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I would like to know what effect changing the material of a set of disc brake pads would have on the rotors if you kept them the same. for example, If I have resin pads and have been using them for the life of the rotors, what would happen if I switched to metal or organic pads, while still keeping the rotors the same? Would they just squeal like crazy? thanks

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    Some rotors (Shimano spec'd under Deore) specifically are not able to be used with one pad type, I don't recall the model or details but there was a question about it. I'd simply use the rotors as is and bed the new pads in properly. (Don't forget to push the pistons all they way back when installing the new pads) If you suspected the rotors were contaminated I'd only ever clean them with isopropyl alcohol or similar. No sandpaper. – DWGKNZ Jul 7 '14 at 4:35
  • Nothing would happen, by changing pad type. You may accelerate rotor wear with sintered pads. – DWGKNZ Jul 7 '14 at 4:37
  • My MTB was bought in 2008, I've changed the pads quite a few times and used both sintered and organic pads. How much they wear the rotors has been unnoticeable - I haven't had to replace the rotors yet so I wouldn't worry about wearing them out with different compounds! Unless perhaps you are doing some crazy DH racing or something. – adey_888 Sep 5 '14 at 5:07
  • +1 for pushing the pistons back before installing new pads. Just be careful not to damage them. I scratched the surface of mine pretty badly when a screwdriver slipped - luckily it didn't puncture the rubber grommet! – adey_888 Sep 5 '14 at 5:09
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Just called Shimano to ask this question for a customer's bike. For switching from a resin to a metallic padset, the answer is clean rotor with isopropyl alcohol and do a standard bed-in.

Didn't ask them about metallic to resin

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I don't think you'll need to sand them. I would just change the pads and then go out for a ride on a borderline muddy day and splash through all the puddles you can find. You'll get grit on your rotors, which will accelerate the wear-in process.

If you do decide to sand them, be very careful. Everything I've ever been taught has been that you don't want to sand rotors, because it's very hard to sand in a circumferential direction (sanding as if the rotor were spinning and the sandpaper were still), which is the direction the rotors will be abraded during use. If you sand radially (moving the paper away from and towards the center of the rotor) you may run in to problems.

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    What kind of problems? – DWGKNZ Jul 7 '14 at 4:28
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    Accelerated pad wear, brake noise/singing, that sort of thing. – stranger Jul 7 '14 at 12:30
  • I guess it should be pretty easy to sand in circumferential direction because you can simply removed brake pads and spin the wheel while pinching the rotor between your fingers and sandpaper. Just be careful not to hit your knuckless in the spokes... – Mikko Rantalainen Jul 13 '19 at 9:54
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Depends on a lot of factors such as the rotor and pad make etc. The rotor has memory, left from the remains of pad material over time. What you can do is sand down the rotor surface lightly with medium grade sandpaper and then bed in the new pads as per the manufacturers instructions. Usually 10-20 hard stops without skidding. This will help avoid squealing etc as much as possible.

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