Motorcycles with Synchromesh Gearbox?

Tornado

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Are synchro rings ever used in motorcycles? Our classic Commandos don't have them and shift very nicely. My classic Mini Cooper with 4 speed gear box does have synchros and it has gear clashing downshifting from 4th to 3rd only. Folks are telling me it's likely to be worn synchros.

Just wondering what advantage there is to synchros (since older GB's could work well without them) and if they are used in motorcycles?
 
Most, maybe even all, motorcycle gearboxes are constant mesh so the gears are always engaged with either other. Unlike a car where the gears slide into mesh as you change gear. Hence no need for syncromesh on a Norton. I don't know why bikes and cars do it a different way.
 
Most, maybe even all, motorcycle gearboxes are constant mesh so the gears are always engaged with either other. Unlike a car where the gears slide into mesh as you change gear. Hence no need for syncromesh on a Norton. I don't know why bikes and cars do it a different way.
I would hazard a guess at space considerations, perhaps??
 
Syncromesh widens the gear so no space in a motorcycle gearbox for 4 let alone 5 or 6 wide gears. One temporary fix where a syncro is failing is to double declutch and as on a bike match the revs to the new gear, it was common not to have syncro on first and that same technique was needed then.
 

Syncros allow you to smoothly shift between non-sequential gears. Since motorcycles are sequential and use constant mesh gearboxes there’s no need for syncros.

There are a lot of variables as to transmission selection but the key factors for a motorcycle are size and efficiency. Plus that sequential shifting is required to shift be your foot.

For cars there’s a host of reasons why constant mesh isn’t desirable. Sequential shifting and straight-cut (as opposed to helical gears) noise being just a couple.
 
swooshdave said:
For cars there’s a host of reasons why constant mesh isn’t desirable. Sequential shifting and straight-cut (as opposed to helical gears) noise being just a couple.

All the synchromesh gearboxes I have dismantled have been constant mesh. I’ve seen exceptions, such as a non-synchronised sliding cog for first and reverse, on old vehicles.

How can helical cut gears not be constant mesh? Do you think you can slide them in and out of mesh while driving?
 
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In this (straight cut toothed) example, you can see that the dogs’ rotational speed on the free-running gear and the “splined hub” are synchronised by the cones.

The hub is slid along the shaft. The gear is not.

Motorcycles with Synchromesh Gearbox?
 
All the synchromesh gearboxes I have dismantled have been constant mesh. I’ve seen exceptions, such as a non-synchronised sliding cog for first and reverse, on old vehicles.

Can you provide an example of a synchromesh that is also constant mesh? From my understanding those are contradictory.
 
Gearboxes are described as “constant mesh” when they have constantly meshed gears, but no synchromesh. It’s a convention.

When they add the synchromesh mechanisms to a constant mesh gearbox, they call it a synchromesh gearbox.

Me giving you examples of thousands of gearboxes, would be less useful than you reading something technical on the subject.

Anyway, here’s one. Supposed to be BMW.

Starting at our left, I can see four pairs of helical gears, in mesh. That’s constant mesh.

I can also see two synchromesh hubs between pairs of gears. The gears stay meshed; the synchromesh hubs lock a chosen gear to a shaft.

As you can probably see, the synchromesh hubs do much the same thing as the sliding dog selector rings or sliding (but constantly meshed) gears with dogs cut into them in many non-synchromesh motorbike gearboxes.

Motorcycles with Synchromesh Gearbox?
 
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I edited the last sentence. My ignorance knows no bounds and it’s apparently unusual for a bike gearbox to have separate selector hubs. Must be lack of room again.

Edit again! Some Harleys have them and call them shift clutches.
Motorcycles with Synchromesh Gearbox?
 
The reason for synchromesh is as the name implies, to synchronize rotational speed of two parts in order to mesh them together.
Limiting the stress on cog teeths or dogs and the awful sound when trying to join two parts with different rotational speed.
Have driven a number of cars and lorries (trucks) with nonsynchronized gearboxes, spanning from late twenties up to early sixties, where double clutching is needed for smooth gearchanging.
Simple reason why it's not needed on a motorcycle is that the rotating masses are much smaller.
Another reason is that on most car engines, ingoing axle rotates with same speed as the engine. A motorcycle usually have a reduction between engine and gearbox. Noticed the primary chain on a Norton? :)
So the kinetic energy is much smaller.
 
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I really suggest you watch this video fully. Notice that all gears (including the dog gears) are always moving.





Now watch this synchromesh transmission. Watch the synchro hubs, they do not always move hence the need for them to adjust their speeds.

I think we may have a terminology conflict. "Constant" doesn't describe the action of the gears but rather the dog gears or synchro hubs.
 
I worked on the Mini gearbox assembly line for a week at 29 boxes an hour, that was enough for me and I moved from one assembly station to the next every break, there were guys on the line since its start.
 
Wow what a great number of informative replies. I asked these same questions on a ytouTube video describing the details of car synchro function. The author responded he knew nothing about motorbikes and how their gb"s worked.

I had an inkling it was something to do with sequential shifting and or rotational mass of gearing differences ' tween bike and car. Hadnt though of extra spacing needed for synchros (Mini is a very compact setup, smaller than some bikes perhaps?!)
 
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