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I got a new bike (Scott Egeniues 920) and after a few rides the rear rotor bent/warped due to long downhill riding (I often go down more than 1400m/4200 feet in one go). When the rotor cooled down, it always got a bit better. But it would worsen with each ride. Eventually, pistons would not go back in completely and the brake would lose its power completely. I repaired everything (the rotor got replaced). A few weeks later, the same thing happened again. ( Shimano BR-MT420 4 Piston brake and the rear rotor is a RT-EM300 CL 203mm with metalic pads.)

Strange thing is that this has never happend with my old bike with cheaper brakes, smaller rotor though I rode the same routes (!) in the same way. The brakes on the old bike were Shimano BR-M506 brakes, SM-RT54 Rotor 180 mm with organic resins.

I searched the web and there are just very few mentions that rotors might wrap permanently due to going downhill/ overheating. My point of view is that this should never happen for a rotor.

What do you think? Any personal experience? Is this a warranty issue? Reasons this did not happen for both brakes?

Furthermore, while it seems more common that overheating brakes can result in temporary loss of braking power, it seems that it should not happen that the brake permanently looses its braking power and requires repair.

What do you think? Any experience? Is this a warranty issue?


Update 1 year after the issue I mounted other (much) cheaper but slightly thicker rotors than the (crappy) Shimano ones. A magnet (for speed measurements) and a bit of tinkering was needed. But ever since the brakes went through many bike park rides and other trips. The issue is gone and I saved a good amount of cash.

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    Not a warranty issue. Brakes are generally excluded. More of a braking technique issue.
    – Carel
    Nov 10, 2021 at 14:08
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    End of first paragraph “Metallic resins”. Are you running metallic pads or resin pads? It is unclear.
    – MaplePanda
    Nov 10, 2021 at 19:38
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    @Criggie No current E-MTB has regen braking. The mid drive layout doesn’t allow for it.
    – MaplePanda
    Nov 11, 2021 at 0:01
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    @ChrisH Anything other than fully resin might be an issue. The rotors are of the resin-only type. Metallic-type pads tend to run hotter, so warpage could be a result. That’s what I’m suspecting.
    – MaplePanda
    Nov 11, 2021 at 17:22
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    @Criggie: I have no picture. They were discolored. I could definitely work on my braking technique, thanks for the hint! However, I am really just interested in technical problem with the brakes.
    – J.T.
    Nov 13, 2021 at 15:44

3 Answers 3

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This answer may not offer much new but I'd like to share my opinion.

The new rotors you have on the new bike are a lower quality than the smaller rotors you had on the old bike. Also, as the new bike is an e-bike, there is more mass to slow down on your descent so the brakes have even more energy to dissipate as heat. The rotors being such a large diameter amplifies the visible effect of the warping.

I think changing your rotors to a superior type will eliminate the warping problem for the most part, and allow you to use metallic pads if you need better pad life.

If you look at the rotor ranger for a variety of manufacturers you can see they are available at a number of price points. Magura make basic ones, superlight ones and ones with reinforcement to reduce the possibility of warping in hard use. All of these can use metallic pads (but are thicker than Shimano).

Shimano makes a basic type that is a softer steel, only for resin pads, that you have now. They have some harder types and some on spiders and with sandwiched layers of other metals to help dissipate heat better. The large aluminium spiders are very stiff to reduce the possibility of warping as well as to reduce weight.

Hope (and others) make "floating rotors" that consist of a steel brake track attached to an aluminium spider. The brake track can heat and expand seperately from the spider, and the main design feature is to reduce warping when the brake is used at high temperatures. Again, the aluminium spider is stiffer too, saves a little weight and dissipates heat slightly better than steel.

My main feeling when looking at the spec of your two bikes is that some pennies have been saved on the rotors of the new bike and they are just not up to the sort of riding you do (but probably more than adequate for the majority of riders, who don't push their equipment so hard).

I would ask the shop if they would potentially discount some superior rotors to make the bike useable. They should be happy if the bike is pretty new, to keep you on-side, if you get on well with them.

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Are you sure the pads were properly bedded in when installed?

I'm guessing here, but if the pads where not at the top of their potential performance (i.e. you actually had less braking power) and descended the same routes, you may have unconsciously applied the brakes for longer periods, letting less time for the rotor to cool down between brake applications.

I had an extreme case, even though with rim brakes. I used to descend a stretch of road of roughly half a mile (600 meters) with an MTB using v-brakes. Out of curiosity, I did put my fingers to the rim to see how hot it was. Never too much, barely above air temperature (or any other part of the bike). On one occasion, I tried the same descent on an old road bike that had dried up brake pads. When I felt so little braking hability, my only option was to apply both brakes continuously. Rims got hot enough to burst both inner tubes near the valve, luckily, not at the same time and I was going relatively slow. When I managed to stop, the rims where noticeably hot.

In other anecdote, I have recently experienced a problem with disc brakes on an MTB where the pads where getting old. I noticed I could brake but after a descent, I perceived the smell of "hot metal". After I changed the pads, I tried my best to bed them in, braking performance got much better and now the rotors are much cooler after the same descents as before. I also got confident enough to descend faster, as now a short brake pulse could reduce speed much more.

I have some DH riding experience and always brake in "bursts" right before turns and such. I did the same with the afore mentioned bike.

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    I think this is an excellent point. Another related way it could go is front brake isn't fully bedded, leading to a tendency to need the rear brake more, but the rear brake is inherently less effective at stopping the bike, so it gets jammed on and overheats. Nov 11, 2021 at 18:06
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'Durability' is rarely the metric that increases with price for any performance bicycle part....! Yes rotors warp more easily with heat as they get larger (in exchange for being able to handle a more demanding single descent where they warp a bit instead of fade out and send you into a tree.) Needing to check everything over and do some tweaks after a challenging 1400m descent does not sound unusual.

Manufacturers seem to have created this expectation in consumers that adding the weight of a battery and motor to a bike means it needs the most insanely overpowered braking system they can fit into the budget. If you completed the same ride before safely on 180mm rotors, adding 20lbs of low slung cargo may not necessitate jumping up a size.

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    OP’s old bike had 180s, not 203s. More braking power is almost never a bad thing regardless.
    – MaplePanda
    Nov 10, 2021 at 19:37
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    Maybe the question is more specifically about highly technical downhill riding than I am qualified to answer, but imo there are many people out there who have been done a tremendous disservice by the industry "upgrading" the bullet proof V-brakes on their commuters and hybrids to finicky mechanical disc brakes that require constant tinkering and tweaking to maintain the superior braking power.
    – Affe
    Nov 10, 2021 at 20:30
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    The bike in question is a $5200USD 150mm travel full suspension E-MTB. Not the same category as V brake-equipped commuters and hybrids I'd imagine.
    – MaplePanda
    Nov 10, 2021 at 21:34
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    I guess that was my point? When you go from a Kia to a Toyota you expect it to last longer and require less maintenance. When you go from a Toyota to a Ferrari this is no longer true. 203mm rotors are a fairly niche high performance part for a specific kind of riding that come with other tradeoffs. OP seemed to feel that by definition since it costs (a lot) more it should require less maintenance.
    – Affe
    Nov 10, 2021 at 22:24
  • Ah, yeah that’s a fair point. A little more info from OP will help find the solution.
    – MaplePanda
    Nov 11, 2021 at 0:02

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