rsbob 

16 CM. Pretty standard on a road bike

cyccommute 

Boiling of brake fluid really isn’t that much of an issue. Cars and trucks use DOT fluid and put much more heat into the brake system but “boiling” never really is much of a concern. DOT 3 and 4 fluid boil around 200°C (400°F) and 230°C (445°F) when dry. Most systems are going to remain dry. If you use an old, open bottle of DOT fluid, it might be wet which reduces the boiling point by about 25% to 140°C and 155°C, respectively. All of these temperatures would be difficult to achieve on a light vehicle like a bicycle. The rotors can certainly get hot but to boil the fluid, you’d need to heat up a lot of parts to the boiling point. That’s not likely to happen even dragging the brakes for a mile and a half.

phughes

Well, despite the arguments in this thread, I just want to say I am happy all turned out well for you. No matter the reason, losing one's brakes can be disastrous, and I am glad you didn't have an accident as a result.

Now, back to the bickering.

Koyote

He did not lose his brakes.

grantelmwood

In my race cars, I use ATE racing brake fluid which has a much higher boiling point.

phughes

I see that now.

Pop N Wood

Agree with your comment about hypothetical. Race cars boil brake fluid all the time, even autocross type cars. Would be nice to know if this is common on bicycles.

But water still gets in brake systems, mineral oil or otherwise. When the denser water pools at the low point (the caliper) it will have a lower boiling temp. Water is also corrosive. Using mineral oil doesn't eliminate the need to periodically flush.

Calsun

Pro riders in races like the Tour de France for example go many kilometers at high speeds and make use of their brakes when necessary. These would be the first people to experience brake failure from overheating of the brake fluid. I do not see how heat from the disc rotors is going to heat up the fluid in the brake lines. Losing the fluid from the front brake line would be serious but that is a different matter.

jsdavis

I have known people to overheat their BB7 brakes on a tandem bike on a -18% descent and I have known a different couple who overheated their cantilevers also on a tandem bike on the same descent. Cables don't boil, things can melt on BB7 that permanently damages them. Not sure if the cantilevers had permanent damage.

This happened on Hawk Hill for those from SF.

CrimsonEclipse

Pure curiosity:
What was damaged and how?

Edit:
Never mind, looked it up.
Apparently you can melt the adjustment knobs (who knew?)
Only happens with tandems and extended 15%+ descents.

drlogik 

The OP's observation is one that I also have pondered. If I had a very long, very steep descent, especially with a touring bike all packed-out, which brakes would I want? I really don't know.

I've gone down long steep descents with rim brakes but not with disc brakes. I know from past experience what the limits are with rim pads, not so with disc pads.

On some of the down hills I've taken over the years, I'd be more worried about the disc heating to the point that it warps because it gets soft and lose its temper. Then again, an aluminum rim gets VERY hot also. I imagine that weight on the bike can greatly influence the heat generated on either brake type.

Watching this thread for more applicable experience and expertise to weigh-in.

Jughed

I tested the limits of my disc brakes this weekend. Did a loop multiple times over the weekend that went over the same 2 gravel passes that were 800-1000' @ 12-24% grades on all 4 of the descents. With the last being in the rain. Poor roads, switch backs and a 190# timid rider - I put the brakes thru their paces.

My rotors were smoking hot, the rain was sizzling off of them on the last downhill, I could smell the pads... no brake fade or noticeable loss in performance. These are bottom tier Sram Apex hydraulics...

mpetry912 

I have an HP infra red pyrometer that needs a new battery.

I'm going to load up a couple of wine bottles and do a descent of the Big Hill using the brakes.

I'll see what brake temps I get. Commercial aircraft routinely see brake temps of 500-700 deg C after landing.

doubt we'll get that hot

/markp

Atlas Shrugged

We are well beyond that unfortunately. In the hearsay and cycling myth category for while now.

Salubrious

Boiling the brake fluid on a bike really is a thing. But not on a 1 and half mile descent!

The Colorado Trail can be ridden with mountain bikes. These bikes are typically loaded with food, water, camping equipment and clothing- whatever is needed to get from one end (Denver) to the other (Durango) so they might weigh 45-50 pounds. There are some descents on that route that try the best of brakes. There are two bike shops along the way (one in Lake City, the other in Leadville) that will convert your hydros to cable operated so as to get around that problem. Both are quite good at setting up the cable operated brakes properly. That's the tricky bit about cable operated brakes- most people don't get them right. Most road descents simply do not offer a challenge by comparison...

I also ride the Tour Divide where there can be trying descents as well. On the Galton Pass, the route descends 4000 feet over ten miles. To reduce time having to replace pads on the route, I use 203mm rotors. This reduces the pad temperature. My bike loaded up is about 55 pounds if I'm being honest. I have plenty of braking power with cable operated brakes and the pads last about 1500 miles despite the mud and dirt.

Water has almost no effect at all on the braking power of cable or hydro operated brakes. The idea that it does is a myth. Things it does do is contribute to rotor squeal until its cleared off and also allow dirt to gather on the caliper...

Redbullet

Just wondering: how the carbon fork blade of a light road bike (caliper area) would manage repeated situations like that?

mpetry912 

I loaded up two wine bottles in the panniers and came down the Big Hill using the brakes fairly hard.

At the bottom I stopped and used my infrared pyrometer and measured the brake temperature.

I showed about 250 deg F on the front brake caliper which is not that hot. 140 rear.

the disk temps are almost impossible to measure because they cool down very quickly due to their large surface area.

I am not using finned pads. ambient temp today was about 74 deg.

which is the whole point

/markp

CrimsonEclipse

ok, I'll bite.
What's so tricky about it?

elcruxio

The heat can't transfer to the fork blade.

mpetry912 

C'mon, the only transfer path is thru the caliper mounting bolts. not a lot of surface area there

it's ambient air cooling that cools down the brakes. That's why you can get fins.

I don't see how you guys get 'em so hot. worried about warping the disk ? Like in F1 ? Just don't see where that energy is coming from

how do those downhill guys do it on the ski runs in the Alps ?

/markp

Salubrious

Setting them up...

The clearance between the active pad and the backing pad should be about 2:1- no more than 1/2mm between the rotor and the backing pad and no more than 1mm between the rotor and the active pad. The rotor is designed to be flexible, fundamentally different from other brake rotors in this regard. This is why active pistons on either side of the rotor don't increase the braking power; that just reduces the squishy feel the brake will have otherwise. At any rate, getting this bit correct seems to be beyond most people- probably because they didn't look at good instructions. The backing pad does need to be far enough out so that as it wears the rotor is not pressed into the caliper body.

So when the pads are new the adjusters are backed out all the way and then adjusted to satisfy this requirement. However the caliper has to be aligned to the rotor- I'm sure you know how to do that already. Then the cable tension has to be adjusted to allow the brake to be worked easily by the rider. The levers should be adjusted so that the lever itself is easily reached. There's usually an adjuster on the brake lever body for this purpose.

So that's five adjustments which are all critical for getting the brakes to work properly- both in terms of breaking power and how squishy they feel.

The disadvantage of course is that the brakes need to be adjusted as the pads wear. The advantage is if there's a problem they are far easier to service in the field! The biggest problem I've run into with them in riding the Divide since 2016 and also the Colorado Trail is dirt and mud can make adjusting the inside pad a bid difficult. On a road bike the advantages are less since there's less dirt and you can ride higher speeds on descents. Some of the descents on the CT or the Divide are so treacherous you're taking your life in your hands to go 15mph.

Salubrious

On a road bike you're probably not going to get them so hot.

Last month descending the Galton Pass my rear rotor got pretty hot. I could not use an icetech rotor because the rear hub is a Rohloff. So it looked pretty baked by the time I crossed the border and got to Eureka MT. The top of Galton Pass is pretty steep and the road is rough. My bike weighed about 55 pounds with all the gear on it and I weigh about 191 so the brakes had some work to do. Add to that its a race and not just touring so I was pushing as fast as I could while actively working to also maintain control on the dirt road. Its not one you could descend on a road bike and you'd be in trouble on a gravel bike.

That's how you get them so hot.

cyccommute 

Herein lies some of the problem with people’s fears of brakes overheating. I don’t disgree with much of what you’ve said but I take umbrage with this statement. I’ve haven’t ridden all the the Great Divide nor the Colorado Trail but I’ve ridden many sections of it over the years on many different bikes with many different brake configurations from cantilever to disc with and without touring loads. I’ve also ridden on trails and dirt roads throughout the mountain west (mostly Colorado) that are more demanding then either of those trails both with and without loads. Of course there are places where going too fast is stupid but they seldom last for more than a few feet to a few hundred feet. Certainly not long enough to cause overheating of brakes.

This route, for example, is one that I did last year

On this bike which 63 lbs with a full Camelback. I weigh in at 230lb.



Even at that weight, I didn’t cook the brakes on the downhill off Weston Pass despite hitting speeds of up to nearly 30 mph on the unpaved bits. I never overheat disc rotors nor the rim brakes (this bike has one of each) despite the fact that I let gravity have its way with me on most any downhill. People who overheat brakes do so because they are afraid of the speed and think the way to control it is to latch on the brakes at the top of any hill and drag them to the bottom. I’ve been involved in discussion here on Bike Forums with people who stop in the middle of a 1.5 mile hill to let the brakes cool. The gnarly section in the map above is 11 miles long and I may have slowed at various points but I never stopped to let my brakes cool.

The problem is with the “latching on the brakes”. People just don’t seem to understand that on a bicycle you should practice pulse braking. Let the bike fly for a bit, get on the brakes…hard…then get off them. Rinse and repeat as necessary. This can be as often as several times per minute or can be every 5 minutes depending on the need for control but the brakes should never be dragged all the way down a hill. The pulse allows them to radiate a bit of heat before more heat is added. That’s how you can ride hellbent for leather down hills for more than 40 years and barely get a brake, rim, or rotor warm, much less hot.

Salubrious

It really looks to me as if you have a 203mm rotor on the front. That does a lot to keep things cooler! I've never had to stop to let my brakes cool either. But I do what you are describing also.

FWIW the more serious descents on the CT occur when you get into the San Juans. A lot of people run hydros on that route and never get into trouble. And there are those that do- hence the two bike shops actually near/on the route offer conversion from hydro to cable operated. I don't doubt that part of that has to do with how people are actually operating the brakes as you say.

My driver boiled the brake fluid in my Chevy Blazer years ago when attending a hang gliding fly-in at Silverton. She was descending the mining road on K2 which overlooks the town- its pretty gnarly. Should have made more use of 1st gear... When I released the bleeders on the calipers they actually hissed!

Not on the CT but a serious descent on the east side nevertheless.

CrimsonEclipse

Ah I gotchya.
Never had much trouble setting up cable disks.
Rim pads always took me a couple of tries