Cunninghambikes

Charlie invented and refined the dual compound brake pad, which is used to “break-in” new rims and eliminate brake squeal. The compound at the leading edge of the pad contains a special abrasive material that cleans and removes high spots from the rim and, because it has a slightly higher wear rate, it produces a toe-in condition that improves braking and reduces pad squeal.

Cunningham started making dual compound in his shop in the mid-eighties. He made them by gluing abrasive-containing Cratex material in front of standard pad material, into Mafac pad holders.  He still have some of these original pads! The idea was to put a short portion of slightly faster wearing, abrasive material in front of the standard material so that as the pad wears, two things happen. First, the abrasive cleans the rim and removes surface dings and smoothes out "washboard in the aluminum". (Bikes that had already been squealing for a while actually develop a washboard ripple in the aluminum that make squeal really hard to eliminate.) Second, because the abrasive material wears faster, it gives the effect of a slight but constant state of toe-in. Cunningham found that both the abrasive rim cleaning and the slight toe-in resulted in more uniform braking and less squeal, even on bikes with "problem brakes" (bikes that had a history of  difficult to solve brake squeal). Cunningham never patented the idea.  Today dual compound pads are available from manufacturers including Kool Stop.