A friend has a stumpy with stock deore hydraulic discs. On even short descents (100-200m) his rear brake gets so hot that it smells powerfully of burning and spitting water onto it produces an impressive blast of steam. After such an episode they lose almost all power and squeal like mad – I’m assuming this is because the brake pad surface has been glassified.

He’s switched pads (regular shimano ones used each time) and changed discs twice but it still happens. His brakes have been bled and they don’t rub. It happens in all weathers including red hot summer days and (yesterday) in the snow. He is of reasonable weight and a fairly confident descender; he doesn’t obviously descend dragging his brake for the whole way which was my first thought.

Given all that, any thoughts on a mechanical cause of this or avenues to explore? Perhaps a slight/intermittent calliper seal leak?

Hmmm so….

“they don’t rub”

and…

“he doesn’t obviously descend dragging his brake for the whole way”

Both of these statements CANNOT be true. It has to be one or the other.

To get that hot on such short descents the pads must be contacting the discs, either through unwanted rubbing or dragging the brake.

Is the rotor true, and are all fasteners secure?

It’s possible the brake is only rubbing when the bike is weighted, so it seems fine on the stand. Frame or hub bearings maybe.

“Insanely hot”, lets say go as fast as you can from 800 m to 600 m (the 200 m difference you describe) and stop at the end of this run as hard as you can:

the brake discs on all of my bikes get insanely hot then.

Vacation thing in the Alps, lets say 500 m difference, very fast run: the braking sound totally changes, ugly rubbing sound, surface of the discs will get quite dark and burned. But – up to now – this dark surface stuff will go away again when continue with normal trail biking. The brakes never failed.

“squeal”: only when mud and water is around. Pads rubbing on the burnt surface sound not like squealing… At least with my bikes.

And yes – the rear brake discs are the ones which turn quicker dark. Might be also due to the smaller disc diameter on the rear and more “constant” braking?

Used to run also sintered pads. But all my bikes have now organic low cost pads. Had more problems with the sintered ones (also peak force when braking is higher when using sintered).

“After such an episode they lose almost all power and squeal like mad – I’m assuming this is because the brake pad surface has been glassified.”

Sounds like it might be the slight oil leak type thing or whatever it is that happens to old shimano brakes sometimes, I’ve had two rears do this, they just squeal and have no power. I think if you search for squealing shimano brakes with no power it’s probably not uncommon.

Some deore level discs need resin pads only. Not using sintered pads on those maybe?

The heat build up is quite normal.

…the strange thing is the loss in braking power…

If there is an oil leak: shouldn’t you see signs of oil UNDER the brake pad after such an event?

Possible to repeat the braking thing, then:

while the brake is still hot push very hard on the brake lever for a minute or so, then letting it cool down, take the pads off and have a look. Oil under the pads?

When the brake cooled off: still no brake power? Once lost – the brake / pads are done/ need replacement?

Others: you mentioned that you splashed water onto the hot brake. I wouldn’t do this. Sounds like this could cause problems as well.

Are the rotors old?

Iirc Shimano rotors are 1.5mm thick from new and Shimano say to replace them at 1.3mm thick. I changed mine at 0.9mm thick and the performance upgrade was amazing.

Brakes are supposed to get hot when you use them, that’s how they work. If the pads were glazed or contaminated with oil and weren’t generating friction, then they wouldn’t get so hot (friction >>> heat).

If only the rear brake is getting hot, then your friend needs to use the front brake more. It sounds like he’s relying too much on the rear and not using the front enough. The front brake should be doing most of the work. It’s possible to descend very quickly with only a front brake, but much slower with only a rear brake.

Larger discs will also help as they can absorb more energy (being larger) and have a larger surface for cooling. However, if your friend is dragging the rear brake and not using the front, he will always have an overheating rear brake.

A little bit of rubbing won’t get discs insanely hot, only braking does that. Is the bike of a size that you can ride? I bet 10p if you ride the same descents without dragging the issue goes away

The working temp for a disc brake is in the few hundred degees C area.  Dragging a disc brake at low speed for even a couple of hundred m can be enough to overwhelm its ability to absorb and radiate the heat leading to over heating.  Over heating pads can result in glazed pads.  simply sand off the glazing and rebed in and stop dragging them

I just want to discuss here rather than offer solid advice, so asking for others to correct me if I’m wrong:

Working on the premise that as you say, your mate isn’t dragging his brakes, the 2 things I can think of are either using a bigger rotor if he’s not already done that, or using one of the ice-tech rotors – as far as I know both of these things are supposed to improve heat dissipation and reduce the chance of overheating. As I said before, please correct me if I’m wrong.

In terms of whether the brake is rubbing, I can’t see how you wouldn’t notice that your brakes are causing drag before it would get to the point where this is causing them to overheat – it’d be like riding with your brakes on surely?

correct got bike – the only possible answer I can see is he is dragging the brakes

He should use his front brake more. All that energy has to go somewhere, best to share it out a bit!

He could also go a bit slower (more time for heat to dissipate).

The brakes don’t drag under normal riding, disc is true and secure, hubs are fine. Given that no one can suggest an otherwise mechanical explanation for this I grilled him on thinking about which brake he relies on; he sheepishly admitted that he uses his back brake predominantly.

We’re going to work on his braking technique before he sets fire to yet another set of pads and rotors!

+1 for swapping bikes for a while and seeing if it’s the bike or rider

Front brake should be doing more of the work, as the issue is with the rear it suggests he’s favoring the rear brake.

Takes a bit of getting used to if in a bad habit, at least get him to try using them evenly if nothing else.

Then in time you can learn to bias front /rear depending on what’s needed.

Try switching his levers round (right on left side and vice versa) without telling him and see if the front one burns instead .

*only if he’s got good dental insurance

You’ll need to video that from frame by frame analysis….

Can I just point out that discs getting hot in use is perfectly sane. Who decided that “insane” was an acceptable substitute for “very”?
Just.
Stop.
It.

NOW!

“Insane” is a bit hotter than “very hot”.

Insanely hot: nearly red hot.

Or?

“Insane” is a bit hotter than “very hot”.

Not sure that they are the same scale in a technical sense.

As above, disc brakes getting hot when used is not really ‘insane’.

Expressing surprise at it might be considered to be somewhat eccentric, though…….

haha. Like that.

Eccentric brakes. Great.

Had once eccentric brakes. Didn’t like them so…

Cheers!

Have we had a brake dragging confession yet?

I’d look very carefully to determine the calipers is inline with the disc,look for uneven pad wear,sticking piston common to either poor adjustment or out of alignment frame mounting not unusual no matter how much the bike cost !

I replaced some deore pads last year but within a few weeks where contaminated,still loads of meat left though ! after swapping the shit shimano resin pads for uber race matrix never looked back what a difference,amazing power in all conditions.

Is the following possible guys?

Small amount of air in caliper  heats up whilst braking, so expands and makes the pads rub even whilst not braking. In turn that air gets compressed again when using the brake making the brake loses power until everything is cool again?

Just an idea…

Pads rubbing won’t get the brakes hot enough to notice. For them to get that hot, they have to be binding enough that you can’t ride on a flat road. Misaligned calipers and pads shouldn’t make any difference to overheating. Overheating like the OP reported comes from sustained hard use of the brake on a descent. If the rear brake is overheating and the front one isn’t, then the rider is relying too much on the rear brake and not making use of the front.