Originally Posted by
FBinNY
There are a number of possible causes for the error, but start with using the string method correctly. The string must be run symmetrically to both dropouts. The loop and knot you have on the right side can throw off results. Repeat that measurement threading the string as you did on the left for both sides (through dropout and tied to opposite side. Then measure carefully.
Also confirm the centering at the bridge using a known to be correctly dished with a straight axle. Be sure both ends of the axle are against the top of the dropout. Confirm my flipping the wheel, and reconfirm by rotating the axle with a cone wrench on the left side (a bent axle will push hte wheel to one side).
Now understand what you're measuring. The string method is a quick (but non-difinitive) diagnostic that the dropouts are at equal distance from the central plane. Even if they are, it's still possible for the wheel to be off center at the brake bridge. For example, the stays could be slightly curved, or one dropout may one slightly higher than the other. So go slow before "correcting" anything until you have a good sense of what the problem really is.
I have tested it with another wheel which I know to be dished properly and by flipping the original wheel. The dish of the original wheel isn't great, but there is definitely an issue since all of the above combinations weren't aligned properly. So in addition to the string method, what else can I do at home?
The rear wheel (which originally came with the bike when it was built in the 70s) has a 120mm hub, but the dropout spacing appears to be a couple of mm wider than this (not as wide as 126mm). I presume the rear spacing is supposed to be 120mm since it was a custom frame and would have fit the wheels, so I guess at some point in it's life the previous owner had an accident and bent the rear end?
In the picture I posted, I have a spare wheel from another bike on there. It is a 126mm hub and obviously doesn't fit properly. However, I knew this wheel was correctly dished and was just using it to see how much the incorrect dish of the original wheel was affecting the alignment measurement. As you can see even with a correctly dished wheel it is still out. The reach of the brakes appears to be fine for the 27" wheel it came with ( I think they're Weinmann 500s)