fairly well to shift up on the power plateau, but a bike is faster and more fun to ride if the power continues to build at least slowly all the way up to the shift point. If the power band is so narrow that it is necessary to rev the engine past the power plateau to where power output is actually dropping off then the bike will feel like it is missing something. If it is an open class bike with 50 or 60hp on tap it is still of course going to be wicked fast, but that dropping off of power before the shift point is disconcerting and not at all confidence inspiring.
Just where the shift point is located depends on how the engine runs down at the other end of the operable speed range. Generally speaking it is always necessary to (up)shift a dirt bike down to no less than 6,000RPM because the power output of gasoline engines universally drops off dramatically at engine speeds less than about 6,000RPM. In order to shift down into 6,000RPM with 75% gear jumps the shift point will be located slightly higher than 8,000RPM. The shift point ends up located about 5-10% past the theoretical shift point simply because there is some delay during shifting. For racing, dirt bikes are often flat shifted wide open without using the clutch, but this does not always work out well for casual ridding where the transmission is expected to continue shifting for hundreds of hours. A stab at the clutch and a slight blip of the throttle assures low stress engagement of the gears and is necessary on many dirt bikes if extended use is expected.
Down between first and second and second and third the jumps between gears are usually somewhat wider, meaning that the engine has to be revved out even farther to shift down to 6,000RPM. It is also true that 6,000RPM is the absolute minimum engine speed to shift down into, and many dirt bikes actually need to be shifted down to no less than 7,000RPM. Particularly the 250 four stroke dirt bikes with direct acting overhead camshafts (cam and bucket valvetrain) don't make power at less than 7,000RPM and actually end up being shifted down to no less than 8,000RPM for maximum acceleration. The KTM 250 with it's slightly more competent finger follower valve train runs over an enormously wide range of engine speeds from 7,000RPM to 14,000RPM with the power continuing to increase all the way up to the rev limiter at 14,000RPM. With the shift point all the way up at 14,000RPM the KTM 250 can be shifted down into more than 10,000RPM where it is already making 40hp. That is a fast 250 four stroke! Because the power band of a 2014 to 2015 KTM 250 SX-F, a KTM 250 XC-F or a 2015 Husqvarna FC 250 is nearly linear from 7,000RPM to 14,000RPM any section of that power band can be used depending on how much power is required. There is of course some curve to that broad power band, and the power does in fact build quite rapidly from 7,000 to 8,500RPM. For slippery conditions shifting down into this more steeply angled segment of the power band between 7,000 and 8,500RPM is just the ticket for maintaining traction while staying on the throttle and accelerating hard.
Power delivery can of course also be modulated with the throttle and the clutch on most dirt bikes, but hard acceleration from second or third gear happens so fast that there is actually very little time to do anything other than concentrate on where the rocket sled is about to take you. Back in the era of 2.85 inch stroke 250cc two stroke dirt bikes the power band of a race prepped 60hp bike tended to be extremely narrow. With the long and heavy two stroke piston on nearly a three inch stroke maximum engine speed was around 9,000RPM. When these engines were tuned to make big power for racing all the power came at the top of the power band and there was very little down below 7,000RPM. This extremely narrow power band, not quite wide enough to cover a shift, necessitated extensive use of the clutch. As a result two stroke dirt bikes were normally raced with the clutch slipping through much of the period of acceleration. This was not as fast or as controllable as the wide power band of a four stoke, which is why 50hp four strokes quickly took over wining all the races in the early 21st century despite their higher weight and less refined chassis dynamics.
The traditional cornering technique used by motorcross racers in the 1980's and 1990's was to brake hard into a corner and come nearly to a stop before accelerating hard out the other side of the corner. This was not the fastest way around a turn, but it was the strategy that won races in that era. What was going on was that the bikes were big and unwieldy but able to accelerate hard which meant that passing in corners was all about being in a position with a clear line for accelerating up into the next straight. The fastest way around a turn is actually to carve the turn under power.
Aside from the stop and go cornering technique of the late 20th century there are in fact four ways to carve a turn. The easiest to perform is the power slide, and this can in some instances be a fast way around a turn. The most difficult to perform is the neutral carve where only enough power is applied to maintain a constant speed around the turn. The neutral carve is the fastest way around turns in challenging conditions with irregular obstacles, but most riders find that neutral carving of turns leads to an excessive number of front end wash outs. The absolute fastest way around turns is a power carve where most or all of the weight is on the rear tire and substantial power is being applied to slightly break traction. The power carve is difficult in that it requires commitment to the maneuver, and if the apex is calculated incorrectly or something changes during the maneuver there is only a very limited amount of correction that can be applied.
A pure power carve where the front tire is off the ground through the entire turn is the absolute fastest way to change from going one direction to going another direction while maintaining momentum. Because this pure power carve is so difficult to accomplish under any circumstances most turns in racing end up being carved with some combination of the three carving techniques. The easiest combination is neutral carving in the first part of the turn and a power slide out of the turn. The way to do this is to brake just before the turn, dive in hard on the front tire with no power applied and carve to a position where the exit of the turn can be seen. Once the exit of the turn can be seen acceleration can be started with a power slide culminating in a weight transfer at the exit of the turn that un-weights the front wheel. This combination of a neutral carve followed by a power slide is what happens much of the time because the body position for a neutral carve is similar to the body position for a power slide. That is, weight is rather far forward on the bike. Because the rear tire does not grip and accelerate as fast as it can in a power slide though this is a bit slower than a power carve. A combination of a neutral carve followed by a power carve is both extremely fast while also being flexible enough to accommodate changing conditions or unseen obstacles. The weight transfer in the middle of the turn is however not usually easy to pull off smoothly. The typical result of an attempted power carve after a neutral carve is an excessively lifted front wheel that requires the power delivery to be interrupted momentarily to get the front end back down. This interruption of power delivery costs valuable time and means that the neutral carve followed by the power slide often wins out.
The reliable way to quickly get around a turn is to neutral carve the whole way until a weight shift for acceleration is performed all the way at the exit to the turn. This is however tricky for racing because it precludes acceleration in the late part of the turn. With no opportunity for acceleration late in the turn the entry speed must be perfectly matched to the turn and the traction conditions. Because the bike will be slowing down throughout the turn there also has to be some compensation made to keep the front tire just on the edge of traction throughout the turn. An easy way to make this compensation is to come into the turn "hot" and then turn in as the bike slows towards the middle of the turn. Turning in is however not the fastest strategy because it makes for an irregular shaped apex as well as a significantly slower exit speed. The fastest way to neutral carve a turn is to smoothly modulate a small amount of power delivery throughout the turn in order to maintain speed. This is difficult to accomplish because it requires modulation of the throttle at the same time that the front tire is being danced through the turn over and around small obstacles just on the verge of traction. In any case neutral carving tends to be unpopular because the loss of traction on the front tire means that the bike goes down. Crashing in a race is not the way to win. A miss calculation during power carving that requires the power be rolled back for a fraction of a second might be the difference between passing and not passing or being passed and fending off the pass, but at the end of the race those fractions of a second lost while throttling back are insignificantly small compared to the long seconds required to pick up (and often restart) a downed bike. "Washing the front end" and crashing might be a gentle sort of a crash, but it can still cause injury or damage to the bike.
The oldest technique for entering a turn was developed back in the dark ages of weak drum brakes and bikes with only rear brakes. This corner entering technique is preserved in dirt flat track racing, where the bikes still have no front brakes. In the absence of good brakes the technique is to throw the bike sideways well ahead of the turn and slide sideways with no power applied. If done correctly with no rear brake applied this sideways sliding can generate braking forces equal to or even in excess of the hardest straight up braking possible. The reason that more braking force can be applied with a sideways slide is that both the front and the rear tire are biting, where straight up braking is largely on just the skinnier front tire. Sideways sliding with no rear brake applied for slowing into a turn can easily be converted into a power slide for carving the turn simply by applying a bit of power. Because sideways sliding into a turn can be done with the rider's weight farther back than neutral carving changing directly to power carving can also be accomplished. Sideways sliding into a turn followed by power carving the entire turn is the absolute fastest way around a turn. As with power carving though sideways sliding for braking is difficult to pull off and requires commitment to the maneuver. Particularly when multiple bikes are entering the same corner at the same time the extra space taken up by sideways sliding bikes is usually considered incompatible with other cornering techniques. Riders do sometimes sideways slide into turns on motorcross tracks, but it is generally not the preferred corner entering technique.
The long 12 or 13 inches of suspension travel on a dirt bike can be very useful for slamming into bumps at speed, but what it is really for is coming down out of the air. When motorcross or suppercross races are watched it might be thought that flying so far through the air is what requires competent long travel suspension, and this is in fact partially true but not for the reasons first assumed. What really requires large impact absorption capability is landing flat off of smaller jumps. The big "timed" jumps on a motorcross track have sloping landing ramps that help to absorb the impact of the landing. Landing on a downward slope is much more gentle than landing flat. Even rather small bumps can easily send a dirt bike so far up in the air that coming back down to earth pushes the limits of the best suspension systems and the best riders.
The most effective way to land flat from going over a bump is the powered landing. A powered landing involves allowing the rear tire to drop far down below the rider with the front tire up high in front of the rider. The rear tire is accelerated before it contacts the ground and power is applied as the front wheel is coming down to make contact. Applying power before the front tire touches down absorbs the impact, and large drops can be taken in this way. In this maneuver the rider is also absorbing the impact with his legs by dropping from a standing position upon impact to a seated position as the front suspension fully compresses. In this way a flat landing is actually taken in four distinct phases, each with about a foot of effective suspension travel. The first stage is the compression of the rear suspension just after the rear wheel touches down. As the rear suspension is finishing compressing the bike pivots forward which drops the center of mass of the bike and rider another foot. As the front wheel touches down the rider begins to squat into a seated position which represents about another foot of effective suspension travel. The final energy absorbing phase is the compression of the front suspension which happens as the rider comes to a seated position. If this is all done correctly dropping from as much as 10 feet up in the air can be fairly smooth and painless if not exactly easy on the rider. If however the bike does not get rotated back enough in the air or if the power is not delivered soon enough the suspension of the bike has to take the full force of the impact and violent bottoming can occur.
For trail and off road ridding jumping is usually a matter of clearing obstacles. If a particularly nasty rocky section of trail has a smoothly angled rock at the beginning this smooth rock can be used as a stepping stone of sorts to leap over the worst rough section. Making this work requires finding a smooth area to land and aiming for it before leaving the ground. Coming down out of the air in a rough rocky section is about the worst thing that can be done, and tends to lead to bloody and painful crashes. Jumping with flat landing also often occurs in off road riding or racing simply because slowing down so much that the tires stay on the ground would take more time. In these cases the goal is to keep the bike as low as possible while carrying speed over the bump.
On a motorcross tack the big jumps with landing ramps are a spectators delight as well as a weightless thrill. While actually racing the jumps are difficult obstacles that have to be delta with, but if everything is going well the sensation of flying through the air is pleasant and relaxing. The timed jumps are indeed all about timing. Leaving the jump with just the right amount of speed to land on the most heavily sloped part of the landing ramp. Overshooting a big jump means flat landing past the sloping ramp. On tracks with only medium size double jumps for beginners and low powered bikes many riders routinely do overshoot the landing ramp, but this is both hard on the rider and hard on the track. What happens if many riders are overshooting and flat landing the biggest double is that a pit gets dug just after the landing ramp. This pit is from everyone power landing in roughly the same spot. The big problem with the pit is that the fastest riders end up landing just at the end of the pit where the upward sloping ground is worse than a flat landing. Traditionally a 60hp race prepped 250cc two stroke or an open class bike could clear the pit and flat land in clear real estate on the far side. Bigger tracks for serious riders have bigger jumps, and overshooting is not so much of a problem. Bigger jumps however cause other and even worse problems. If something goes a bit wrong on the takeoff and the jump is not cleared then a very violent impact on the face of the landing ramp can occur. It is these impacts with the face of a jump that most severely test riders and suspension systems, and on big tracks they often lead to serious injury.
Safer jumps are big table top doubles where the landing ramp follows a high plateau. This high plateau is long enough that the worst that happens from a bit of a mishap on takeoff is a flat landing up high. Landing flat up on the table top before the landing ramp is actually not too bad. Not only is the distance jumped less than for overshooting a jump, but landing up high means that the impact is much less severe than landing down at track level. For spectacular high altitude and long distance flights to impress spectators or the camera the table top landing gets in the way. From the perspective of hang time and the sensation of flying though big table top landings are just as good as fully open big doubles.
Jumping is not as simple as just timing with the takeoff speed to land on the ramp. There are a lot of other things that can happen while flying through the air second after second. If the bike begins to rotate back while in the air closing the throttle or tapping the rear break slows or stops the rear wheel and adds a forward rotating impulse to bring the bike back level enough to land. Likewise if the rear begins to lift while in the air application of power to accelerate the rear wheel adds a rearward rotating impulse to bring the bike back level enough to land. On a downward sloping landing ramp there are actually a number of different landing techniques, and landing on the front wheel can work perfectly well.
If the bike begins to rotate sideways or roll to one side or the other corrections are in the form of aerodynamic adjustments (real flying). If the bike begins to rotate to one side then going with the flow and allowing rear end to kick out can actually bring the bike back to flying straight before the landing. This tail whip maneuver has become a stylized trick, but it has it's origins in the frantic flapping of a creature thrown through the air. The basic idea is that the rear wheel square to the wind will slow down more than the front wheel which is kept pointed into the wind. The wind drag on the rear wheel will counter the original rotational velocity and the bike will come back to land straight on the landing ramp. Even if the bike lands somewhat sideways recovery without a crash is likely as long as the front wheel comes down pointing in the direction of travel. Landing with the rear wheel out to one side certainly is a bit violent, but it does not always spell disaster.
Roll to one side can also occur after takeoff, and similarly there are aerodynamic corrections that can be applied. In this case it is also the flat side of the dirt bike that is used as a wing, but the aerodynamics are a bit more sophisticated. Essentially the bike is thrown out away from the direction of roll and twisted over so that it slices through the air on an angle. With the flat side of the bike acting as a foil a rotational force can be generated to counter the original rolling velocity.
Much like a cat dropped upside down, motorcross riders are often able to twist themselves and their bikes around in the air to land rubber side down. These aerodynamic corrections have been taken to an extreme of late with the "scrub". The goal of the scrub is to keep the bike low over the jump so that more speed can be carried without overshooting the landing ramp. A scrub involves leaning the bike way over on the lip so that the center of mass of the bike and rider stays as low as possible coming off of the jump. Coming off of the jump low and leaned over then requires extreme corrections in the air in order to land reasonably straight on the ramp. The corrections for recovering from a scrub are similar to the angled bike toss for recovering from undesired roll coming off a jump.
Of course the best way to land straight is to be set up for it coming off of the jump, and even in the case of the scrub recovery is easiest if the bike is already headed in the right direction before it becomes fully airborne. Small last minute jabs at the lip with the rear tire can be used to set recovery roll in motion, and each rider has his own little tricks for getting the flight maneuver to work out. Most riders learn how to do these things by trial and error with quite a lot of crashing over many years of ridding. Figuring out how to fly well and land straight off of a big jump really is a lot like the frantic flappings of a terrestrial creature thrown into the air. We all seem to have some innate ability to manipulate the air as we fly through it, but learning how to do it while holding onto a dirt bike is not the most natural thing in the world.
What does however tend to come very naturally is flapping ones arms like a chicken after ditching the bike in mid air. In the worst kind of messed up takeoffs the last ditch strategy to land straight on the ramp is to throw the bike off in one direction or another and land on ones feet. This would not generally be considered a good idea, but many horrendous crashes have been bought down to minor tumbles by ditching the bike mid-air. Once free of the bike chicken flapping can be used quite effectively to correct flight and straighten up for a square landing feet first. The other thing that throwing the bike can do is to adjust the point of landing. If a jump is hit crooked and it looks like the landing ramp will be hit just on it's side then throwing the bike sharply out away from the landing ramp can get the rider to land gently on the sloping ramp instead of violently crashing into the side of the ramp. Throwing the bike is also a last ditch strategy for recovering from severe roll or twist coming off of the jump. Throwing the bike away and landing on your feet is far preferable to rolling over and having the bike land on top of you.
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