Unanswered What would eventually happen?

[Beekeeper]

I have Shimano XT BR-M8000 brakes and have often experienced brake fade on long descents.

What could ultimately happen if heat continued to build up? What damage could I do to components etc and what is likely to happen first?

 

[steve_sordy]

The discs get hotter and hotter, they go blue, the heat is conducted back to the brake fluid, it boils and you lose all braking power! There may be some noise and shaking. As for permanent damage, I'm not sure. The blueing on the discs will fade, the pads may need replacing, again not sure. Boiling the brake fluid can't be good for it. And what happens when it boils, where do the gasses go? It must put the system under huge pressure. Has anyone actually done this?

 

[KenX]

This happened to me initially on long descents on my Tues DH bike, due to dragging the rear brake....
It would get to the point of having zero brakes, lever back to the bar, the only way to restore some braking other than a 30 minute rest was to pour water over the calliper and disc.
Spoke to a pal of mine, bike and ski guru Darren Turner who taught me to use both brakes together, relying on just the rear is poor riding!
Once learned, I find myself using the front brake more and more, as soon as weight transfers onto the front tyre, it grips more and you can increase the brake pressure, the opposite happens on the rear.
Obviously there's a learning curve involved and fear of locking the front/going over the bars etc, just need to practise!

 

[RickBullotta]

Do you have the pads with heat dissipation fins? If not, get them. Also, why are you using your brakes? If the trail/road is straight, let it fly!

 

[Gyre]

As Steve said: brake fade, mushy braking, possibly including brake failure. If the fade is on the rear, bias your braking more to the front. If that doesn't work, switch up to larger rotors.

Spoke to a pal of mine, bike and ski guru Darren Turner who taught me to use both brakes together, relying on just the rear is poor riding!
Once learned, I find myself using the front brake more and more, as soon as weight transfers onto the front tyre, it grips more and you can increase the brake pressure, the opposite happens on the rear.
Obviously there's a learning curve involved and fear of locking the front/going over the bars etc, just need to practise!

In moto classes they teach you that the front brake has 75-80% of the total system braking power. On extremely steep trails, it's probably more like 90+%. With modern long/low/slack bikes and long travel dropper posts, the risk of OTB is pretty small. You just have to be more conscientious about modulating your front brake as the trail gets rougher and the fall line from your center of mass gets closer to the front hub.

 

[Beekeeper]

Do you have the pads with heat dissipation fins? If not, get them. Also, why are you using your brakes? If the trail/road is straight, let it fly!

Thanks, yes I’ve seen pads with heat fins on them. Can I fit them to my Shimano XT BR-M8000 brakes?

 

[Slowroller]

I have a brake engineer friend in the industry and asked this exact question. Unless you have water in your system, your fluid won't boil. Your pads however will get so hot that they lose their coefficient of friction and effectiveness. They reach a set point of temperature, and since they aren't creating increased drag, they don't get any hotter, or hot enough to melt. Stop pulling your levers for a second and they will recover. I rarely run into this on a mtb/emtb since I'm on and off my brakes a lot, even on sustained descents of many thousand feet. On a road bike however, it's really easy to do ripping into multiple switchbacks. You can warp your rotors easily, but they'll straighten out again with use. Big, thick rotors and fins do help somewhat.

 

[RickBullotta]

Thanks, yes I’ve seen pads with heat fins on them. Can I fit them to my Shimano XT BR-M8000 brakes?

Yes, you can. If you expand the bottom of this page, you'll see a list of the compatible pads. These include both finned and regular, as well as metal vs resin. The pads that begin the "J" are finned.

BR-M8000 (shimano.com)

 

[Beekeeper]

I have a brake engineer friend in the industry and asked this exact question. Unless you have water in your system, your fluid won't boil. Your pads however will get so hot that they lose their coefficient of friction and effectiveness. They reach a set point of temperature, and since they aren't creating increased drag, they don't get any hotter, or hot enough to melt. Stop pulling your levers for a second and they will recover. I rarely run into this on a mtb/emtb since I'm on and off my brakes a lot, even on sustained descents of many thousand feet. On a road bike however, it's really easy to do ripping into multiple switchbacks. You can warp your rotors easily, but they'll straighten out again with use. Big, thick rotors and fins do help somewhat.

Thanks, that’s helpful. I was worried the fluid would boil and escape or the cable melt.
Pumping the brakes seemed to work well with Shimano pads fitted but I’ve changed to cheaper Clarks as an experiment but they are not so grippy which means I’m on the brakes for a longer duration.

 

[Beekeeper]

 

[Mikerb]

Apart from improving technique in your use of the brakes, you need also to have brakes that match the sort of terrain you ride. So for example, sintered pads, 4 pot callipers, large/larger rotors, better rotors, if you are doing long fast descents. Otherwise you really need to ride within the capability of the bike which includes its brakes.
A lot of people prefer the "on/off" characteristics of most Shimano brakes but it does make modulation rather more difficult compared to SRAM brakes ( and others) for example.
My trail bike is a Levo......brakes are Guide RE which is a combination of a fairly basic lever with Code ( older version/smaller diameter pistons) 4 pot callipers and 200mm rotors.
My Enduro bike is a Whyte.....brakes are Code RSC which are levers with a higher pressure ratio, bigger reservoir, stronger cables, and 4 pot callipers and 220 rotors.
Horses for courses.

2 part rotors are supposed to dissipate heat better. Ally dissipates heat better than stainless steel and a 2 part rotor is a SS ring mounted on on an ally centre..........may be an option?

The other thing to consider considering you stated you changed pads is that you have mismatched the pad material with the rotor. If you use pads with different material make up than the original pads you need to remove the conditioning layer on the rotor laid down by those original pads ( wire wool or wet and dry, then IPA). Then bed your new pads in to what should then be virgin surfaces on the rotors. Overall it is best to stick to the same compound and preferably the same brand to a void that hassle.