rsbob 

Was on a 1.5 mile descent yesterday which was relatively steep with pitches ranging from 10 to 17% with lots of tight switchbacks. There were few opportunities to get off the brakes lest shooting off a corner. So what immediately comes to mind is boiling the brake fluid and not being able to stop. I did find a run-off spot and 3/4s of the way down where I stopped for two minutes to let things cool a bit. In the future, I will avoid that descent.

Which begs the question, has anyone here, of know of any reading material, of brake fluid overheating to such an extent as to being unable to stop? I shudder to think…

Not interested in hearing from the inevitable, “I will never use disc brakes” people. I already know your bias. Thanks


A pic from my vantage yesterday.


Seattle, looking over Mercer Island, Lk Wa

Koyote

I don't quite understand your situation: were your brakes starting to fail, or were you just worried that they might fail?

indyfabz

My rim brake cables never boil. Just sayin'.

tomato coupe

I've never heard anyone say they lost their disc brakes due to overheating on a mountain descent, but I know a few people that have destroyed carbon rim-brake wheels on mountain descents due to overheating. Pick your poison. YMMV. C'est la vie.

Koyote

At any rate...

SRAM hydro disc brakes use DOT fluid, which is hygroscopic (absorbs moisture), while Shimano uses mineral oil, which starts out with a higher boiling point and is also hydrophobic (does not absorb moisture). All that means is the old SRAM fluid could conceivably boil on a long steep descent, while Shimano fluid would probably not ever do that. The only other problem could be glazing of the rotors from intense heat, but that's unusual.

Honestly, I think you've got too much time on your hands if you're worrying about this. From your description, your descent is not really a big deal.

tomato coupe

I want to hear Bauke Mollema's comments if that happens to him.

PeteHski

My SRAM Force brakes were fine last year on the L’Etape du Tour with epic non-stop descents of the Galibier and Croix de Fer in 40C ambient temperature. I was riding the brakes for most of the descents too. So I wouldn’t worry too much about it.

Polaris OBark

I used to worry about brake fade, especially on some of our steep hills (that get up to 22%). I descend like a clinically paranoid old lady, and have gone through an embarrassingly high number of Shimano pads. I have never once experienced brake fade.

Use the finned pads, finned high-quality rotors, and feather your brakes. Stopping along the way to let things cool and to regain my composure is worthwhile as well. But if I improved my descending technique, it would be for the better.

skidder

I've never heard of brake fluid boiling on a bicycle. I'd expect to notice the pads getting a little soft from heat (thus loss of braking ability) before worrying about brake fluid getting to its boiling temperature. How were the brake rotors themselves? I'm thinking they were not glowing with excessive heat.

Polaris OBark

I've seen discoloration on cheaper rotors.

wolfchild

It's possible depending on your braking technique and how you use your brakes. Dragging your brakes on long descents can overheat them. In worst case scenario the extreme heat would have more negative effect on the pads and the rotor than on the type of fluid your brakes have.

georges1

If you are using hydraulic disc brakes, when was the last time that you purged them and put new oil in them ? Normally, well maintained disc brakes shouldn't give you any troubles.

base2 

I have had fade failure. It was with properly adjusted Tektro Aries using the OEM organic pads that were about the size of a penny. I would get fade descending that city block towards Seward Park. Truly awful. The worst was one time descending into Mukilteo at 30-ish MPH. I watched the stoplight about half way down cycle from green to yellow to red. I slowed from 30 to 20mph and there was no more. No matter how hard I pulled the lever there was simply no more braking to be had. I had plenty of lever. What I didn't have was any more ability to convert kinetic energy to heat. As I rapidly approached the intersection, wonder turned to panic, turned to quick thinking. I stuffed my foot into the rear brake bridge BMX-style, hastily stopping in the intersection in front of a bunch of cars that very plainly had the green light. They were very kind in and stopped for me. Whether or not they recognized the state of emergency is another matter.

I chucked the blued 160mm rotors in the trash & upgraded to Avid BB7's and 180mm rotors the next day.

For myself, if the pad is smaller than a Shimano B01S, I'm not interested. Even with full-on hydraulic brakes, I continue to be unimpressed with the lever squeeze effort required on 160mm rotors, unless they are made by Carbon-Ti. (Carbon-Ti, IME are particularly grabby but wear quickly) For all other offerings, including Shimano's IceTech 180/160 is my preference. Which IIRC is the bike industry recommended rotor size for a person of my weight, anyhow.

I weigh 180 pounds, FWIW.

CrimsonEclipse

I've never had the cables on my BB7's boil...

sarcasm aside, consider larger rotors.

Using Mineral or DOT?

Chuck M 

I don't know that I have made a descent like the OP posted, but when I do go down a steep decline, I worry more about my helmet than than my disc brakes.

LarrySellerz

isn't one of the advantages of disc brakes over rim brakes that you aren't heating the rim? I thought prevailing wisdom was that rim brakes are more likely to fail during mountain descents. Disc brakes were engineered with your situation in mind, youre using them as intended. Id be more worried about ruining cheap brake pads, but again those were engineered with this in mind.

LarrySellerz

I think mountain descents are the one case where disc brakes totally beat rim brakes out, bad weather can **** badly adjusted mechanical disc brakes more than rim brakes

big john

I had severe fade on a mountain bike with Avid Juicy brakes with DOT fluid. Lucky there was nothing to run into at the bottom. The lever went all the way to the bar.

The hill was so steep it would be hard to actually stop on it. With a loose surface I would have been better off to just lock the rear wheel and slide all the way down.

I've had rim brakes fade a number of times but it's not the same effect, they just lose power from being hot and likely glazed.

phughes

Yes, but a disc on a bicycle has less area to dissipate the heat. No matter which type of brakes you have, you cannot ride the the brakes excessively downhill. You have to let off and coast to allow the disc to cool. The disc will cool faster than a rim, but a rim will take longer to heat up enough to be an issue.

I love blasting downhill, so I don't brake much unless I get behind a car going slowly. To help keep your speed down though, sit upright so the wind hits your body, acting as a brake.

indyfabz

Get back to me after you’ve ridden a fully loaded touring bike with rim brakes down long descents in rain, sleet and snow a few times. And don’t forget to wear a shirt. It gets cold in those conditions.

rosefarts

Early 2000’s, I’m young and fit and in a peloton of 100 or so other like minded people. We’re ripping through the descent. The smell of burning brakes and sunscreen is ever present.

”Boom, crash” behind me, “boom crash” behind me. Then “boom crash” in front of me and I’m on the ground. Out of the race. And needed a new wheel.

I’ve reached down off epic descents out of curiosity and melted part of my glove on the rotor. Funny thing, I’ve had my rims nearly as hot on other bikes.

It’s almost like big descents generate heat. Funny how that works.

CrimsonEclipse

Explain to me how this happens.

rosefarts

Water is a lubricant. Rims are much closer, sometimes in the water. Rim brakes are a bigger rotor but they can’t squeeze as hard since a crushing force would deform the rim. With the size of rotors and the clearance of disc calipers, it’s much more likely they’ll get dried quickly if they get wet. Put it all together and there is less stopping power.

LarrySellerz

How the bad weather messes with the disc brakes? IDK but my cheap bike with cheap disc brakes straight up won't stop in the wet. As for mountain descents killing rims with rim brakes, I was under the impression that this is accepted knowledge but am open to learning otherwise. I was also under the impression that accepted knowledge is that disc brakes are superior in adverse conditions, and im trying to push back on that idea with the case of cheap mechanical disc brakes.

Russ Roth

Mine never did either, just got hot enough to melt the vinyl rimstrip allowing the tube to puncture and overheated latex tubes to blow. Love our modern age when I can toss liquid latex in a latex tube and not have to worry about punctures or blowouts for my kid's bike while giving her a better rolling experience.


OP: You don't say what brand of brakes. Something to be aware of with Sram brakes is that they recommend DOT4 fluid which expands less due to heat and has a higher boiling point but trades off with being more susceptible to water contamination and has to be changed more regularly. With DOT3 cars shouldn't but can go 8 or 9 years without changing the fluid, mine made 165k miles in 9 years before I changed the fluid since the calipers were getting replaced at the same time. DOT4 should be done every 2 years, 3 at most due to its ability to absorb moisture. Gotta watch the contamination, that's what'll get you most times.

Course, at the end of the days brakes shouldn't fail, after all; to brake is to admit defeat. Just let em go and let it ride and the brakes will never fail.