IMO, there are a couple of items of misinformation in this and the linked threads. Although reluctant to jump in the middle of this, as a retired drive train engineer, I feel a couple of comments may be in order. This is REALLY long, feel free to skip it if you’re not into technical details!
In a thread in a COG forum that is linked to in reply #1, “Greg” says “Motorcycles are a constant mesh transmission. That means everything is turning the same speed. This enables shifting like this.” This statement is just plain false. Whether a transmission is sliding gear or constant mesh, the elements within are turning at a variety of speeds. And being constant mesh does not enable clutchless shifting, I used to be able to do it on my Model A with a sliding gear transmission, with the proper technique, and the attendant risk of damage.
Within that COG thread, Fred Harmon links further to a page that, in part, says “Most all of the gearboxes on motorcycles are "constant mesh". That means all the gears are constantly meshed with one another and are always spinning. Because of this, there are no synchros to speed up the gears when they engage, like there are in automotive gearboxes.” This is misleading. The fact that the gears are in mesh has nothing to do with using a synchronizer or not--any constant mesh gearboxes, whether used in motorcycles or autos, may use synchros if desired to achieve the performance and durability objectives of the application.
A sliding gearset is one where a pair of mating gears is each splined to its respective shaft, and therefore each is limited to rotate only at the speed its shaft is rotating. When not in use, one of the gears must slide via splines on its shaft to a point where the teeth of the gears are no longer in mesh.
A constant mesh gearset is a pair of gears whose teeth are always in mesh and where only one of the gears is splined directly to its shaft, the other may turn on its shaft at a speed other than the shaft speed, via a bushing or bearing. In order to transmit shaft torque, the gear that can spin freely on its shaft must be connected somehow to its shaft when needed, usually by a collar with spline teeth with or without a synchronizer, or by mating dog teeth or pins and mating holes that are engaged when the collar is slid laterally on the shaft. Note that one gear of a constant mesh gearset may be slid on its shaft, but the teeth will not go out of engagement with each other, so they stay in constant mesh.
The gearsets in the Concours transmission are all constant mesh. Some of them serve double duty since they are made with dog teeth on their faces, and when slid closer to their neighboring gear, are used to connect that gear to the shaft they share. But they are still in constant mesh, none of the gear teeth ever go fully out of or back into engagement during operation. Any grinding or clunking you hear during shifting is the dog teeth on the face of one of the sliding gears trying to get engaged with its neighboring gear on the same shaft.
When you shift, what is happening is that the speed of the input shaft is being changed to a new speed appropriate to the new gear ratio being selected (slower for an upshift, faster for a downshift), and the force to do this comes from the momentum of the output side (which includes the momentum of the whole motorcycle) through an impact at the moment the dogs engage the new gear, accelerating the much smaller momentum of the input side. Despite anyone’s best skills, some impact cannot be avoided, especially using a shift system typical of a motorcycle, where there is no pause for neutral. When the shift lever is preloaded and the current gear releases at the moment of zero torque, those dogs will try to engage the next gear almost instantly, long before any effect of letting off the throttle, or revving for a downshift, can have much effect.
This is where the clutch comes in. If the clutch is disengaged, then the moment of inertia of the input side is only the shaft, connected gears, and the clutch output plates. This is relatively small and the impact to do this is modest. But if the clutch remains engaged, then the inertia is the total of the previous items, plus the whole clutch, flywheel, and the rotating inertia of the entire engine, since these are all connected! This is probably ten to fifty times the inertia. In short, it takes a much much larger impact force.
I’m not saying it is for sure that clutchless shifting will destroy a transmission. But it will surely result in much higher impacts, and greatly increase the chance of leading edge chipping and eventual dog-skipping (grinding) and with fatigue damage, the chance of complete failure of the dog teeth, as MOB has pointed out.
A hint of what is happening here is the “clunk” you hear and mild “kick” you feel if you quickly put it in first gear from neutral quite soon after pulling in the clutch lever. That’s the output shaft bringing the input shaft to a stop, and you feeling and hearing the impact of that on the bike itself. In clutchless shifting, clunks and impacts are also there, you just don’t hear them or feel them because you’re at speed.
OK, lecture over. Test on Friday…
