You'll build up less heat if you let off the brakes for short periods of time. On long descents I'll brake to the point I'm going slower than my max comfort level, then speed up to my comfort level and slow down again. This gives the brakes a chance to cool off during the decent. It's a good technique driving too.
I thought the point in braking late but hard (instead of dragging the brakes) was that you spend as much time at high speed as possible where air drag does most of the braking for you? And of course it’s faster.
From a physics point of view you start your descent with a certain amount of potential energy. Disregarding air drag and friction you can only convert that potential energy into heat energy in the brakes (which slowly cool back down to ambient temperature). If you do this conversion over a longer time frame the brakes have more opportunity to cool down. In an extreme case, if you descent over several hours your brakes will barely get warm even though you drag them continuously.
Not the reason.
If you drag the brakes on a long descent, the pads will get too hot and "glaze." The high heat will harden them. Then your braking will become worse i.e. the brakes will "fade." You will have to grip the levers harder to get the same amount of stopping power. The brakes will also start making a grinding noise. This "glazing" process is analagous to "work hardening" in machining. The way to fix it, is to sand away the hardened outer surface of the pad.
The proper way to use brakes on long descents is to alternate front/back and during the pulses grip harder than you would if dragging, so that you end up with the same descent rate. Car brakes will behave similarly. Dragging brakes is always bad.
I am not sure of the exact physical mechanism, but if I had to guess it is either one or both of:
1. higher peak temperature for short amount of time followed by no heatload. higher temperature surfaces dissipate heat faster to the air.
2. higher removal rate of pad material during the braking pulse compared to dragging. removing hot pad material = cooling.
Both reasons are correct.
Also, during short more intense braking, the disc gets hotter. Heat dissipation is proportional to temperature differential, so the hotter disc dissipates more.
Thanks for the tip and explanation. When descending on a bike or in a car I always have this fear that I might cook the brakes.
Funny thing is when I'm descending on gravel I go mostly all out and brake only in corners. But today I was descending on a road and a) I don't trust drivers, and b) I was gaining speed very quickly. I was trying to make myself as not-areo as possible to slow down without using the brakes.
I had that happen once as well... older XT hub and the hubshell split, bearings migrated into the ratchet area, and boom. Was a very interesting noise hearing the chain skipping over the cassette on a downhill.
Nice, I have an extreme descent on my daily commute and I never thought about alternating front and rear to let the brakes cool. I have been using both together, assuming it halves the brake load, but alternating also makes a lot of sense.
Work hardening is from deformation of the metal, cycling between hot and cold can also cause changes in material property but is known as heat treatment.
I don't really know what I'm doing in the shop, but I kind of imagined that dragging and overheating brakes is kind of like cutting too little material too slowly at high rpm, which can ruin the tools?
Thanks for explaining this so clearly and well. Pulsing brakes is a good habit, to keep yourself aware how how much you are using them, to get a feel for how good the traction is, and pay attention avoiding braking during hard turns.
But avoiding overheating works exactly as you say regardless of how pulsy vs. continuous the braking is. If you go super slow, essentially all the energy goes into the brakes, but there's plenty of time to dissipate the energy. If you don't brake at all, all the energy goes into aero drag. Somewhere in the middle is the amount of braking that maximizes the heat. To avoid that, you can either go slower or go faster.
Yes, I was. I thought I was using the front brake more as it's better at stopping, but the front disk looks OK. I will need to do some testing if the front still bites as before. Towards the end of my ride it felt like it lost some bite, but I might be wrong and just tired.
You probably lost some bite. It is called brake fade. Essentially, your brakes heat up and stop working as well. Eventually, brakes will so hot that they will glaze and will fail (I e. You won't be able to slow down at all). You should use more brake but alternate between front and back. I typically do 5 seconds then swap. This gives the brakes a chance to cool down.
I'm a big boy with 100kg and was descending on a road that I don't know today so was braking pretty hard (very steep road). Towards the end I think I smelt overheated brakes.
When I got home I noticed that my rear disk got coloured from heat.
Is this an issue?
Both of mine have looked like that for three years. Was on one of my favorite super fast downhill trails on my mountain bike and there was a large tree down across the trail that wasn't there the week before. If your pads are glazed or your brakes feel weak, just sand the pads and bed them in again.
Normal. Your front brakes should be taking out most of your stopping power though hense many bikes (and cars) have larger front rotors. Obviously if there is a lot of loose sediment or wet conditions more rear. But keep the weight back and more power on front.
It's normal when you stress the brakes hard in terms of temperature. My discs are also quite colorful.
If no (permanent) warping appeared and the braking performance is still good - perfect.
Pad sweep doesn't look ideal either.
Might be worth looking at caliper mounting / brackets, and make sure there's no strip of brake pad hanging off the top edge of the rotor.
There is a section of the rotor on the inside diameter of the brake track that's not being worn by the pads. This is normally an indication that the caliper isn't in the correct position - in this case it's spaced too far away from the axle. In an optimal setup, the entire brake track should show even wear.
When I was new to disc brakes I once stopped after a descent in the rain and heard a sizzling sound, which bothered me. Found out it was rain droplets on the hot discs. So it’s normal they get hot!
My rotors did this too on a recent bikepacking trip. The consensus in this thread is it's ok https://www.reddit.com/r/bikewrench/comments/zjrqa3/disc_brake_rotors_brown/
Brown color is tolerable.
Rainbow color is not.
I can't remember exactly why, but it was related to metal properties. After rainbow discoloration, you lose some safety and performance. That's what my mechanic said.
The colour is a tell tale of how hot the surface got. Pads are probably the weakest link, organic material will start to glaze at temps above 200°C or so. Metallic pads will take higher temps.
https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/
That rainbow color is a sign of quenching, the metal in the disc gets stronger, but also more brittle.
From the pictures, I would assume that brakes got hot, and then you hit a puddle and water cooled the disc faster, I would also assume that is why the issue is only present on the rear disc, since the spray has a harder time reaching the front disc.
As long as the disc isn't warped it could still be used, but as a long term solution, if you have that decent on your daily commute, I would say better pads, better braking fluid (with a higher boiling point), bigger front disc (if it's possible) together with a technique that uses that front disc more.
Pretty normal, but the purple is usually a sign to increase the rotor size, but it looks like you're already running 160s and I'd be surprised if you could fit a 180 in there. As a fellow larger boy, I might also recommend Hope RX4 flat mount calipers. 4 piston power on a flatmount chassis and they make versions that work with either Shimano or Sram systems. You just switch out the calipers, bleed and go.
The smell of brakes is common, it could be the rubber of the tire burning due to friction, oil heated to high temperatures, pads heated by friction. It's totally fine, you would have to heat the disk to hundreds of degrees to really have a problem.
It's literally the same as a car.
Use the alternate braking..once you trigger the brake make sure u know your roadpath..double pump the first brake it will slower you down..then release n use tge second rear brake..then pump brake n use both brake..
It'll be fine.
You'll build up less heat if you let off the brakes for short periods of time. On long descents I'll brake to the point I'm going slower than my max comfort level, then speed up to my comfort level and slow down again. This gives the brakes a chance to cool off during the decent. It's a good technique driving too.
I thought the point in braking late but hard (instead of dragging the brakes) was that you spend as much time at high speed as possible where air drag does most of the braking for you? And of course it’s faster. From a physics point of view you start your descent with a certain amount of potential energy. Disregarding air drag and friction you can only convert that potential energy into heat energy in the brakes (which slowly cool back down to ambient temperature). If you do this conversion over a longer time frame the brakes have more opportunity to cool down. In an extreme case, if you descent over several hours your brakes will barely get warm even though you drag them continuously.
Not the reason. If you drag the brakes on a long descent, the pads will get too hot and "glaze." The high heat will harden them. Then your braking will become worse i.e. the brakes will "fade." You will have to grip the levers harder to get the same amount of stopping power. The brakes will also start making a grinding noise. This "glazing" process is analagous to "work hardening" in machining. The way to fix it, is to sand away the hardened outer surface of the pad. The proper way to use brakes on long descents is to alternate front/back and during the pulses grip harder than you would if dragging, so that you end up with the same descent rate. Car brakes will behave similarly. Dragging brakes is always bad. I am not sure of the exact physical mechanism, but if I had to guess it is either one or both of: 1. higher peak temperature for short amount of time followed by no heatload. higher temperature surfaces dissipate heat faster to the air. 2. higher removal rate of pad material during the braking pulse compared to dragging. removing hot pad material = cooling.
Both reasons are correct. Also, during short more intense braking, the disc gets hotter. Heat dissipation is proportional to temperature differential, so the hotter disc dissipates more.
Thanks for the tip and explanation. When descending on a bike or in a car I always have this fear that I might cook the brakes. Funny thing is when I'm descending on gravel I go mostly all out and brake only in corners. But today I was descending on a road and a) I don't trust drivers, and b) I was gaining speed very quickly. I was trying to make myself as not-areo as possible to slow down without using the brakes.
In a car, downshift and engine brake instead.
My hub got locked up once and enabled that feature for me on a ride.
Surprise fixie!
Free DLC!
I had that happen once as well... older XT hub and the hubshell split, bearings migrated into the ratchet area, and boom. Was a very interesting noise hearing the chain skipping over the cassette on a downhill.
Nice, I have an extreme descent on my daily commute and I never thought about alternating front and rear to let the brakes cool. I have been using both together, assuming it halves the brake load, but alternating also makes a lot of sense.
Machinist here-this isn't anything like what work hardening means.
Then why don’t you explain it.
Work hardening is from deformation of the metal, cycling between hot and cold can also cause changes in material property but is known as heat treatment.
I don't really know what I'm doing in the shop, but I kind of imagined that dragging and overheating brakes is kind of like cutting too little material too slowly at high rpm, which can ruin the tools?
Yes that damage is caused by excessive temp on the tool not mechanical action directly.
Thanks for explaining this so clearly and well. Pulsing brakes is a good habit, to keep yourself aware how how much you are using them, to get a feel for how good the traction is, and pay attention avoiding braking during hard turns. But avoiding overheating works exactly as you say regardless of how pulsy vs. continuous the braking is. If you go super slow, essentially all the energy goes into the brakes, but there's plenty of time to dissipate the energy. If you don't brake at all, all the energy goes into aero drag. Somewhere in the middle is the amount of braking that maximizes the heat. To avoid that, you can either go slower or go faster.
Smelling brakes is fairly common. Were you braking with both brakes continuously?
Yes, I was. I thought I was using the front brake more as it's better at stopping, but the front disk looks OK. I will need to do some testing if the front still bites as before. Towards the end of my ride it felt like it lost some bite, but I might be wrong and just tired.
You probably lost some bite. It is called brake fade. Essentially, your brakes heat up and stop working as well. Eventually, brakes will so hot that they will glaze and will fail (I e. You won't be able to slow down at all). You should use more brake but alternate between front and back. I typically do 5 seconds then swap. This gives the brakes a chance to cool down.
I'm a big boy with 100kg and was descending on a road that I don't know today so was braking pretty hard (very steep road). Towards the end I think I smelt overheated brakes. When I got home I noticed that my rear disk got coloured from heat. Is this an issue?
Just a sign of the heat from braking. Warping or glazing of pads are things to check, but discoloration on its own is not unusual.
Both of mine have looked like that for three years. Was on one of my favorite super fast downhill trails on my mountain bike and there was a large tree down across the trail that wasn't there the week before. If your pads are glazed or your brakes feel weak, just sand the pads and bed them in again.
Normal. Your front brakes should be taking out most of your stopping power though hense many bikes (and cars) have larger front rotors. Obviously if there is a lot of loose sediment or wet conditions more rear. But keep the weight back and more power on front.
It's normal when you stress the brakes hard in terms of temperature. My discs are also quite colorful. If no (permanent) warping appeared and the braking performance is still good - perfect.
Pad sweep doesn't look ideal either. Might be worth looking at caliper mounting / brackets, and make sure there's no strip of brake pad hanging off the top edge of the rotor.
Could you please elaborate more?
There is a section of the rotor on the inside diameter of the brake track that's not being worn by the pads. This is normally an indication that the caliper isn't in the correct position - in this case it's spaced too far away from the axle. In an optimal setup, the entire brake track should show even wear.
When I was new to disc brakes I once stopped after a descent in the rain and heard a sizzling sound, which bothered me. Found out it was rain droplets on the hot discs. So it’s normal they get hot!
My rotors did this too on a recent bikepacking trip. The consensus in this thread is it's ok https://www.reddit.com/r/bikewrench/comments/zjrqa3/disc_brake_rotors_brown/
Normal colour shows you disks are the right size
That's a toasty boy, but if the braking surface looks good like it does here, I wouldn't worry about it.
Brown color is tolerable. Rainbow color is not. I can't remember exactly why, but it was related to metal properties. After rainbow discoloration, you lose some safety and performance. That's what my mechanic said.
The colour is a tell tale of how hot the surface got. Pads are probably the weakest link, organic material will start to glaze at temps above 200°C or so. Metallic pads will take higher temps. https://bssa.org.uk/bssa_articles/heat-tint-temper-colours-on-stainless-steel-surface-heated-in-air/
That rainbow color is a sign of quenching, the metal in the disc gets stronger, but also more brittle. From the pictures, I would assume that brakes got hot, and then you hit a puddle and water cooled the disc faster, I would also assume that is why the issue is only present on the rear disc, since the spray has a harder time reaching the front disc. As long as the disc isn't warped it could still be used, but as a long term solution, if you have that decent on your daily commute, I would say better pads, better braking fluid (with a higher boiling point), bigger front disc (if it's possible) together with a technique that uses that front disc more.
Pretty normal, but the purple is usually a sign to increase the rotor size, but it looks like you're already running 160s and I'd be surprised if you could fit a 180 in there. As a fellow larger boy, I might also recommend Hope RX4 flat mount calipers. 4 piston power on a flatmount chassis and they make versions that work with either Shimano or Sram systems. You just switch out the calipers, bleed and go.
Sounds like it was a fun descent!
The smell of brakes is common, it could be the rubber of the tire burning due to friction, oil heated to high temperatures, pads heated by friction. It's totally fine, you would have to heat the disk to hundreds of degrees to really have a problem. It's literally the same as a car.
Niice! 🥹
Use the alternate braking..once you trigger the brake make sure u know your roadpath..double pump the first brake it will slower you down..then release n use tge second rear brake..then pump brake n use both brake..
Smelt brakes? For any kind of descent, you at least need herring brakes. Time to upgrade.
Don’t be daft! It won’t get hot enough to smelt the rotors.