I just finished building a single speed bike and I think my chain line might be a little off. Is there a tool or trick to sighting the chain line?
You should be able to "sight" this fairly well. Get close to the front sprocket and align your eye such that you're just looking down one side of the front sprocket towards the rear. Moving your head back and forth sideways very slightly the same side of the rear sprocket should flicker into and out of view.
Or you can use a long metal ruler, placed against the side of the sprockets. With perfect alignment (which isn't really necessary) the ruler will fit flat against both sprockets simultaneously.
My preferred method is the following:
With the bike upright (left handlebar end against a wall), with one hand apply some pedalling pressure to the right pedal, while it is pointing to the front.
With the other hand, press a straight ruler (with length preferrably around 1 foot or more) against the upper part of the chainring, making sure it touches the ring in two points of its outer circumference (one ahead of the bottom bracket, and other behind). This way you can be sure the ruler is aligned with the chainring plane.
With the chain tensioned by the pressure on the pedal and the ruler as described, look from above to the alignment between the chain and the ruler:
- If the back part of the ruler hides under the chain, the sprocket is further from centerline than the chainring.
- If there is a gap between the rear part of the ruler and the chain, chainring is further than centerline relative to the sprocket.
- If the ruler runs along the chain in a rather parallel fashion, the alignment is fine.
It is worth noting that a perfect alignment is not strict necessary if the error is, say, less than half the chain width (I am being arbitrary here).
Also, this procedure just assure alignment to the chainring plane, since the sprocket could be rotated. This happens if a badly dished wheel requires the rear axle to be SLIGHTLY vertically rotated in order to get the wheel aligned inside the frame, but I don't think this would matter much unless it is too extreme.
In my experience, this alignment is meant mostly to avoid premature/asymmetric wearing of the drivetrain, and also to prevent the chain from falling off if it stretches too much before being retightened. If you don't notice an abnormal wearing pattern even if the alignment is not perfect, I would think that's just fine.
Hope it helps.
The best way is to measure the chain line distance at the chain ring and then measure the chain line at the rear cog. You require to measure sizes on your bike as accurately as possible and do some calculations. This site explains how to do it for geared bikes but the same principals apply. http://www.parktool.com/blog/repair-help/chain-line Hope that helps.