Rim brake pad interchangeability? Working on rehabbing donated bikes for Christmas Anonymous, I regularly encounter bikes where the rim brake pads have hardened to the point that the brakes are ineffective.  We have a stock of salvaged pads that we've generally drawn from as replacements, but that stock is getting pretty thin.

So I took a pad from a bike (a BSO with relatively conventional cantilever brakes, which is the most common config we see) and visited a local bike shop.  Here they sold me a couple of pairs of Shimano BR-CT91 pads.  These were a pretty close match in terms of appearance -- the actual pad is a hair shorter, and the mounting shaft lacks the ribbing of the old unit, but otherwise pretty close.

But when I went to install the pads they wouldn't go in.  The mounting shaft of the new pad was ever so slightly larger in diameter than the old one, so it simply wouldn't fit.

I'm suspecting that the Shimano pads are intended for V-brakes, not cantis.

So, the question is:  What different configurations of brake pads are there, and how do you tell them apart?  And is there any terminology (other than obscure manufacturer's part numbers) that will distinguish one type from another?  (This is particularly important if we want to order some replacement pads online.)
I've made some observations checking out different bikes here and there and at today's workshop for Christmas Anonymous --

We will assume, for the sake of this discussion that brakes fall into three categories, calipers, cantilevers, and V-brakes.  (Yes, there are others, but not common enough to worry about unless you're really into odd bikes.)

Calipers appear to (almost) always use simple pads with threaded studs.  I'm vaguely recalling from the past that there are some variations on this scheme, but I haven't observed them of late.

I have observed at least four variations for cantis:


(Updated) A pad with a female thread, designed to accept an Allen-head bolt.  This pad appears to be very much similar to the pad for V-brakes.
A pad with an unthreaded stud with striations running parallel to the stud's length.
A pad with an unthreaded stud with "ribs" (shallow grooves) about every 2mm along the stud's length.
A pad with a smooth unthreaded stud.


(Note that I've been shown/sold pads of both the #3 and #4 variety from local shops, and both are labeled simply as "canti" pads.  It's not clear how one differentiates.)

From what little I can tell, there is a slight variation in diameter between the three varieties of unthreaded studs.  The stud with parallel striations (#2) has the smallest diameter, while the smooth stud (#4) has the largest diameter.  The #3 is somewhere in-between.  This variation is probably not enough to make a #2 unstable in the mount for a #4, but a #4 stud will not fit in a #2 mount, and a #3 stud is borderline fitting into a #2.

(Updated) V-brakes appear to always use a brake pad with a female thread, designed to receive an Allen bolt.  On the bolt are typically washers and bushings that allow the pad to swivel, after a fashion, for adjustment.

(Everyone, of course, is encouraged to reply with your specific observations on this topic.)