I just came across this video about a motorbike gearbox, and have a couple questions.
Firstly, she says the detent for neutral is between first and second gears. Why isn’t it between fifth and first? That would seem to make more sense to me. Are you expected to shift down to first when setting off, then shift back over neutral to get to second? And presumably the grooves in the shifting drum have gaps between fifth and first to stop you shifting too high and ending up back in first, or is there another mechanism for that?
Secondly, and probably more importantly for my understanding of the transmission, can someone elaborate on how the ‘constant mesh’ transmission means it doesn’t need synchronisers? For example, before the shift from neutral to first is shown, the input shaft, and the first free-wheeling gear with it, are rotating while the output shaft and the corresponding dog clutch are not. Surely, when the dog clutch is moved to connect with the free-wheeling gear, they wouldn’t be able to mesh, unless it happened right when the recess and pin were in line, and even then that would cause a jolt. What am I missing here? I also watched another video with a physical gearbox, which seems to confirm that it works like in the first video, but doesn’t explain it much, and I can’t really see why it works.
I just posted this at [email protected] but realised it doesn’t have much activity, so hopefully it fits here.
Hey, motorcyclist here.
The difference between a moto gearbox and a car’s is that the synch is needed on a synchronous transmission, to allow for smoother shifting as a stated design goal. It also potentially increases overall longevity, at the expense of adding consumable parts (the synchro rings) and their corresponding complexity.
Motorcycles use “crash” (aka nonsynchronous) transmissions, the operator is expected to know how to rev match, to get the engine and gear train close to the right speeds before engaging the shift lever. Some people are better at it than others. There is absolutely a jolt when shifting, particularly on Harley’s when going down to first.
Ultimately, it leads to a simpler system which weighs less, and also gives total control to the operator, a common desire amongst most of us bikers. Ignore the guys on bikes with automatic transmissions, they’re weird 😋 These transmissions are great, I can even shift gears (carefully) without using the clutch at all, mainly because it’s non-synchronus.
As for why neuch is between first and second, it’s a user experience decision. We always shift down to be ready to accelerate at a light, and don’t want to look at a dashboard to figure out where in the shift range we’re currently at. So, just gently thump the shifter down till she won’t go no mo, and you’re in first. Slight bump up and you’re in neutral, or you stall your engine.