Sport Touring Corner By Norm Kern, MSTA Editor / ReasonsToRide.com Contributor
“Should I buy heated grips or heated
Transmission Problems, and Why They Are Always With Second Gear Shifting from one gear to another
The sport touring world is buzzing right now with news of a transmission recall for Yamaha FJR models from 2016 onward. According to NHTSA, there have been two reports of second gear failure in the USA, although there are additional instances in other countries and the recall is worldwide. This is a major repair, as it involves replacing both shafts and all the gears in the transmission. (See the sidebar for details on the Yamaha FJR recall.)
As we accelerate, engine RPM increases and we want to shift to a higher gear to drop engine revs down to a reasonable range. Shifting gears changes the gear ratio between the clutch and output shafts, so one or both of them must immediately change RPM. Since the output shaft has the momentum of the bike's rear wheel and drive train connected to it, its RPM stays nearly constant during the shift. Therefore the clutch shaft must suddenly drop RPM on an upshift.
Since the recall includes my FJR, many have asked me what the problem is, and especially WHY it occurs. It turns out that lots of bikes have similar problems and large sport touring models are among the most vulnerable. Let's begin with how motorcycle transmissions work. Motorcycle transmissions have two shafts The clutch shaft is connected to the engine through the clutch. It has a gear for each transmission "speed" that meshes with a mating gear on the output shaft. One gear of each pair has "dogs" on the side that can be engaged to lock a gear to its shaft or let it spin freely. The shift lever operates a cam that moves the shifting forks to engage and disengage the dogs on the gears. One set of dogs must disengage before another engages to avoid locking the shafts.
If the clutch is disengaged before shifting, it's just the clutch shaft itself and some of its gears that have to slow down. That reduces, but does not eliminate the shock force on the gears and dogs. Reducing throttle during upshifting slows the crankshaft but has no effect on the clutch shaft, as it is disengaged by the clutch- it only affects stress on the dogs and gears when the clutch is released. Remember, the moment the gear ratio changes is always the moment of greatest stress. Why are most transmission problems related to second gear? Look at the chart, which shows the drop in clutch shaft RPM when upshifting between gears on an FJR. By far, the greatest RPM drop is between first and second gear. That's where the most stress occurs. Another factor in second gear damage is the neutral detent between first and second gear, which makes it easier to miss the 1-2 shift by not getting the transmission all the way into second. When this happens, the clutch may be released and power applied, only to find the transmission is in neutral. A quick stab at the shift lever can engage second while under power with the clutch already released- a recipe for disaster. Look again at the chart. The drop in RPM between shifts is proportional to the engine RPM at the shift point, so the greater the RPM, the more stress on the transmission gears and dogs.
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