Why isn't chain slack controlled automatically?

Hello org,

It's probably a stupid question, but why don't bike chains have a tensioner?

I understand the need for slack in the chain, since the sprocket is not concentric with the swingarm pivot point, but why can't this slack be controlled by a tensioner?

Mihai

If it could be done, someone would have done it. If you accellerate hard then the force on the tensioner would be massive & the chain would go slack at the bottom.

I don't really have a clue of course but that makes sense to me

[NTECUK]

Simply put its cost over function

If the drive only went on one side of the chain, it would probably be possible (like on a pushbike with a freewheel).

But since motorbikes engine brake, the tension can go to both the top (accelerating) and bottom (engine braking) of the chain, thus the tensioner cannot go on either side as it always has to be on the 'slack' side (like a fixed wheel pushbike).

[Bibio]

i think its called an idler wheel and if i'm not mistaken a trials bike used to have one as i'm sure i have seen one before.

Presumably the pressure required to keep the chain taught would cause a lot of wear on whatever was doing it, plus you wouldn't get any warnings that your chain was getting old and tired.

[NTECUK]

Yes trail enduro bikes used to have them My DT 250 had one.
But you need a combination of the sprung loaded action to keep the tension right,Plus a ratchet mechanisum to step out to take up wear.
Same as cam chan tensioners.We know how well they behave don't we,And they are sheilded from muck and grit.

[Jayneflakes]

I know that a chain drive is a fairly efficient transfer of power from the engine (providing that it is well maintained), but why has shaft drive not dominated the scene, the benefits must surely outweigh the negatives?

No need of a chain oiler if you have a shaft spinning in a lubricated gear box and no dirty wheels from chain fling either also no need to check chain stretch once a week and adjust for slack. Opinions?

[yorkie_chris]

Shafts don't generally run in oil, the bevel drive at the back end has gearbox oil. The front bevel drive runs in the engine oil.
The only bit in the middle is the UJ which on most is enclosed inside the swingarm with a rubber boot to keep the crap off.

I'm not particularly concerned about my XJ wearing it's final drive out, I think they nicked it from a tractor.




Nothing wrong with having a chain tensioner but it couldn't have a ratchet, this would snap something right sharp due to suspension movement.

And also I'm fairly sure I've done 15,000 miles or more without adjusting the chain on the SV, don't really see the need for more modification!

Originally Posted by Jayneflakes

I know that a chain drive is a fairly efficient transfer of power from the engine (providing that it is well maintained), but why has shaft drive not dominated the scene, the benefits must surely outweigh the negatives?

No need of a chain oiler if you have a shaft spinning in a lubricated gear box and no dirty wheels from chain fling either also no need to check chain stretch once a week and adjust for slack. Opinions?

Because it cost more to manufactor, less spare part sale, less power(us boys are terrible hung up on bhp) just some reasons that came to mind before the coffee kicks in