chain falls off on fat tire bicycle

Jerry SantaMaria

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This is my second 4 stroke kit install. Both were purchased by spooky tooth cycles. For some reason I cannot get the chain to stay on this time. I purchased a fat tire mountain bike but no matter what I do I cannot keep the chain on the bike. I hope someone out there has experience with fat tire bikes and can help. I can get pics if needed.

Jerry
 
This is my second 4 stroke kit install. Both were purchased by spooky tooth cycles. For some reason I cannot get the chain to stay on this time. I purchased a fat tire mountain bike but no matter what I do I cannot keep the chain on the bike. I hope someone out there has experience with fat tire bikes and can help. I can get pics if needed.

Jerry

Does this mountain bike have rear suspension? If so it will cause oscillation like slack and tightness in the chain which was causing my sprocket to warp and flex. And when it did that the chain easily derailed off of it.

The way I fixed this is to replace the chain tensioner like this: http://www.niagaracycle.com/categories/eclypse-single-guy-bicycle-chain-tensioner-mt-95c to absorb the oscillation. I mounted it as close to the sprocket as I could. Keep note that the 415 chain is heavier than the normal bike chain so it might be necessary to use another mid way between the sprocket and the engine due to the weight of the chain.
 
I think it is due to your drive sprocket and rear wheel sprocket alignment due to the fat tire bike. I have a similar issue my bike is small so I have an imperfect tilt to the motor at about 5-10 degrees. What it does is make the chain twist slightly and want to jump off the drive sprocket. For irregular builds, we have to juggle possible alignment to two sprockets versus one (rear). I have a chain tensioner both at the drive sprocket and rear sprocket to compensate the imperfect tilt. It works great but ideally you would want to check your alignment and try to fix it and not use two tensioners to fix the imperfection. The top chain has tension and usually is fine, its the bottom chain right at the rear sprocket where most of the problem comes from. If you cannot get the imperfection hammered out and the chain is jumping from the rear sprocket, then place the chain tensioner as close to the rear sprocket as possible and the roller as low as possible. This will provide the best guide to the sprocket. again, ideally you fix the imperfection but u can always brute force it with two tensioners. see pic

** that nice brutalization of English...
 

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The bike does not have rear suspension. I do want to say thank to all that did reply with advice and very helpful information. I will try 1 and then 2 chain tensioners if needed. Just to be correct the rear needs the tensioner on the bottom right?
 
I also wanted to add that it does depend on your bike bar width. Some bikes have a very narrow bar closer to the axle. So, if there is not enough of a bar to properly grip the bar then move it further away from the axle. If you move it further away, I actually like to put the roller on top of the chain and then move the tensioner along the bar to provide tension. Finally, lining up the chain so that the links fall in the middle of the teeth of the rear sprocket when the bike is up right (not resting on a kick stand).

Try one tensioner on the back and then finding an isolated road and slowly picking up speed and looking at the drive train. If it falls off again, you want to know where the problem is either the drive sprocket or rear sprocket. Hope that helps.
 

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being a fat tire bike, you will need to put the chain out with a jackshaft type system, to clear the tire. otherwise the chain will be on an angle and will fall off, no matter HOW many tensioners you have on it.
 
being a fat tire bike, you will need to put the chain out with a jackshaft type system, to clear the tire. otherwise the chain will be on an angle and will fall off, no matter HOW many tensioners you have on it.

I am assuming that his setup does not have the chain rubbing against the tire because not only will you have chain alignment issues but the tire is going to have a flat soon. Depending on the angle, you can prevent the chain from falling off using chain tensioners. I have an imperfect 5-10 degree tilt and my chain does not fall off due to tensioners.
 
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