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The lower side of the chain is the slack side, it is not under a load. That is why it flops around so much. The top side is where the load is being carried and ALL slack has been removed so long as the load does not let off. All that slack goes to the bottom of the chain where it is not under tension. So you aren't seeing stretching, just slack flop. It would be interesting to see a vid showing that side during a similar run.
There's way too much movement to just be the slack that was previously on the top run. However, I'm sure it's more than just chain stretch as well. Are drag bikes hardtails? If the rear suspension is compressed under acceleration, that will change the chain tension. Also, there could be deflection of the swingarm - those guys run really long swingarms.
Also, I've seen yellow plastic chains at Home Depot. Maybe it's a plastic chain for weight savings.
Generally, compressing the suspension will increase the length between the two sprockets, and thus reduce the amount of slack. The location of the swing arm pivot relative to the counter shaft sprocket also affects this. Basically, when the swing arm, pivot point and the sprockets are all in a straight line, the chain is at its max tension location. This is why you don't want to over tighten the chain when the bike is sitting on the side stand with the rear suspension uncompressed. If you don't leave some slack, it will pull hard on the counter shaft and can damage the seal and/or shaft itself. If the suspension were compressed to the point where the swing arm went PAST the straight line position, the chain would start going slack again. Absent some kind of restricting device on a drag bike, I could see the bike squatting under heavy acceleration to the point where that might happen. ALL materials deflect/deform under a load. So the chain does indeed stretch under hard acceleration. However, the amount of actual stretching is likely trivial as the chain material is extremely hard and strong. It also doesn't really have much time to heat up during such short loadings and thus thermal stretch is probably non existent. Where you get the slack in the chain itself regardless of adjustment is in the clearances between the links and pins. Even if they are relatively tight clearances, they can add up on a chain that is long. So I would expect to see a lot of slack like that on a drag bike because their chains are usually much longer than those found on regular bikes. A big danger of having too much slack in the chain is derailment on the bottom of the rear sprocket. This is why a lot of dirt bikes have chain guides on the rear sprocket. The LONG suspension travel of dirt bikes requires a good deal of slack at the uncompressed suspension positions. The guide keeps the chain lined up with the sprocket so it cannot go off to either side.
That's what I noticed.... Yet TM is also correct in that is the slack side we are watching.
My opinion? Just get a shaft drive and forget about chains.
Ah! You mean trading one kind of final drive problems for another kind of FD problems? Any one here ever have a problem with a BMW GS FD?
...My opinion? Just get a shaft drive and forget about chains.
Me
I was just REAL fortunate that neither incident resulted in a loss of control/accident.
Oops I'm not supposed to call her "little" anymore!
