Why did the hydraulic disc brake stop working when riding down a steep mountain?

Question

I rode my mountain-bike with a bicycle trailer attached. Three kids (age 4 to 8) were inside the trailer (trailer made of steel). We went down from about 500 m above sea level to 150 m above sea level with an inclination of around 15 to 20 %.

My hydraulic disc brakes (Avid Juicy 3 with 180 mm discs) got very hot and something began to smoke and stink. When I was at about 250 m above sea level I stopped the bike, because the kids were complaining about the bad smell.

When I wanted to continue the ride, the front brake didn’t work at all. I could easily pull the brake lever with no resistance! As if the cable was broken and the fluid inside was gone. I stopped again. It was in the evening (dark), and I couldn’t find anything.

I continued the ride very slowly, only using the rear brake, pulling the front brake lever again and again to understand the problem. The more often I pulled the lever the resistance to pull it grew and the brake started to work again.

To summarize the problem: The brake stopped working completely after I had stopped the bike (but before it was working like a charm, only with a bad smell). And started to work again, when I pulled the lever a few times.

Now the questions:

• Why?
• What happened?
• Since then the brake is working as usual again. How is that possible?

Answer 1

What you experienced was brake fade. There are two reasons why it happens:

1. The fluid inside the brake boils releasing gas into the system which makes the brakes feel spongy and then non-existant.
2. The pad and rotor heat up sufficiently to not work, this feels like you are applying the brake but it doesn't work.

You look to have experienced the first. This is exacerbated by water in the brake line as that boils at a lower point, the DOT fluid in the Avid brakes is hygroscopic and will attract and retain water in the brake fluid. You may have also had air in the line which would have also exacerbated the problem.

The additional weight of the trailer and kids (I'd estimate an additional 100kg nearly) put more work on the brakes to keep speed low. This is the main cause of the brake fade and why you may not of experienced it before. Braking technique plays a big part although in this situation outside of your control. If you drag the brakes (long continuous periods of moderate braking) rather than short bursts you will see the brakes heat up much quicker.

In the first instance let the brakes cool to get you home. After that it would also be worth bleeding the brakes with new DOT fluid as well to purge the system of air and water. Also I wouldn't try to take that much weight downhill on a trailer.

Answer 2

Lets put some physics to play (mostly because I want to do some numbers....) Assume the brakes did all the work.....

200kg all up weight of rider, bike, trailer and kids. 350Meter vertical descent means you need to dissipate 700kJ of energy = 200WH.

I found a reference to "Road and mountain brakes must absorb 75 watt-hours of energy over a 15-minute period without failure to be legal for sale in Europe" so will use it for my calculations. 200WH at 75WH/15Min is 40 Minutes.

So assuming you spend 40 minutes doing the descent not to exceed the 75WH/15m limit of the brakes, and the the angle of the slope is 15 degrees, 350Mters is about 1.5km - you could safely descend the hill at no more than 2km/hour -seems too slow. Maybe that standard is 75WH/15m per brake - so you could twice as fast 4km/h. Even if other friction, tire scrubbing and aerodynamic losses (At that speed ?) were 1/2 of the energy loss - you cannot exceed 8km/h without over heating the brakes. But a cold set of brakes stores energy as heat - so what is the acceptable rate of energy dissipation after 15 minutes?

As others have said, you boiled the fluid - replacing it is not going to fix the problem. If you do this again with the kids.... "Your gonna need a bigger brake"... duh da, duh da.... (...Movie reference...)

In researching this answer I am now far more aware of limitations of my brakes - thanks....

Answer 3

Though there are some very good answers here already, I get the impression you are more interested in why the brakes suddenly stopped working after you stopped riding for a moment.

I am not a brakes expert, but I have a solid background in thermodynamics. From that perspective, it sounds like what happened (as explained above) is that you had boiling occurring, which means that as you continue to apply the brakes there is additional heat energy put into the system. Once boiling occurs, the additional input of energy leads to an increase in pressure and temperature in an enclosed system like this one until the rate of leakage reaches an equilibrium with the heat energy input. This means that even though there is a problem, there is pressure in the system so that when you squeeze the handle, the brake is applied.

As soon as you stop, the addition of heat energy to the system ends. Think of a pot of boiling water on a stove: if you lift it off of the burner it will instantly stop boiling (though the temperature will not instantly drop) because you are lacking the energy required to sustain conversion of liquid to gas (latent heat of vaporization), even though the temperature is still high.

In your brake system, the boiling may continue for a short time as the pressure decreases due to your leakage (this lowers the required temperature to boil the liquid, so the liquid will continue to vaporize and sacrifice more of its heat energy to do so, and temperature and pressure will lower together until the pressure is no longer high enough to leak or pressure has reached ambient atmospheric pressure).

While you were riding the brakes, the expanding gases filled the void created by the leakage and maintained pressure high. Once the boiling ceased, the gases either condensed or returned to normal pressure (if there was any non-condensing gas in there) except that now you have less liquid in the system that you started with and a vacant space filled with gas at low pressure.

The next time you try to apply the brakes, the response will be almost nothing because of the huge void in the system.

Answer 4

I suspect you overheated the caliper so it expanded a little causing a leak. The smell was either plastic parts of the caliper or brake fluid burning off. Once it cooled a little your pumping the lever cause the self-adjusting feature of the brake to adjust to the loss of fluid. That's what makes me think it was fluid loss rather than the caliper itself being damaged.

I strongly suggest you take the brake to a bike shop and have them service the brake - pull it apart, check for damage and bleed/refill the fluid. It's possible that the seals inside are damaged, and if they've become brittle from being heated they'll probably work ok for a while, before they shatter and stop working altogether. So it's worth having someone pull them out, play with them, then put them back. Or replace them, if they break from being handled.

When you get the bike back do the normal brake test - squeeze the lever as hard as you can and make sure nothing gives way. It's better that it breaks now than when you're in the middle of a panic stop.

I've had a similar experience with cable disks and they didn't seem to work any less well afterwards (they were poor to start with). I stopped because I wanted to know whether I could, as I could see black fluid bubbling out of my front brake caliper. When I got to the end of the ride I took them apart and they seemed ok. Since that was on a bike tour and they seemed to work I left them on the bike for another 2000-3000km, but once I got home I binned them. Thing is, if a brake fails after you took some parts out it's all your fault, even if you burned those parts out by over-heating the caliper. Perhaps especially if you did that.