Why isn't chain slack controlled automatically?

[Sebulba]

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

[HoL]

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

[ClunkintheUK]

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.

[Spank86]

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? :D

[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!

[speedyandypandy]

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? :D
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

[Sid Squid]

You could run a a idler wheel - but why? It's not a matter of cost, it's just not needed, simply modern motorcycle geometry does not require extremes of chain tension and slack, so any mechanism for controlling such slack is superfluous.

Trials bikes run idlers as they have much greater suspension travel, which means greater differences in chain slack.

If you had an idler on a more powerful motorcycle, or perhaps a V-twin with lots of engine braking, when the throttle were shut at any significant engine speed the torque fed into the lower run of the chain would back off the tensioner, this could mean lots of slack in the upper run, that might be unfortunate.

[embee]

Bike shaft drives are very expensive to make and very heavy relative to a chain. I like shafts, I have 2 bikes with them and for touring it's great, simply no maintenance required at all, just forget about it, but they are heavy. Chains are simple, light, cheap to make/build, and last well these days. There actually isn't usually any significant difference in transmission efficiency between a gear pair and a chain/sprocket, a couple of percent loss in both.

Just for info, the old BMW (/5,6,7 etc) shafts ran with oil, not a huge amount, just enough to get splashed around. It was separate from the final drive and gearbox. The front UJ bearings weren't sealed types, and the rear spline was open to the shaft/swingarm and was oiled by what was in the swingarm tube. The shaft on the Deauville is dry as YC desribes.

Purely as a thought, it could actually be advantageous for a power transmission chain to run with slack on the non-drive side since it allows the grease to return to the load side between the pins and bushes. If it was tight all the time it might accelerate the wear. Camchains in engine oil are getting plenty of runny oil on them so it will get round the pins anyway (plus the dynamics are such that there is often slack in the chain anyway between cams etc due to torque reversals over cams).

[punyXpress]

The bike I mentioned in the ' other thread ' also had a tensioner, but never found out why.
Adjustment never varied.

[Dave-the-rave]

A fully enclosed chain would be ideal but I guess they just don't look sexy.

[embee]

A fully enclosed chain would be ideal but I guess they just don't look sexy.

That's absolutely true, just like fork gaiters.

The only issue with a fully enclosed chain is that unless it's truly maintenance free, which the bike versions aren't usually, then it's usually just totally neglected. That's fine on a Honda Cub but not so good on a high power bike.

The MZ system was nice and just needed an occasinal squirt with an oil can. I've read that some did crack after a few years but that's just a material issue.

http://t3.gstatic.com/images?q=tbn:ANd9GcR4KY13cp6t8Q1NmQjgWOu4EPgGPXbzV xpC-aAVOQW8AQp0Qf7SutUTMdeT

[Viney]

Buells have them on their belts

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