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Discussion Starter · #1 · (Edited)
This rotor was on an e-bike that was used for general commuting by a non-cyclist who has a habit of continuously dragging the brakes. This cooked the rotor to the point where there was severe discolouration and embrittlement of the stainless steel, and it snapped under moderate force while bedding in new pads — fortunately at low speed without damage to either the mechanic or the bike. The lines also needed to be bled because lever was spongy, presumably from boiling the fluid.

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The rotor still has its full 2-mm thickness; there isn't even a perceptible ridge at the edge of the braking surface, so wear isn't the issue here. The pads looked a bit glazed but didn't seem different than normal used ones other than that.

I played with the busted rotor on the workbench and was it was obvious that the metal was much more brittle than normal, almost certainly from prolonged overheating. I've never seen discolouration this extreme even from downhill racing applications.

Seems most likely a case of this, though I'm not sure which specific grade of stainless is used on bike rotors:

400 to 500 °C embrittlement.
Fine-grained, high-chromium ferritic stainless steels, normally ductile, will become embrittled if kept at 400 to 500 °C (750 to 930 °F) for long periods of time.
Personally, this gives me even more justification to keep springing for the biggest rotors I can fit and as much caliper as possible to go with it.

So has anyone seen a case like this before? :unsure:
 

· Out spokin'
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Judging from the rust on the QR and the oxidation/discoloration on the rotor spokes, I'd say the owner ought to find a less humid place to store his/her bike. Not to mention learn to not drag the brake. Finally, how about fitting a rotor larger than 180mm if s/he needs so much braking performance.

OP, you say the rotor isn't worn (still full 2mm thick) but what's with the heavy scarring at the edge of the brake track? It's hard to tell from the photo but from here it almost looks like the abuser ran the bike with pads worn down to the backing plates.
=sParty
 

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Discussion Starter · #5 ·
So misuse by someone else gives more justification for some action on your part?
Doesn't make sense.
It doesn't change anything for me, as I've been running the biggest rotors available since disc brakes came out.

My current use case is pretty extreme, too: I ride a fully-loaded cargo bike (often >200 kg total weight) at speeds up to 70 km/h on steep mountain roads, so why wouldn't I go as big as possible on the brakes? :D
 

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Discussion Starter · #6 ·
Judging from the rust on the QR and the oxidation/discoloration on the rotor spokes, I'd say the owner ought to find a less humid place to store his/her bike. Not to mention learn to not drag the brake. Finally, how about fitting a rotor larger than 180mm if s/he needs so much braking performance.

OP, you say the rotor isn't worn (still full 2mm thick) but what's with the heavy scarring at the edge of the brake track? It's hard to tell from the photo but from here it almost looks like the abuser ran the bike with pads worn down to the backing plates.
=sParty
It may look like that but the pads still had backing material and they were still the set that came with the new bike. I also measured the rotors with a caliper and there's no perceptible metal loss, plus the ridge isn't even deep enough to catch a fingernail. It's just a change in colour from where the brake pad ends.
 

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Discussion Starter · #8 ·
@mtnbkrmike
Winter commuting can and does destroy bikes here on the west coast of Norway, but not like this.

Although corrosion is extremely common year-round in this salty and wet climate, this is the first case of brittle failure I’ve ever personally come across, as well as the first in which a rotor has become so heat-darkened.

You’d think that the rider would lay off the brakes from all the drag, but I’m always amazed by the ways that non-cyclists manage to be oblivious to their bikes, especially when there’s an electric motor to do the work.
 

· high pivot witchcraft
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@mtnbkrmike
Winter commuting can and does destroy bikes here on the west coast of Norway, but not like this.

Although corrosion is extremely common year-round in this salty and wet climate, this is the first case of brittle failure I’ve ever personally come across, as well as the first in which a rotor has become so heat-darkened.

You’d think that the rider would lay off the brakes from all the drag, but I’m always amazed by the ways that non-cyclists manage to be oblivious to their bikes, especially when there’s an electric motor to do the work.
Interesting. Are you able to share the brand of rotor?
 

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@mtnbkrmike
Winter commuting can and does destroy bikes here on the west coast of Norway, but not like this.

Although corrosion is extremely common year-round in this salty and wet climate, this is the first case of brittle failure I’ve ever personally come across, as well as the first in which a rotor has become so heat-darkened.

You’d think that the rider would lay off the brakes from all the drag, but I’m always amazed by the ways that non-cyclists manage to be oblivious to their bikes, especially when there’s an electric motor to do the work.
It could also be a poorly aligned brake caliper that is constantly rubbing. I've seen this several times where the local shop does the "adjustments", but things are still rubbing when riding. Just tossing that out as well.
 

· Elitest thrill junkie
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If only he had used a larger rear rotor...
 

· Elitest thrill junkie
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Interesting. Are you able to share the brand of rotor?
brake and rotor look to be Magura?

In any case, I would join in saying 180 is crazy inadequate for an e-bike. It's adequate for my self-powered 4" travel XC bike.
 
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Discussion Starter · #13 · (Edited)
@mtnbkrmike
Of course; it’s a Magura MDR-C, which is extra-reinforced for e-bike use. Top-quality disc in my opinion.

@Redlands R&C
Yeah that’s always a possibility, but there wasn’t any pad rub, plus the owner specifically stated that his wife is a timid rider who always keeps a bit of pressure on brake levers because she thinks it’s safer. 😬

The rotor snapped at service during the first firm stop to scrub in the new pads.
 

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Could be sticky pistons. I've experienced sticky pistons with magura mt5 calipers twice. Looks like these are mt30 which are mt5 calipers minus the silver beauty rings. Once it was so bad I had to remove the wheel to push the pistons back manually. With a motor you could power through sticky pistons without a care.

You can't do this to a rotor riding trail, or straight DH'n the biggest vertical drop tracks in the world at WC speeds. Fade would be SO bad well before this point you would know something is way wrong. Even if fade didn't scream at you that something is wrong, your hands would be cooked well before this could happen. I would not look at this as a reason to run super big rotors. I would look at this as a mechanical failure of some sort that wouldn't happen even under the most extreme DH situations. The only time we see rotors fail like this is when tested on machines. The temps it takes to bring a rotor to failure like this are well past the temps we can generate with gravity. When was the last time you saw a WC DH rotor blow up? If the fastest riders in the world aren't cooking rotors to failure, you don't need to worry.
 

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Discussion Starter · #16 ·
@Jayem
180 is small for an off-road bike ridden aggressively. However, it generally more than does the job for how most people ride these commuting bikes on pavement.

I personally go with 220s whenever possible, but I ride hard and like to hit the big trails.

@slimat99

Sticky pistons happen, but this is simply a rider who’s totally oblivious to how she’s misusing the bike. 😋

That’s what I find so amazing about this failure: even though competent users can’t break them under the most extreme racing conditions, an over-cautious commuter managed to cook them to death by dragging the brakes for such extended periods that they apparently changed their crystalline structure.



400 to 500 °C embrittlement
Fine-grained, high-chromium ferritic stainless steels, normally ductile, will become embrittled if kept at 400 to 500 °C (750 to 930 °F) for long periods of time.
 

· Elitest thrill junkie
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@Jayem
180 is small for an off-road bike ridden aggressively. However, it generally more than does the job for how most people ride these commuting bikes on pavement.
If they are self-powered.
 
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