thinking...

I've noticed that every motorcycle in existance uses a sliding rear axle to adjust chain tension, whether it be a hardtail or a full suspension, yet every multi-speed mountain bike in existance, whether it be hardtail or full suspension, uses a spring-loaded chain tensioner. wouldn't it make more sense to have a fixed rear axle and run either a spring loaded chain tensioner or one that adjusts using a small turnbuckle? that way your pivot-to-axle length stays the same, and you can keep more or less constant chain tension with full suspension rigs, regardless of where the pivot is in relation to the front sprocket. Am I completely off my rocker and wanting to make everything way more complicated than it needs to be, or is the motorcycling world just being lazy?

My bike runs a belt, but I think it uses the same sliding axle arrangement. I'll have to check.

I'm fairly certain that everything on the market today that has an engine using the sliding axle arrangement, I'm just curious as to why nobody has tried a fixed axle with either a spring-loaded or screw-adjustable tensioner (even though I have seen spring loaded tensioners in chopper magazines, just never seen one in use)

Because it's simple and cheap, probably. Some dirt bikes have chain tensioners, but that's mainly because they have more travel in the rear suspension so the chain can get excessively loose. Ideally, the centerline of the swing arm pivot would be in line with the drive sprocket, but most bikes don't have that design feature.

At any rate, I think the small amount of change in wheelbase you get when adjusting the chain tension isn't enough to make that much difference in how the bike handles.

edit: most bicycles have derailleur gears so a chain tensioner is necessary.

I think the torque gerated by motorcycle vs torque generated by pedaling would whip the chain out of shape. Maybe a tensioner with the ass of a valve spring?

Dan

well, what I was thinking was put the tensioner on the return side of the chain (bottom) so that the torque of the engine keeps the chain taut, and the tensioner just takes up the slack but isn't really affected by engine torque, even on a bike with rear suspension. as far as keeping the tensioner, well, tensioning, as long as you could keep it out of coil-bind on bikes with a lot of suspension travel (and thus a lot of chain growth, depending on the suspension design), a stiff RC touring car spring should work reasonably well, in leiu of that, a valve spring could work (like 914 suggested)

I'm going with: "If those clever SOBs at Honda haven't figured it out..."

Dan

stuart in mn wrote: Because it's simple and cheap, probably. Some dirt bikes have chain tensioners, but that's mainly because they have more travel in the rear suspension so the chain can get excessively loose. Ideally, the centerline of the swing arm pivot would be in line with the drive sprocket, but most bikes don't have that design feature. At any rate, I think the small amount of change in wheelbase you get when adjusting the chain tension isn't enough to make that much difference in how the bike handles. edit: most bicycles have derailleur gears so a chain tensioner is necessary.

Also bicycles have "Freewheeling" to allow coasting without pedaling... the tensioner may be happy because of that ! and "Cranky" (sorry, couldn't resist...) without it

Because we've always done it that way!

914Driver wrote: I'm going with: "If those clever SOBs at Honda haven't figured it out..." Dan

Actually they did, they went to shaft drive.

Do fixies have a spring tensioner?

Fixed gear bicycles, which are closest to motorcycles in terms of not using the chain to shift gears, use the same system:

That said, I'm pretty sure that Buells have (had) tensioners - but they run belts and not chains.

I believe another reason is for rear wheel alignment.

Slyp_Dawg wrote: well, what I was thinking was put the tensioner on the return side of the chain (bottom) so that the torque of the engine keeps the chain taut, and the tensioner just takes up the slack but isn't really affected by engine torque, even on a bike with rear suspension. as far as keeping the tensioner, well, tensioning, as long as you could keep it out of coil-bind on bikes with a lot of suspension travel (and thus a lot of chain growth, depending on the suspension design), a stiff RC touring car spring should work reasonably well, in leiu of that, a valve spring could work (like 914 suggested)

What happens when you go down a steep hill and use engine braking? All the slack would be on the top-side of the sprockets. Give it a little acceleration and the chain will probably jump off.

Dirt bike have spring loaded tensioner that will remove your fingers. They also have slots in the rear. Mostly do to swing arm pivots not being in-line with the output shaft so as swing arm goes up (suspension compression) the distance gets longer thus you need twice that change in chain length.

You forget that Bike often change there sprocket ratios so you need movement.

Dirt bikes used to have them.

They don't last.

Yeah, the oldie tensioners had to be under such strong preload that they were not only dangerous but wore quickly as well. About the swingarm to drive sprocket relationship: lots of newer bikes have the swingarm pivot running through the back of the engine cases for just that reason. AMP Research experimented with a swingarm pivot system that placed the pivot point dead in line with the drive sprocket, but the linkage necessary to make this work was large, cumbersome and expensive. The guys that rode the experimental bike said it did help with torque input chassis movement but was not all that much different from conventional setups.

About shaft vs chain: shaft drive is maintenance free, but it has a drawback for high performance use. When you whack the throttle, the pinion gear has a tendency to try to 'climb' on the ring gear. This has the VERY unsettling effect of raising the bike when you least need the CG change, such as in a turn. My Seca 750 did that and it scared the shi! out of me. I learned quickly that if that bike was leaning way over, I did NOT get on the throttle. Chain drives do the opposite. The back of the bike tends to lower on acceleration, thus preserving the operator's underwear.

I always wondered why you couldn't run a short chain from the engine drive sprocket to a set of double sprockets on jackshaft which would be part of the swingarm pivot, then run another long chain from that back to the rear wheel. One of these days I may chop up an old junker to see if I can make that work.

Patent pending.

Curmudgeon wrote: About shaft vs chain: shaft drive is maintenance free, but it has a drawback for high performance use. When you whack the throttle, the pinion gear has a tendency to try to 'climb' on the ring gear. This has the VERY unsettling effect of raising the bike when you least need the CG change, such as in a turn. My Seca 750 did that and it scared the shi! out of me.

And when you cut the throttle the bike drops, grounding out the frame and spitting you into the weeds. Some bikes, like newer BMWs with their paralever rear suspension, have mostly eliminated that problem.

Curmudgeon wrote: I always wondered why you couldn't run a short chain from the engine drive sprocket to a set of double sprockets on jackshaft which would be part of the swingarm pivot, then run another long chain from that back to the rear wheel.

Nothing new under the sun...I remember seeing that setup in an old bike magazine somewhere, I think in the late 1960s.

BMW's Paralever has some passing resemblance to AMP Research's setup. Can you find a link to the chain setup? I'd be interested in how they did it.