Do mechanical disc brakes suffer from heat-induced fade?

Question

The other day I read a question about heat induced brake fade in hydraulic disc brakes caused by too much braking on long, steep descents, which is something I have also experienced when cycling in the alps. (A quite unpleasant experience, I might add.) This made me wonder if mechanical disc brakes would suffer from the same problem. There is no fluid to start boiling, so at first thought they should be heat-fade-proof.

If so, I think an optimized disc brake system would have hydraulic activation of a short mechanical lever or wire connected to the caliper itself. This way the system would optimize modulation (by minimizing elasticity) while still being safe to brake for extended periods. I realize that even with mechanical brakes, you can't brake indefinitely, as the pads, calipers and discs would overheat at some point, but if the margin is large enough to cover most practical uses, the idea might be worth exploring.

So I'd like to hear from people with mechanical disc brakes if they have experienced heat induced fade?

Answer 1

From my experience, the answer is no. I have been cycling across the Great Glen and have no problem with mechanical disc brake.

My bike was moderately loaded with camping equipment. So the load (including me 70 kg) was around 110 kg or 242 lbs. There were continuous downhill sections but the brake works fine, thankfully.

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From a scientific stand-point, there are three types of fade

1. Green fade - caused by resin boiling off of new pads
2. Fluid fade - brake fluid boiling and system losing pressure
3. Pad fade - pad overheats and loses friction

Ignoring 1, 3rd cause starts to occur at much higher temperature and require time than 2nd cause (which occurs at around 120 oC for moderately contaminated mineral oil*). That is why you need to check your mineral oil before setting out on a trip.

Pad fade is the main factor in Mechanical Disc brake fade. Unless you are not braking heavily and continuously for more than 10-20 minutes, you are fine. In number, if you are going down 20% gradient hill, braking continuously to keep your constant speed at 18 kmph (11 mph), the braking power is around 1078 Watts for 110 kg (bike+load+rider). That means, referring to **, you can safely brake for hours, if not hundred of hours for the iceTech.

Refer

*http://www.epicbleedsolutions.com/blog/dot-brake-fluid-vs-mineral-oil/images/shell-dot-fluid-boiling-points.png

**http://cycle.shimano-eu.com/publish/content/global_cycle/en/nl/index/news_and_info/Velotech_Disc_Brake_Test.html

Answer 2

According to my experience, some combinations of disc-pad materials do, and some don't. (Or maybe rotor/pad contamination does allow it to occur)

In some cases, the heat can cause a "glaze-on" coating to develop in the active surface of the pad, and this glazing has lower friction coefficient, low enough to be unable to lock up the front wheel while pushing the bike.

I once saw this happen to a another person's bike on a on a long, steep MTB descent (XC leisure ride). The rider asked for help about the brakes because more force than usual was required at the lever, stating that the brakes where "normal" before the descent. We cautiously checked the brakes, which by the time where not hot enough to cause skin burns. and where able to somewhat restore performance, removing the glaze with a bit of sandpaper (I carry sand paper as part of my patch kit). No thing was done to the disks at this time.

Other riders of similar skill level who descended at similar pace did not experience the issue, also, those other brakes felt cooler to the touch, although many of them had higher spec brakes. My guess is that the overheating/glazing happened either due to the pad state/material or due to contamination.

I've also experienced quick glazing on rim brakes (single pivot caliper brakes) on an old road bike. The dried up brake pads had little stopping power but felt OK while riding on flat terrain. I had scrubbed the pads and rims surfaces recently, something I regularly do on V-Brakes, but this time, during a steep descent I felt diminishing braking power until I could not use "burst brake" technique. Due to continuous braking, rims got hot enough to pop the tubes near the valve (I think new tubes could have withstand the heat). Luckily, I did not lose control. After the fact inspection showed the pads heavily glazed and rims smeared with black residue. This was somewhat an extreme case where many things combined to that outcome, but definitively, fresh, good condition pads should not have caused the issue to begin with. Old rubber pads seem to be more prone to develop a similar "glazing" than new ones. (Same bike performed OK once pads, tubes and tires where replaced)

Bottom line: Keep your brakes (pads and rotors) clean, as oil or other contaminants can also enable glazing to occur, even though in less demanding working conditions they may seem OK. Perform thorough inspection regularly and do not let pads get too old (I think this applies for both disc and rim brake pads).

Answer 3

Yes - I recently had a brake-fade problem with cable pull hydraulic-assisted hybrid brakes.

The bike and I weigh in at 120kg and I was braking from 60 km/h on a steep slope, but I experienced nearly total loss of braking.

It was very scary, the calipers were making contact to the disc, but I was not slowing down, even using all my force in gripping the levers it did not help. I guess the brakes had boiled, the discs had changed color a bit and steamed when I added water from my water bottle !!

I imagine this is Brake fade at its most dangerous. I am now trying to go back to normal cable discs brakes.