I can only assume that the aluminium is being gouged from the rims for some reason
Hi Greg,
Yes, most likely that is exactly what is happening. Shimano pads are notoriously hard on rims, and in the first few brake applications on my new Sherpa's Andra rims, I noticed immediate evidence of scoring.
The "cure" for me and many others with the same problem was to replace the Shimano pad inserts with some Kool-Stop Salmon inserts. Problem solved. These are specifically billed by Kool-Stop as "rim friendly" brake pads (blocks) because they cause very little scoring.
The Kool-Stop salmon pad compound was first developed by Kool-Stop for the Scott-Mathauser finned-back, bonded brake pads back in the late-1970s. I still have a set on one of my bicycles dating from 1979 with well over 35,000mi/56,000km and they work as new. The rims are in fine shape as well. What makes the Kool-Stops salmon-colored? The secret ingredient is iron oxide or rust in the pad formula.
All that said, I have owned some rims where -- once the process started -- there was no end to it, and no matter what pads I used, they would soon "load" with aluminum oxide from the rims and braking became completely ineffective. Biggest offenders were the Araya 16A-3, 16A-5 and 20A rims from the mid-90s. Oddly enough, I have never had the problem of endless oxide loading with a French rim, and Mavic and Rigida have always given me a good result.
So, yes, most likely your problem is caused by the Shimano pads. And, most likely, a switch to Kool-Stop Salmon pads will fix the problem and greatly extend the life of your rims.
Best,
Dan.
