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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
    Commented Oct 8, 2013 at 14:20
  • @BPugh Sorry, I read the question too fast! You are correct. I removed the dup flag.
    – Aaron
    Commented Oct 8, 2013 at 14:38

3 Answers 3

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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 of 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
    Commented Oct 8, 2013 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. Commented Oct 9, 2013 at 8:02
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    @trailmax, a reasonably priced caliper (as in a vernier caliper or dial version, not a brake caliper) can reliably measure 0.05mm
    – stib
    Commented Oct 9, 2013 at 11:17
  • @stib yes, I know about calipers. I bought mine for 20 pounds and a rotor usually comparable in price.
    – trailmax
    Commented Oct 9, 2013 at 11:18
  • @BenediktBauer Good to know, never seen things like this given with rotors
    – trailmax
    Commented Oct 9, 2013 at 11:19
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Shimano recommends that its rotors, which start out 1.8mm thick, should be replaced when the braking surface has been reduced to 1.5mm. Credit https://road.cc/content/feature/when-should-you-get-new-disc-brake-rotors-257623

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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
    Commented Oct 9, 2013 at 11:13
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    It depends a lot on your ride style, terrain conditions and also indirectly how many km you do. For commuting, I do about 50km in a day, so I have an e-bike. I need to change the disc brake rotor every 1 to 1,5 year (about 4 to 5000km). I’ve disc brakes from Shimano, 160mm (I think something like SM-RT66, but don’t recall exactly), maybe the mileage might be better or worse with other brands or model. So the other answers are more helpful, one need to measure wear, often after 3mm wear from origin, one needs to replace them. Check manufacturer documentation.
    – Huygens
    Commented Oct 21, 2023 at 11:25
  • @Huygens How many pads do you go through per rotor? And what type of pads do you use? You replace rotors as frequent as I replace pads :\
    – WornChain
    Commented Oct 21, 2023 at 23:25
  • @WornChain Indeed with the originsl semi-metalics on my TRP Spyre the rotor was done at a similar time the pads were. So one pair of pads per one rotor. The price is similar anyway. Commented Oct 24, 2023 at 5:00
  • @WornChain: I had my front fork broken at the same time as the rotor was done (it was a no-brand rotor which lasted 9month), so as I was going to the bike mechanic for the fork, I asked he changed the rotor and all. He put new rotor from Shimano and also changed the pads at the same time (I suppose from Shimano, but which type I do not know). Now, both rotor and pads are almost used (I'm just above the 1,5mm that Shimano recommends as minimum for the rotor), I expect to do another 1000km at most; so as I changed them about 10months ago, they will have lasted a bit more than a year I expect.
    – Huygens
    Commented Oct 24, 2023 at 9:51

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