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Old 04-24-19, 02:53 PM
  #11  
redlude97
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Originally Posted by shelbyfv
Could you explain a little more? A fellow I ride with had rubbing on DA discs after a short downhill. His thought was that the fluid expanded as it heated. Wasn't much of a hill and I know folks ride these in the mtns w/o issues. Thanks
especially for road levers and calipers, the master cylinder is pretty small. The way hydros work is the fluid moves as the piston is pressed by the brake lever. Then it retracts when the lever is release and the fluid moves back in the master cylinder. Since this volume is small, and the calipers also have minimal volume and size, the amount of piston retraction is tiny. So the gap between the rotor and pad is very small even for a perfect system. When it gets hot the fluid and/or caliper expands so now the pistons retract even less so the pads sit a bit closer to the rotor. When people say their rotors warp after descending its actually the pistons retracting less and there happens to be a slightly out of true spot on the rotor and that one spot will rub. With mechanical discs you can change the distance between pads/rotors and since the pistons don't automatically move out with wear generally this gap will be larger than on a hydro system so its less of a problem.
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