Impossible Chain alignment?

sudjim · Feb 8, 2011

Good morning everyone,
I am helping a guy our with a bike he built that the motor chain keeps coming off the rear sprocket. I have built about 10-15 bikes so far and this one has got me puzzled. It is a beach cruiser. He took off the rear wheel and replaced it with another one without the coaster brake. It has a 5 speed cassette on it. I had to re-do the rag joint and get it trued up. It is well within spec (no noticeable wobble). Motor is mounted pretty well, and is not noticeably tilted or cocked, seems about centered between the bars.

But if you were to put a yard stick or straight edge on the motor sprocket when the rear wheel is properly on the bike, it seems to aim NOT at the 10T gear on the motor, but about 3 or 4 inches off the bike. I adjusted the rear wheel to where it seems to align with the motor, and the rear wheel almost rubs on the fork near the bike chain side. I got the bike to start and seemed to work, but then the pedaling side seems to jump teeth because that side is out of alignment. It never worked well and that situation lasted about 1/4 mile at most and then the chain jumped the motor sprocket again. Has anyone found a bike that just would not align? I've tried everything from bending the chain tensioner, adjusting the rear wheel, running no tensioner, switching chains. Should I try a new wheel or a new bike? Thanks mucho

The_Aleman · Feb 8, 2011

1), You probably have to adjust angle of motor. Centering the motor on the bars is not necessarily the optimum. In fact, none of the 2-strokes I've run were center, they all had to be cocked a bit in the frame.

2), used cassettes with single-speed chain is risky business. Cassettes (and multispeed externally shifted bikes in general) use a narrower chain than singlespeeds. The sprockets wear different and so do the chains. If that 5-speed cassette is well worn, it's not going to like that singlespeed chain under load. And multispeed chains don't like singlespeed sprockets as a general rule. YMMV.

Venice Motor Bikes · Feb 8, 2011

You can file the sides of the new sprocket teeth into a 'bevel' so that they feed smoothly into the chain without snagging it & causing it to jump off.

professor · Feb 8, 2011

I prefer aligning sprockets with a straight edge against the large sprocket (checking and rotating the sprocket in case it runs out a bit) and see where the straight edge ends up. Good word from Venice too.

bairdco · Feb 8, 2011

i'd check to see if the frame's straight. if it's off by 3-4", something's definitely wrong.

2door · Feb 8, 2011

I'm with Bairdco and Venice and I'll add that the motor SHOULD be centered in the frame. Cocking the motor to get chain alignment is not the right way to do it. The engine drive sprocket is exactly parallel with the engine mounts. If the frame is straight and true then there should be no reason to "cock" or offset the engine to get the chain to align. Yes, there can be some lateral misalignment betwen the rear sprocket and the engine drive sprocket but it usually isn't enough to make a chain jump off.
Find out what is bent and either fix it or try a new frame. Smooth the edges of the rear sprocket teeth and shoot for no more than 1/2" of side to side alignment, preferably less.
Tom

The_Aleman · Feb 9, 2011

*snip*...Cocking the motor to get chain alignment is not the right way to do it. The engine drive sprocket is exactly parallel with the engine mounts. If the frame is straight and true then there should be no reason to "cock" or offset the engine to get the chain to align...*snip*

You cannot say with absolute certainty that cocking the engine isn't the right way, when for some people it's the only way to make it work on their setup. We all know there are many different hubs out there, many widths of tires, lots of different frames.

I would agree to try any other method first, but don't overlook cocking the engine. A few degrees can make all the difference.

Lowandslow · Feb 9, 2011

Before you cant the motor in the frame, step back and examine the frame to make sure its true and not bent. An easy way to check is to use a string tied around the steering tube and to the rear dropouts (should form a triangle if viewed from the top). Then you can measure at different points to check if everything is aligned correctly. The string provides a straight line as a reference point. Getting everything square and in alignment is by far the most critical part of the whole build. I would only cant the motor as a last resort.

sudjim · Feb 10, 2011

Thank you everyone for your comments and suggestions. It is humbling to run into something like this. You think you know what you are doing and you get a curve ball. Personally, I enjoy that part of the hobby. Minor fabrication, modification, tinkering and sometimes a EUREKA moment. When I see this bike back again, I'll try the string and alignment trick. Jim

corgi1 · Feb 11, 2011

I had a small bike once that got hit in the side or something and the rear wheel alignment was off so that the pedal chain wouldn't stay on,cheap bike ,layed the rear triangle on a cement block,steped in it and bent it back ,but it was not ever right

samuel tapahuasco · Aug 20, 2017

I still have Almos same problem . i put the chain in place and when i star test the bike is off right away to the outside size of the rear sprocket?.i really need some help.

crassius · Aug 20, 2017

properly aligned, the chain tensioner should have the chain pointing directly at the rear sprocket

Chaz · Aug 20, 2017

samuel tapahuasco said:
I still have Almos same problem . i put the chain in place and when i star test the bike is off right away to the outside size of the rear sprocket?.i really need some help.

It would help if you can post a few pictures from different angles so we can see how you have things set up. And of course, the rear sprocket must be true with no wobbles.

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Impossible Chain alignment?

sudjim

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The_Aleman

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Venice Motor Bikes

Custom Builder / Dealer/Los Angeles

professor

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bairdco

a guy who makes cool bikes

2door

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The_Aleman

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Lowandslow

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sudjim

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corgi1

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samuel tapahuasco

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crassius

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Chaz

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