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I've probably done about 20k on the rotors I bought the bike with. They still work fine, but are visibly worn and slightly grooved from me not replacing the pads soon enough a few times.

How do I know when it's time to get new rotors (apart from by finding out the hard way, of course)?

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@Aaron, not a dup. This question involves the rotors which are fixed to the wheel, the pads are a separate piece that engages the rotors. –  BPugh Oct 8 '13 at 14:20
    
@BPugh Sorry, I read the question too fast! You are correct. I removed the dup flag. –  Aaron Oct 8 '13 at 14:38

2 Answers 2

up vote 2 down vote accepted

In my 14 years cycling career only once I had to replace rotor due to wear. And that rotor was 6 years old and used for heavy downhill riding in all conditions. I've gone through a lot of pads on that rotor (like 20-30 pairs). And only when I could actually feel with my fingers the groove on the surface, I replaced it. Also it started eating pads like mad - uneven breaking surface wore pads much quicker.

I'd say rotors is something you would not replace quite often. And rotors would not fail on you like a worn-out rims - rotors won't split easily. So for a sake of measurements, 30% thickness reduction would be a good indication to replace the poor thing.

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Since I can feel grooves on these, and as they're eating pads maybe I should start thinking about new ones (I'll take better care of the next ones). Good to know they're not going to collapse on me though. –  stib Oct 9 '13 at 11:13

Normally the manufacturer of the brakes gives some minimum value for the rotor thickness. For higher prized brakes they often even provide some gauge that lets you easily check if the rotor is still thick enough.

Typically the minimum thickness might be somewhere below 2mm (I believe to remember that the absolute minimum for Magura discs should be 1.7 or 1.8 mm). But as said, this should be an information that should be in the manual fo your brakes.

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never seen a gauge to measure rotor thickness on sale together with a rotor. And to measure 2mm or 1.7mm these things must be pretty accurate => expensive. –  trailmax Oct 8 '13 at 20:38
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@trailmax With my Maguras whipped a spacer to put into the caliper for bike transport when the wheels are removed. This spacer has a groove that tells your that your rotor is too thin if it fits into this groove. The trick is that you don't have to measure exactly, you just need some reference to "compare" (mechanically) with. The latter is much easier to achieve than manufacturing a tools that measures the exact thickness. –  Benedikt Bauer Oct 9 '13 at 8:02
    
@trailmax, a reasonably priced caliper (as in a vernier caliper or dial version, not a brake caliper) can reliably measure 0.05mm –  stib Oct 9 '13 at 11:17
    
@stib yes, I know about calipers. I bought mine for 20 pounds and a rotor usually comparable in price. –  trailmax Oct 9 '13 at 11:18
    
@BenediktBauer Good to know, never seen things like this given with rotors –  trailmax Oct 9 '13 at 11:19

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