Originally Posted by well biked
They should be measured from the center of the brake mount to the center of the brake surface on the rim (or the center of the brake pad) to determine the reach. Here's a link to Sheldon's site showing this:
http://sheldonbrown.com/gloss_ra-e.html#reach
Great, thanks. Useful to me and the OP.
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"c" is not a unit that measures tire width