Are my brakes overheating?

Question

I'm riding a Giance Stance E+1 eMTB and recently had the brake pads and the rear rotor replaced by my Giant dealer, the brakes were worn out and the rotor had gotten damaged.

Only a few rides in on the new brakes. Yesterday when I was descending, 1500 ft in ~20 minutes, I got this terrible loud squealing sound from the brakes. Sounded like something was about to break. After stopping and letting the brakes cool off the squeal was gone.

Looking at the rotor it looks like the braking is uneven:

Are my brakes overheating? Are they incorrectly installed?

Answer 1

At a descent rate of nearly 1000m/h, brake overheating is likely unless braking is well managed. The energy management in the descent is a skill you will need to learn regardless of improvements you could make to the bike.

'Your gonna need bigger brakes' won't work, you already appear to have fairly big brakes. If you have installed organic pads, change these to sintered (metallic) as sintered handle high heat much better. Better brakes might also help (Stance range is Giants entry level MTBs), they will all overheat if pushed too far.

If you 'hit the gas' between corners, every time you add power (human or electric), this energy has to be removed. This happens from friction and aerodynamics, but an more than these the energy is lost by converting it to heat in the brakes. Don't hit the gas to save a half second between corners.

The next is braking technique - you have two brakes, were you using both? If not, you need to spread the energy (heat) between the front and back brakes.

Were you dragging the brakes? This is bad for heat dissipation, you need to pump them, releasing completely for short intervals provides better cooling. Use aerodynamics to you advantage - make yourself 'big' so the air slows you down.

How heavy are you - a big rider on along descent has to much more careful than a lighter rider as we all get the same sized brakes but the energy that needs to be dissipated is much more for a heavier rider.

Stopping an letting the brakes cool off is always a good option. Better to take a minute or too longer than have brakes fail. Be very careful after stopping to check the brakes before riding off. If really hot, the lack of airflow when stopped means more heat makes it into the calipers and can boil the brake fluid. If this happens, you will have no brakes. This effect has been observed in practice (Usually heavily loaded bikes on long road descents).

Answer 2

Braking is hard work for your bike. It has to transform your kinetic energy/momentum into something else, mostly heat and distortion of the pads.

A steep gradient means more momentum, and a heavy bike adds to that. I routinely coast down a hill just as fast as a lighter rider who has to actively pedal.

There are some techniques for you to try once the rotor and pads are replaced.

• Sit up. Be less aero on the descent, so you gain less speed. Do this by opening out your shoulders and arms, and straightening your back.

• Hands - some people like to descend on the drops for better braking. If you choose to use the hoods it incurs more frontal area. Modern brake levers work just as well from the hoods, older styles worked better from the drops.
  If you have a flatbar put your hands as far outward as possible while still able to actuate the brake lever

• Technical Solution - check your bike's manual for a "regenerative brake" If it exists, this should be able to use the power from the rear wheel driving the motor.
  Based on your photos, you have a mid-drive and this would require the chain to be continuously moving so probably not a viable solution in this case.
  A rear or front-drive hub motor could totally be capable of regen braking.

• Braking Method - Absolutely do not drag the brakes. This will heat them and that heat will have time to "soak"
  Instead, do a short hard brake and then get off the lever. You can alternate between front and rear brakes to have one in a cooler state in case of emergency braking need.
  Some tandems are equipped with a third brake specifically designed to drag and therefore limit the top speeds. These are often a set-and-forget brake controlled by one rider, and are designed to work when hot using drum or roller brake designs, not a disk or rim brake.

• Choice of Line - take a wider line when on a corner rather than cutting into the inside. This means you have further to traverse and therefore the effective grade is less for that piece.

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The danger of an overheated rotor is that it fails. This might be binding on the caliper, or in extreme cases the metal can expand and wedge giving you a wheel that won't rotate at all. The rotor might also tear and catch giving the same result.

Also the heat can damage the caliper if its hydraulic. Boiling any water in the oil will give steam, resulting in a soft brake that doesn't work or splits and spills oil. Worst case the hot oil can push the pistons out, engaging the brake and making everything rapidly worse.

As per other suggestions, make your descent more sight-seeing and less race, take some halfway photos at a convenient lookout, and post them on Strava while your brake cools.

Answer 3

There are a few factors affecting how well an MTB brakes which aren't brake related and in turn could reduce brakes overheating. mattnz gave some good pointers directly related to brakes and braking. However, there are some additional things which also affect how well a bike brakes and could help with preventing brakes to overheat (not listed in a particular order).

1: Suspension setup

Suspension help the wheels track the ground better. If the rebound is too fast for example it could cause the wheels to skip across the trail (rocks and braking bumps). It's not possible to slow down effectively when the wheels are skipping across the surface and not tracking the ground. Spend some time setting up the suspension correctly.

2: Tyres

The tyres are the contact point between the bike and the ground and plays a big part in slowing a bike down. Tyre knob patterns and size, rubber compound, and tyre pressure are factors to consider. For extended downhills on rough(ish) or loose terrain more aggressive tyres would likely help "bite" into the ground more and slow the bike down quicker than a less aggressive tyre (xc or general trail use tyre) would. In turn you don't need to be on the brakes constantly. With regards to tyre pressure, less pressure will also improve overall traction and braking compared to a tyre with more pressure. I would go out on a limb and say the rubber compound will likely play a minor in braking traction (within reason and it's open for debate). Also take into account that better braking traction will also slow you down and make you work harder on the ascents (not the biggest worry with an e-bike I suppose).

3: Technique

The front brake is the most effective brake to reduce speed quickly. Don't be afraid to use it. Just don't use it when changing direction. While changing direction you can use a bit of rear brake. Another thing often overlooked is body position when braking. It's a bit of an art more than a science but worth mastering. There are a lot of videos about this topic on YT. I found the one from Ben Cathro/pinkbike especially useful (it also covers other aspects of braking). There are also some more advanced ways to scrub speed but I would suggest to get the basics under control before jumping into the deep end.

EDIT:

Also, make sure the discs aren't bent and/or rubbing against the calipers. Pad/disc rubbing can reduce heat dissipation during long descents.

Answer 4

I started having same problems when I switched to electric bike, Turbo creo SL, compared to my old regular bike they have both same brake rotors, so I think definitely the extra 5+ kg merits different brake pads or rotors. I drag the brakes due to dangerous descends with lots of intersections, so I can't afford just speeding up every time just to cool the brakes. I did same with my old lighter bike with no problems.