Questions about motorbike gearbox
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 !askmechanics@lemmy.ml but realised it doesn't have much activity, so hopefully it fits here.
I'll see if I can't check out the video this evening. That's a fantastic video from what I've seen so far.
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.
Motorcyclist here. The drum doesn't spin directly from 1st to 5th. Although it's round, the usage is linear from the rider's perspective. So functionally, neutral wouldn't be between 1st and 5th, it'd just be below 1st.
The reason neutral isn't at the bottom is because it's not important on a fully functioning bike. It's more important to positively find 1st. In a situation in which you suddenly need more acceleration, you go down a gear. If you need more, you go down more. Imagine you're in some none-1st gear at 10mph coming to a stop, but the car behind you isn't slowing. You mash the shifter down a few times until it stops moving. You know you're in first and can accelerate as fast as possible. It's the same deal if you're stopped - when it doesn't move down, you know it's 1st. With a car gearbox, each gear has a distinct shifter position. With a motorcycle, all of that is hidden inside so all you get is ups and downs.
So why isn't neutral important? Because in all normal situations, your hands are on both bars, meaning you can grab the clutch lever. If you need to decouple the engine from the wheel, the clutch is much faster than fiddling with the shifter. There are many riders that only use neutral when on the kickstand. All other times, they're just holding the clutch when stopped. It minimizes the time needed to get moving in case of emergency. Some riders see this as paranoia, but the potential injury from a rare rearending far outweighs the constant inconvenience for some.