A "drop bolt" is a special centerbolt for a caliper brake, which is off-center to allow the brake caliper to reach farther down from the mounting hole than it is intended to. Drop bolts are commonly used when a bicycle is equipped with smaller diameter wheels than it was designed for, most often when substituting 622 mm (700C) wheels for 630 mm (27 inch) wheels.
Drop bolts are available but for traditional single-pivot side-pull brakes, of the type which use two nuts to secure the arms to the bolts.
Drop bolts are not availalble for side-pull brakes that use a centerbolt with a fixed head, such as all Shimano models, nor are they available for dual-pivot nor center-pull brakes. If you have calipers of this type, or if you need more than a few millimeters of "drop", conventional drop bolts won't help you.
Drop bolts are rather expensive, and difficult to find these days.
I have made home-made drop bolt replacements from aluminum stock or even reflector brackets. For example, a reflector bracket has two holes for the reflector. Cut off the flat part the reflector mounts to, with its two holes. You need two of these. One set of holes is secured to the brake bridge/fork crown with a 6 mm or 1/4" bolt. The center bolt of the caliper goes through the other set of holes, with a stack of washers between the flat pieces of reflector bracket. You may have a problem if the center bolt isn't long enough.
Elegant? No. Practical? Yes.
This system is a bit funny looking, but it works extremely well. I have never done it on a customer's bike, but several of my personal bikes are set up this way.
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Parts & Acc.
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