The Husqvarna six speed transmission was the same for about 20 years, 1984 to 2000 that I know of and probably all the way up to 2004 which was the last year for the Swedish four stroke platform. That is a pretty long run considering how rapidly dirt bikes changed within that period of time. This same transmission was used on many two strokes 175, 250, 400 and 500 through the 1980's and it was also used on all of the four strokes through the 1980's and 1990's including the 1980's Swedish 510 and the 1990's Italian 350, 410 and 610 models. It is a robust transmission with all straight cut gears that rides mostly on cylindrical roller bearings. The total spread of gears is fairly wide at 3.53:1, at least by motorcycle standards. The spread of gears is fairly typical for a motorcycle transmission, with the spacing between first and second much wider than the spacing between fifth and sixth.
It is this variation in spacing that is of great interest. Manny street bike transmissions have an even more extreme variation in spacing, with the jump between first and second even larger and fifth and sixth even closer together. Many dirt bike transmissions on the other hand have more even spacing with less variation in jumps between the gears.
Ratios and Riding
What is it Good for?
Why is it in a Dirt Bike?
What a Dirt Bike Needs
The jumps between the gears in the Swedish six speed Husqvarna transmission are as follows:
First and Second: 69%
Second and Third: 75%
Third and Fourth: 76%
Fourth and Fifth: 84%
Fifth and Sixth: 84%
Motocross models had four and five speed transmissions with slightly different ratios. For 1991 the WMX 610 got a five speed that was nearly identical to the six speed, but with first gear missing. This eliminated the large jump between first and second gear, which is the common feature of all the four and five speed motocross transmissions. There is however another difference in the 1991 WMX 610 five speed transmission, third gear is a bit lower. It is just a one tooth difference from the 23/24 fourth gear in the six speeder to a 22/24 third gear in the five speed. It is a small difference but it makes the spread of ratios look somewhat more even. The jumps between the gears on the 1991 five speed are as follows:
First and Second: 76%
Second and Third: 80%
Third and Fourth: 81%
Fourth and Fifth: 84%
There was also a six speed transmission that went in the same cases that had a tighter spacing between first and second gears, this was used on some 125, 250 and 500 two stroke Cross Country and Enduro models through the 1980s. This smaller jump between first and second was 72%, so it was really only a rather small bit tighter than the 69% jump on the more common six speeder. The jump between third and fourth is also a bit tighter at 80% in this two stroke transmission, but fifth and sixth are the same as on the more common six speeder. The jumps between the gears on the two stroke six speed are as follows:
First and Second: 72%
Second and Third: 76%
Third and Fourth: 80%
Fourth and Fifth: 84%
Fifth and Sixth: 84%
A third six speed transmission was also used on 125 and 250 motocross two strokes. This motocross specific six speed had tighter gear spacing all the way through. The jumps between the gears in the motocross two stroke six speed are as follows:
First and Second: 80%
Second and Third: 80%
Third and Fourth: 84%
Fourth and Fifth: 84%
Fifth and Sixth: 88%
A 125 two stroke motocross specific six speed with yet tighter spacing was introduced in 1986. The jumps between the gears on the 125 motocross transmission are as follows:
First and Second: 80%
Second and Third: 83%
Third and Fourth: 84%
Fourth and Fifth: 88%
Fifth and Sixth: 92%
The four speed used on the 1980's four stroke motocross bike and some of the 500 two stroke motocross bikes has fairly narrow spaced gears and an even spread. The jumps between the gears on the Swedish motocross four speed are as follows:
First and Second: 76%
Second and Third: 80%
Third and Fourth: 84%
The Italian four speed used on later TC 610 motocross bikes has an uneven spread of gears. The jumps between the gears on the Italian four speed are as follows::
First and Second: 84%
Second and Third: 82%
Third and Fourth: 88%
The four speed TC 610 would best be geared up rather high so that only the wider jump from second to third was used on most tracks. With the stock 14/52 gearing the four speed TC 610 would in fact go 81mph in third gear at 8,500RPM. Fast enough to blast past a Japanese 450 rowing the gear box on a first turn straight.
Any transmission tends to benefit from slightly wider spacing between the lower gears and slightly tighter spacing between the higher gears. This is true on just about any type of land vehicle. Just how much of a variation in ratios is ideal depends a lot on the application.
For riding a dirt bike less variation is usually best. The big 69% jump between first and second gears on the Swedish six speed nearly always seems excessive on a dirt bike. This is true on both the 350 and 610 four strokes as well as on the 400 two stroke. Perhaps unsurprisingly the big jump between first and second is the biggest problem on the smallest displacement motor, the little 350 four stroke. The result is that the 350 four stroke tends to need to be geared down quite low with about 13/51 or even 13/52 gearing to work well off road. The stock gearing for the 350 was 12/50, which is oppressively low for general use and the 12 tooth sprocket does not last nearly as well as a 13 tooth.
Again unsurprisingly this big jump between first and second gear results in various levels of difficulty depending on how well the small bore motor happens to be running. If the engine will make good torque over a reasonably wide range of engine speeds then the big jump is of course much less of a problem. For the stock 1991 WXE 350 with a 10.2:1 compression ratio the problem was in being forced to run gasoline for much higher compression ratio engines that would not make torque bellow about 6,000RPM with 28 to 33 degree BTDC spark timing. When the small bore motor is able to run better with a good match between the compression ratio of the engine and the fuel being used reasonably good torque is available down to 4,500RPM and even 3,500RPM making the big jump much more manageable.
It is also true on the 610 four stroke that a better running engine does not suffer as much with the big 69% jump between first and second gears. On the 610 the jump between first and second has a much different feel to it depending on what gearing is used, but the 610 does not tend to get stuck on a technical climb if the gearing is a bit off like the 350 does. The stock 14/50 gearing works fine on the six speed 610, and second gear gets used a lot even on tight single track trails. Going up to 15/50 gearing though makes that big jump between first and second start to seem rather large. Again though it depends on how well the motor will run over a wide range of engine speeds. The 610 motor will make some harsh torque down to 3,000RPM, but for the most part engine speeds above about 3,500RPM are best for any gasoline engine. On the other end the 610 will always rev to 6,000RPM, but sometimes depending on the state of tune and fuel being used the 6,500 to 8,500RPM range of engine speeds where the big power is can be a bit elusive. Even when the motor will only run from 4,000 to 6,000RPM that is a wide enough range to easily get between first and second, but it does not leave any wiggle room for timing the shift when it is convenient. If the 610 motor will only run from 4,000 to 6,000RPM then the shift point has to be hit perfectly each time which is not always so easy to accomplish.
On the 400 two stroke the jumps between the gears tend always to seem somewhat too wide, especially the lower gears. The cylinder port two stroke just has a very hard time pulling over a wide enough range of engine speeds to get between the gears without slipping the clutch. For this reason the big two stroke is best geared down fairly low with the stock 14/52 gearing so that first gear rarely needs to be used. When the big 400 two stroke is running really well it will pull from 5,500 to 7,000RPM, which is just barely enough to get between the higher gears without slipping the clutch. Since a whole lot less power is available from 5,500 to 6,000RPM than at 6,200 to 6,800RPM though clutch slipping is a given. The 400 two stroke will actually pull up to 7,400RPM on just the right fast flame front travel speed gasoline, but the power is always dropping off dramatically up there above 7,000RPM. Even on the best gasoline the 400 two stroke is only making about 35hp up at 7,400RPM compared to 60hp or more at 6,800RPM. The over rev on a good running cylinder port two stroke tends to be very weak. On somewhat slower flame front travel speed gasoline the power tends to end very abruptly, usually at 6,900 or 7,000RPM just above the point of peak power production at 6,800RPM. The slower flame front travel speed gasoline results in essentially zero over rev, the motor makes more and more power until the top of the power band and then it just totally falls flat. On even slower flame front travel speed gasoline, or gasoline for higher compression ratio engines, the power cuts out even earlier around 6,000RPM. Normally once the big two stroke starts making power "on the pipe" at 6,200RPM though it is able to pull up to peak power at 6,800RPM. Regardless of how well the 400 two stroke happens to be running the 75% jumps between the gears tend to seem extremely wide, and some clutch slipping is required to attain maximum acceleration. Often this clutch slipping is just a matter of a bit of a touch of the clutch just after shifting to bring the engine up into the power band. Then once the bike has accelerated a bit more it is possible to completely let off of the clutch as the engine pulls from 6,200 to 7,000RPM. Another unfortunate result of the narrow power band however is that touching the clutch after the shift often results in the rear tire breaking loose. Once the rear tire is spinning clutch slipping is no longer required until the next shift, but spinning the tire on every shift on nearly all trail surface types does result in rapid wear of both rear tires and trails. It is often quite striking to see how much faster a four stroke can accelerate without the rear tire breaking loose than a two stroke can with the rear tire churning wildly.
The big 396cc two stroke tends to make about the same peak power output as the 577cc 610 four stroke. The big four stroke just does this peak output up at a somewhat higher engine speed around 7,500 to 8,500RPM. Obviously the big difference is in the width of the power band, with the 400 two stroke making big power only between 6,200 and 6,800RPM and the 577cc four stroke pulling hard everywhere from 3,500RPM up with over rev to 9,000RPM or even higher. The other big difference is in the mean piston speed where maximum power is attained. The 2.91 inch stroke 400 WR is running at a somewhat lower mean piston speed at 6,800RPM than the 3.01 inch stroke 610 running at 7,500RPM. The shorter stroke two stroke could potentially be setup with porting and a pipe to operate up at 8,000 to 8,500RPM also where it might make nearly 20% more peak power output, but this would be an extreme sort of level of tune for a two stroke. The lower engine speed was chosen for the 400 two stroke because around 60hp is plenty for a dirt bike and because the engine runs better down at 6,200 to 6,800RPM mostly because higher engine speeds tend to result in even narrower power bands on cylinder port two strokes.
If the 69% jump between first and second on the Swedish six speed tends to seem too wide nearly all the time, then the more moderate 75% jump between second and third and 76% jump between third and fourth usually seems just right. At least on the 610 motor this 75% jump usually seems just right. On both the 350 four stroke and 400 two stroke the 75% jump can still seem excessive. On the 350 four stroke it is a matter of how much power is required and how well the engine is running. Maximum power tends to come around 8,000 to 9,500RPM on the stock 350, and this upper power band can seem pretty narrow. The stock 350 motor can rev higher than 9,500RPM sometimes, but there isn't more power up there with the very heavy 313g connecting rod and heavy 270g piston.
The 610 also has a narrow upper power band from 7,500RPM to about 8,500RPM when it is running at it's best, and this is too narrow for even the 84% jumps between the upper gears. Since the amount of power down to 7,000 and even 6,000RPM on the 610 is still very large though the jumps between the gears do not tend too seem to wide. In fact the power on the 610 is so strong all the way down to about 4,500RPM that there tends to be a huge range of engine speeds from 4,500 to 8,500RPM to make use of for heavy acceleration. Especially in second, third and fourth gears the power available down to 4,500 and even 4,000RPM is so big that more is hardly ever desired for real off road riding.
So what is this strange Husqvarna transmission that seems ill suited to any of the bikes it came on any good for? The tight 84% spacing between the upper gears is good for the little 350 four stroke and the two strokes, and the wider 75% spacing between the lower middle gears is good for the big 610 four stroke, other than that though the gears always seem to wide at the bottom and too narrow at the top. So what is it good for? The answer is that it is a road racing transmission. That sounds like a very strange thing to say about parts that came only on dirt bikes, but it is true. Husqvarna was big into road racing back in the 1930's, but by the 1980's the Husqvarna road race bikes were long gone. Long gone in appearances anyway. In actuality the Husqvarna dirt bikes really are heavily biased towards very high speed racing. The very relaxed 29 degree steering head angle and the very long 59.5 inch wheel base both are all about going very very fast. Compare this to a 24 degree steering head angle and a 55.0 inch wheelbase on many of the 200hp sport bikes. The big Husqvarna dirt bike does in fact handle quite well on dirt up to well over 100mph and always seems to have lots more to go in terms of handling the speed.
As far as the transmission goes it is the large variation between the lower gear spacing and the upper gear spacing that points towards road racing. For going really fast a bike has to be geared up very high. The 15/50 gearing on the 610 is not nearly high enough as it still ends up all wound out at 8,500RPM in nothing flat. The dual sport 610 in Europe came with 17/48 gearing in some years, that is probably getting closer to what the 610 can do on a road race course. That is 137mph at 8,500RPM, something the 610 certainly can pull on race gas and it is not even out of the question on pump gas.
To go very fast it is necessary to shift within the power band, so road racing with 17/48 gearing is what the 84% jump between fifth and sixth is really for. The 610 motor always seems to be making maximum power by 8,500RPM, but it will also often rev to at least 9,000RPM. The 84% jump between upper gears allows the engine to drop down from 9,000RPM and just hit the bottom of the power band at 7,500RPM. Top speed might not be 145mph at 9,000RPM, but winding out to 9,000RPM at 122MPH in fifth gear is required to get into sixth above the 7,500RPM bottom of the biggest power. The only problem with all of this is that the early Husqvarna 610 motors do not have pressure lubricating systems and sustained high engine speed tends to toast the rod bearing. Just zipping up to 8,500 or even 9,000RPM with trail gearing works fine, and the short few seconds at above 8,000RPM does not do much damage to the rod bearing. Pulling the long back straight away on a road race course is however a different story. Longer periods of time spent above 8,000RPM are likely to toast the rod bearing because it runs out of oil.
I once made the mistake of top ending the 610 on the beach with just 13/51 gearing. The engine seemed to keep pulling for a long time after it really got going at 7,500RPM and the bike was no doubt going well over 100mph. I could feel the power output dropping as engine speed climbed to 8,000 and 9,000RPM but the power was still quite strong and the bike just kept going faster and faster for a long ways. After just a few miles of this when I slowed down the engine was clanking horribly and sounded like it was going to seize up at 2,000RPM. Amazingly though the rod bearing was able to run in by cruising back through town at moderate engine speeds and that motor is still in operation now more than a dozen years later. That little stunt does however show that even the 610 is not immune to rod bearing failure if the engine is used for land speed record racing or such.
Clearly the narrow 84% jumps between the upper gears are for going very fast, and to go very fast very tall gearing is required. The purpose of the wide 69% jump between first and second is just to get a low enough first gear that casually pulling out in first gear with 17/48 sprockets is not too difficult. For fast starts first gear still would not be used even with the 17/48 gearing, but there is a tradition of street going road race bikes having a rather low first gear so that they can still easily be used casually around town. Especially for a bike that is ostensibly a dirt bike having a low first gear is highly desirable.
So the six speed in the 610 is a sport bike transmission, a compromise setup that can be used both at the track and on the street without changing sprockets. Just don't try it with off road knobbies as they tend to pop off at unexpected times. On a rainy day though a Husqvarna 610 with tough dual sport knobbies could probably clean up in the 600cc class against heavier and more powerful 600cc four cylinder sport bikes on a tight circuit. Even in the rain on a rather tight circuit though top end performance is of paramount importance, and without the narrow 84% jumps between the upper gears the 610 does not stay in the upper power band.
Of course there is the cylinder count disadvantage also. The three inch stroke Husqvarna 610 just does not have the maximum power production potential of an inch and three quarter stroke four cylinder. The three inch stroke 610 makes maximum power at about 8,000 or 8,500RPM, where short stroke four cylinder can have the peak power point up at 12,000 or 13,000RPM even with rod bolts and a cam and bucket valve train. Anyway it is looked at the inch and three quarter stroke four cylinder engine has the potential to make about half again more power than the three inch stroke thumper. The 1.67 inch stroke 2001 through 2016 Suzuki GSXR 600 is rated at 105hp at 13,300RPM with a torque peak of 44 ft-lb at 11,000RPM. The 105hp might seem a bit weak compared to 180 to 200hp from the most powerfull 1000cc four cylinder sport bikes, but the GSXR 600 does hold onto a respectable 38 ft-lb of torque down to 6,000RPM which is pretty impressive for a cam and bucket valvetrain.
If the 84% jumps between the gears are only any good for going very fast with the Husqvarna 610 then it might be argued that the Swedish six speed transmission is in fact better suited to the smaller displacement 350 four stroke. There is some truth to this, as the 84% jumps seem more appropriate on the 350 at much more dirt appropriate speeds than on the big 610. Where this line of reasoning breaks down though is that the huge 69% jump between first and second gears is much more of a problem on the smaller displacement engine. On the 610 the big jump between first and second gears is a slightly inconvenient annoyance. On the 350 though that big 69% jump between first and second is a deal breaker. It gets the bike stuck all over the place when the motor is not running at it's best, and even when the motor is running great all the way from 4,000 to 9,000RPM the big 69% jump between first and second gears is very annoying and slows the bike down a whole lot on tight trails.
If the Husqvarna six speed transmission is only any good for road racing then what is it doing in a dirt bike anyway? The answer has to do not only with the road racing heritage of Husqvarna Motorcycles, but also has to do with marketing. Six speeds sounds more impressive than five. By stretching out the ratios the same total spread could be obtained with just five gears. If the upper ratios were stretched out on the six speeder then the total spread of gears could be even wider, and a wider spread of gears puts more peak loading on the transmission. A narrower total spread of gears keeps loads lower and allows a smaller and lighter transmission to be used.
But what about that very low first gear? It is a 2.6:1 reduction in first gear, and that would seem like it would much more heavily load the transmission than stretching fifth and sixth gears out with a bit more overdrive. It certainly is true that the big 2.6:1 reduction in first gear puts the largest loads on the transmission. This is however mitigated by the fact that first gear normally does not get subjected to full power output from the engine. Even geared way up with 15/50 gearing first gear is just a crawling gear. Any kind of heavy acceleration takes place in higher gears. Even when at a near standstill on the trail kicking up to second before accelerating gets the bike up to higher speeds much faster.
Even at that though second gear is 1.8:1 and sixth gear is just a 0.75:1 overdrive. Second gear still much more heavily loads the transmission than does sixth gear. In reality fifth and sixth gears could be spread out farther without causing any immediate problems with the transmission. What it comes down to though is that the transmission is oversized for street use. It is a big heavy duty transmission that does not wear out quickly. A dirt bike that does not get all that many hours on it does not really need such a big transmission. If the transmission is designed around holding up to large displacement and a long stroke on the street then the bias towards sixth gear durability makes sense. Full acceleration is not going to last long in second gear, but the bike may end up accelerated hard in fourth, fifth or sixth gear frequently. At least that would be true on a smaller displacement engine that did not go as fast. The 120 to 140mph top speeds of the 610 motor are not the kind of thing that most street riders would be using all the time. Still though that is what the transmission is designed for. For cruising at high speed in sixth gear.
So the six speed Husqvarna transmission is what it is for three reasons. One is that six speeds sells better than five speeds. Another is that the engineers were still thinking a bit about road racing when they designed the bike, and finally the bikes are designed around performing well and holding up well to extended use as street machines. That seems strange for a company that marketed their bikes as "Husqvarna Competition Motorcycles", but the bias most certainly is towards longevity and practicality as an all around street and off road motorcycle.
If the Swedish six speed transmission is not quite correct for any of the dirt bikes it came on, then what sort of a transmission would a dirt bike work best with. Really what it comes down to is that the variation in jumps is just too wide on the Swedish six speeder. For any dirt bike it would be better if the jump from first to second was more like 72 or 74%. For the 350 four stroke and the 400 two stroke just bringing first gear up to a 72% jump like was done on the 125 and 250 two strokes would be a vast improvement. Even on the 610 bringing first gear closer to second so there is only a 72% jump would be a dramatic improvement. What the 610 also needs to work as well as it can as a general purpose dirt bike and dual sport bike is wider spacing between the upper gears. The 84% jumps are just right for 140mph road racing, but for anything normal something more like 80% jumps would be much better. In fact the 610 really could stand to just have 75% jumps all the way through the gear box. With bigger jumps between the upper gears the overall gearing could be lowered so that first gear would be very low even without the huge 69% jump between first and second. For all around flexibility it might make sense to go with 80% spacing between fifth and sixth, but this would just be to make the transmission better applicable to a wider range of applications than just the 610 dirt bike. Another option would be even wider spacing and just five gear sets. When the 610 motor is running at it's best it pulls very hard all the way from 3,500RPM to 8,500RPM and has little need for narrow spacing between the gears. Perhaps 69% spacing between first and second and then 72% spacing between the rest of the gears would be good for a wide ratio five speed.
What the 350 needs above all else is a lighter reciprocating assembly. Without so much weight being thrown around good power to 10,000RPM would be much more usable. Of course everything in regard to the 350 tends to be ridiculous because it lacks the pressure lubricating system necessary to sustain 9,000 and 10,000RPM engine speeds. It's not just that the gearing is wrong for the 350, the whole 350 motor is wrong for cases, bottom end and a transmission designed for a three inch stroke motor. Since the 610 is way too much power for just about any sort of dirt bike a small bore version does make sense. The original Swedish four stroke had a smaller 3.60" bore for just 503cc of displacement on the same 3.01" inch stroke. With the Italian small valve 350 head this could be taken to an extreme with an the even smaller 3.31" bore. The Honda XR400 rod goes nicely into the Italian 350 motor, and the 4.4mm shorter center to center length along with the necessity of running higher 11:1 or 12:1 compression ratios on today's pump gas means that going up to about a 2.84" stroke length works well. Even when stroked all the way out to the 2.84" stroke length the small bore Husqvarna is still only 400cc, which is a considerably smaller displacement than the 577cc 610 motor. This modified 400 small bore stroker motor would still do well with narrow spacing between the upper gears if it was to be able to go very fast such as in a desert race, so again the Swedish six speeder looks pretty good aside from that huge 69% jump between first and second gears.
My 386 stroker motor is sort of like this, but I wanted the shortest possible stroke for efficiency and long range so I went with 2.68" which resulted in an extremely excessively low 9.7:1 compression ratio. The 2.84" stroke supper stroker small bore Husqvarna can have whatever higher compression ratio happens to be required, anywhere from 11:1 with a slightly shorter stroke and the stock 84mm Husqvarna piston up to 12.5:1 with an 85mm Honda XR400 pop up dome high compression ratio piston to make a "409 Supper Stroker". The 2.84" stroke length is both more compatible with the rest of the engine and bike in general, and it also does two things to reduce the likelihood of premature rod bearing failure. The longer stroke with the larger big end diameter of the Honda XR400 rod moves the bearing closer to the oil squirter. In fact the 2.84" stroke on the XR400 rod moves the edge of the bearing right out to the same location as on the 610 motor. The longer 2.84" stroke also brings the absolute maximum engine speed down from 10,500 to around 9,500RPM. And possibly even more importantly the longer 2.84" stroke also lowers the normal operable upper power band from 8,000 to 9,500RPM on the stock 2.48" stroke 350 motor to about 7,500 to 9,000RPM which is much more similar to the 610 motor which hardly ever actually suffers rod bearing failure in normal service as a dirt bike.
The much longer 2.84" stroke on the shorter Honda XR400 rod still results in a slightly longer rod to stroke ratio than the 610 motor so it is all around a reasonable engine build strategy for getting less displacement on an early Husqvarna four stroke motor.
The longer 2.84" stroke does not however do much about the excessively wide jump between first and second on the Swedish six speed transmission. The larger displacement does tend to alleviate the problem of a large jump a bit, but just a 17% increase in displacement is not all that much of a change. The longer stroke itself actually only tends to narrow the range of engine speeds over which the engine runs well.
The really significant change that does actually deliver better performance over a wider range of engine speeds is a better match between the compression ratio of the engine and the fuel being used. Going up from a 2.48" stroke to a 2.84" stroke might tend to reduce maximum engine speed by around 10% with hardly any reduction in minimum engine speed, but when that stroke length change also allows the compression ratio to be brought up to where the engine will actually run bellow 6,000RPM the difference can be quite dramatic.