Yamahs SR 125 Chain has come off (quick question)

[nathandenniso...]

My chain came off my motorbike, tightened it before so know what to do, it's still on the front sprocket, is it a case of just loosening the chain enough to get it on the back sprocket then tightening it up to how it should be, thanks

[chris-red]

If it hasn't sanpped then you didn't tighten the chain properly, it should not be possible for a chain to come off of the sprockets if it is correctly tightened.
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[DrDonnyBrago]

[nathandenniso...]

[nathandenniso...]

[binge]

Sounds like it was either;

a, not tight enough
b, adjusted so the rear wheel is not true





Ben

[nathandenniso...]

Is it a case of just loosening the chain enough on the cam to get it around the back sprocket and then tightening it up to the desired tightness?

[DrDonnyBrago]

[Teflon-Mike]

Its pretty much as the book explains, & as said.
There's Four reasons a chain will come off.
A) Too much slack
If so its normally 'thrown' when the wheels turning fast enough for there to be centrifugal forces on it, but when you roll off the throttle, say for a bend or something, taking teh tension off the top run, from the engine tugging it, but the wheel stlll dragging the chain round from the low side, so the tension goes from the top to the bottom.
B) Incorrect Wheel alignment.
If you have the wheel spindle pulled back further on one side than teh other, then the wheel sproket can be at a slight angle to the chain and front sproket. When this happens teh chain will 'ride' to one side or other, then it can pick up on the 'edge' rather than dropping into one of the gaps in the links, and lift off.
C) theres no teeth left on the sprokets to hold onto the chain!
Or at least the sprokets are so worn that there's not enough metal to get proper engagement on the chain, and the 'ramps' on the teeth are so shallow that teh chain can litterally ride up them, rather than bear on them and pull teh sproket round.
D) the chain is SO stretched, that the gaps between the links is bigger than the width of the sproket teeth (agrivated by worn, hence narrower sproket teeth). In this case, the chain will bear on the teeth, but on the top side, when being pulled, the chain is stretched, pulling teh links apart. When they get to the engine sproket, the teeth pick up in the gaps, but getting closer and closer to a link pin, until at some point a tooth missed a gap altogether and lifts the link pin, up and the chain straight off the sproket.
Can happen on the 'low side, o the back wheel too, but as the front sproket is smaller, and the angles more acute, and teh chain in tension, normally happens that end.

So, there you have it. all about chains and sprokets!

Check the condition of the sprokets. There should be a small area of flat 'land' on the top of each tooth. If they have worn to a point, then the sprokets shot, and needs to be replaced.

Check the condition of the chain. the side to side 'waggle' test is probably the easiest. But more accurately, google the links per inch for the guage of chain you have, and measure over the full length, the chain, off the bike, stretched out on the floor, pulled tight at either end. Renew is in doubt. Chains cheap, legs harder to replace!

Lube chain, and refit it all up, paying careful attension to the rear wheel alighnment, as well as chain tension, turning teh wheel by hand to check the chain runs smoothly and isn't snagging or catching, through misalignment.
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