Shimano M system pads and their others are made of a very hard material!,they have good power in the dry if set up correctly,but in the wet is a different story, and they are extremely abrasive on the rims.. My tip, as Hunter recommends is to use Koolstop : thinline Eagle 2's,these come in a dual compound and offer powerful braking and are kind to your rims! and I think you'll agree It's easier and cheaper to replace the pads a little more often than to get a wheel rebuilt with a new rim !. P.s the Koolstops are made in the USA ! buy local product and keep an American in a job!
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Velosophy#1: It is better to have a bicycle and no money , than money and no bicycle ! Velosophy # 2 : "Winning is simple, but not easy." #3: "Give a man a fish and he shall eat for a day , teach him how to fish and he will sit in a boat and drink beer all day"