Disk Brake Heating Up

I’m riding with a Shimano BL-M445/7L Disc Brake from UDC. It’s heating up on some longer/steeper hills (10% to 13% grade for ~1/2 mile) to the point that the disk gets warped and starts to rub on the caliper. About 10 minutes after the hill, the disk will cool off and go back to being flat enough that the rubbing stops.

Is this OK? Will I eventually ruin this brake?


Disclaimer: I don’t know the answer to your question.

I was going to comment that both of my disc brakes rub the calipers, but that doesn’t change between heat and cold. :frowning:

What I know about automotive brakes is that they get very hot very fast, but they also cool down relatively instantly. But this probably has little or nothing to do with a skinny rotor being applied for half a mile or more. That’s a lot of constant use.

A disc brake shall not rub. If your brake heats up too much, it is too weak for your use. What you can do is the following:

  • don't brake continously, give the brake some time to cool down as you ride.
  • bigger rotor/disc: bigger diameter, more braking power / less friction needed for same braking power, more material to heat up, more time of the revolution to cool down
  • ice tech rotor: better heat conduction
  • higher grade brake that allows ice tech pads with cooling fins
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Yes, it’s a problem. As Eric said, you are overstressing the brake. Aside from the immediate issue you are experiencing, the excessive heat apparently leads to the seals degrading, and the brake leaking over time.

A bigger two piece rotor will help dissipate some more heat, and also not warp. Also, as mentioned, a higher grade brake with cooling fins will help.

But, as an immediate fix, intermittend braking, or just using it less hard will help too. I’ve seen people get their brake disks so hot that the steel discolored, if you just stay of the brake a bit you can probably prevent that.

A little rubbing of disk brakes however is normal, and in my experience doesn’t wear down the brakepads much.

Thanks very much, everyone. It’s a really fun hill. Just at the edge of my abilities. Next time I ride, I’ll try stopping part of the way down.

Makes me wonder how people ride down ski slopes or other long downhill trails…

On ski slopes, the snow will cool down the brake :stuck_out_tongue:

Good rotors help a lot. I’ve never had overheating issues this far, even in the alps. But those have always been group rides, so every few miles at least you take a break to let the rest catch up, and technical on the downhills, which you go relatively slow on, giving the brake chances to cool.

Where I’ve seen people get them very hot (including the mentioned discoloured disk, and a very distinct smell of overheated pads) is things like ski slopes, or gravel roads downhill, where you go downhill fast. But I think most people just don’t ride those as much as you think, because they get boring after a while. If you spend all the time hiking up a hill, you either like to enjoy the view going downhill, or ride something challenging.

What’s your wheel size? And rotor size?

and your pad quality ?

My brake continues to heat up on this downhill section. It’s a fun hill that I want to keep riding.

To answer the question above, I’m riding a 2018 29” KH from Unicycle.com (with the Nimbus hub to get the disk inside the frame). My disk rotor size is 180mm diameter and 1.8mm thick. Pad quality is probably pretty good – I’ve had the muni for less than a year and the brakes have not seen a ton of use.

Does anyone have any experience with putting the Shimano Ice Technology disk brakes on a muni?

If so, I’d really appreciate knowing exactly what you bought that worked with the muni.



actually the 180mm rotor is fine.

However it is a bit more complex. The BL-M445 is using a BR-M447 caliper. This caliper has plasic pistons which only allow the usage of resin pads. Sintered pads could actually take more heat, but also heat up the pistons and calliper more which will not work with the plastic pistons.

I am not sure on all the Shimano models, however at least from the SLX and higher grade, they all use ceramic pistons. Then you can use sintered pads as well.

Bottom line: If you significantly want more brake power and way better fading resistance, you will likely have to go for an upgrade of your brake.

As an example: We found the same issue when going down a steeper, long decent on a Muni tour. While my Shimano Saint did not have any issues, another brake (BR-M447) showed the discoloured rotor and started smelling, although the riders weight (she) was at least 20kg less than mine.

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My starting point would be a two piece rotor. I have a Formula 180mm one, but I don’t think the brand matters. It allows movement between the friction surface and the center, which helps it not warp. Also the aluminum center is better at transfering heat than steel. When I stop with hot brakes, I can hear the two parts moving (little ping sounds) as the outer part cools.

If that doesn’t work, it’s likely time for a new brake. If you want to do Shimano Icetech, I’ve heard of great experiences with the Shimano Saint M820. Pricey, but good.

I’d expect any “downhill” mountainbike brake to work well, you just need to choose one you like. Mountainbikes go way faster than us, so even though they have two brakes, they put theirs through much more stress than we do. Anything you read on mountainbike reviews, other than ergonomics can be directly applied to unicycles. With modern brakes, they all seem to fit well on the caliper side with the inboard brakes, the only concern is how the master cylinder fits under the saddle. For example, I recently test fitted a Sram Guide R, and the handle just doesn’t really work on a unicycle.

so you’re using a d’brake and thebrake on the left, right?

I run a SLX with IceTech (Pads as well as rotor) on my 24" Muni with 160mm rotor. It works fine for me and i never had any issues. (But i do not ride the hardest downhills.) I’ve one of the first generation IceTech rotors that were one piece, so the whole thing was plug and play with my Spirit cranks. The newer ones have some plastic parts on the spider that do not work with spirit cranks without filing/grinding something off. For inboard disc, the new gen. rotors shall also work plug and play.

This is all super helpful, everyone. Thank you!!!

Eric, I am using the d’brake on the right side (I think it’s right. It’s the side that the d’brake was designed for). Sort of a bummer because I can’t use the little cable tie brackets built into the left side of the KH frame.

So you have the rotor on the hub (inside the frame) but on the right side of you uni? So you brake points to the front?

The caliper must always be on the back side of the frame and on the right side of the d’brake. Therefore, internal (on the hub) disc brakes are always on the left side of the uni, external (on the crank) always on the right side.

I fear, that you run your brake backwards

Eric, my apologies if I described the setup wrong. Your picture is exactly what I have set up. The disk is mounted internal to the frame. It is mounted on the left side. So, the D-bracket holds the calipers behind the disk (in back of the rider).

I’ve used disc brakes on very long descents without noticing a problem. I haven’t checked the temperature, because there was no problem and I assumed it would be too hot to touch. Hot is not a problem on its own; it’s just friction. So my point here is that a disc brake should work for very long steep descents.

So if your brake is getting wobbly, there is probably something wrong with your setup, somehow.

My first suggestion is to ignore it. It’s not a wise suggestion, but one that is wrong less than it is right.

I don’t have a next suggestion. I can say that there should be no noise when you hold the wheel in the air and spin it, and the wheel should not slow and suddenly stop on its own after a few revolutions. If those things do happen, it might be because the brake pads are passively in contact with the rotor. Try loosening the bolts on the brake (not the bolts to the frame, but the bolts connecting the brake housing itself to the adaptor which connects to the frame), then pull and hold the brake lever, then re-tighten the bolts. This aligns the brake to the rotor. This can reduce or eliminate any rotor-brake pad contact when you are just riding. If this had been happening, fixing it could help with downhill performance problems.

Sorry, that’s just blatantly wrong. Excessive heat is a problem with brakes.

It causes:

  • Brake fade from pads getting too hot
  • boiling brake fluid, leading to spongy brakes
  • Seals in the brake starting to leak quickly
  • Brake disks overheating and changing their material properties leading to rust
  • as mentioned in this thread: brake disks deforming. The material expands, and since it has nowhere to go radially, it will become wavy.

If you are not experiencing problems, it’s because you are not going fast enough to get the brake to it’s limit. It’s simple physics: when you go down a hill at constant speed, you are converting potential energy from height into heat by using your brakes (and some rolling resistance, leg muscles and wind resistance.) The faster you go, the faster you produce heat, and the hotter your brake gets, since it can’t transfer the heat to the atmosphere fast enough.

“a disk brake”. Look up some tests, and you will find that different models of brakes have very different capabilties, especially when it comes to when they start fading from heat.

That could be a problem, and should definetely be done. But the problem of a disk getting wavy is a real one, and while readjusting the brake may reduce it, it will remain a problem.

Thanks very much, Bristlecone and Finnspin! I appreciate your help.

I’ll double check tomorrow, but I’m pretty sure that my disk is not warped and that my pads are not touching the disk when disengaged (maybe there is just a tiny bit of rub, but it’s imperceptible and it’s #$%&@ hard to adjust to the point that there is zero rubbing).

I think I’m going to simply take the muni to a quality mountain bike shop in San Jose and have them double check the setup and the durability/quality of the brake system that I have relative to better options that may be out there.

I’ve been starting to ride down more and more hills, and it’s only this one really steep hill that gives me problems. But, it gives me problems every time and I’m starting to ride down this hill three times a week.