7

As I've mentioned elsewhere I recently had quite a bad crash - broken collarbone, shoulderblade, and a rib. The circumstances are rather unusual: both brakes failed totally.

Crash details for those that want to know (TL;DR: I'd been using both heavily, let them off, they didn't come back on):

I was descending a mountain pass, as I often do, taking it rather slowly as the light was fading and the road was damp with some loose material. My habit when picking my way down that sort of road is to use the back brake lightly and continuously to control speed, and use the front brake for significant slowing before bends. Coming out of a bend I released both brakes as the next 100--200 m was straight and not too steep. When I was fast enough, I squeezed the brakes but carried on accelerating. I spent a few seconds trying to slow down, braking hard while rolling over the strip of grass up the middle of the road in the hopes of losing some speed, but knew I couldn't make the next bend, let alone the next few km of unbroken descent, so I headed for the grass off to one side to lose speed/hit something softer. I did manage to drop from around 35 km/h to more like 25 km/h but hit a rock with my front wheel and another with my body (and the aerobars with my helmet, hard enough to bend the aluminium armrest). Just before I crashed the brake started squealing, and the ceramic pads are normally silent.

The ride on Strava shows that only 20km earlier I'd had enough of a descent to put the brakes through their paces; at the top of the big descent I tightened the barrel adjusters and fixed pads a little as there was more slack than I would have liked -- and I tested them. A few days post crash the brakes stop the bike while it's being pushed, but the front wheel isn't rideable to test.

My bike has done about 16000 km and I recently had it serviced, including new brake pads. They're ceramic (I usually use metal), and normally have plenty of stopping power though I've had to tweak them a few times in the 1000 km since they were fitted -- new cables settling in? The brakes are Promax Render-R cable discs (BB5-style pads) and are normally good, barring one case of contamination.

I haven't taken the back wheel off but the front came off to get it home and I took the chance to inspect the pads:

enter image description here

(L) moving pad, (R) fixed pad.

The wear on the moving pad is strange. The pale patches are shiny metal backing and the actual pad is worn into a wedge shape. Update: I've measured the pad diameter, it comes in about 0.1 mm bigger than an old set of metal pads I had lying around, and the rotors are perfectly flat.

Further update: There are possible signs of melted material at the back, in the rotor air holes and the edges of the pad, and the pads turn out to be 70% ceramic (and 30% organic I assume). The front rotor has a groove worn into it to match the pads shown above, and has worn more in the month/~1000 km since these pads were fitted than in the previous 6 months since it was new. The rear rotor has also taken a lot of wear recently.

I'm as sure as I can be that I wasn't skidding the back - I was steering normally, not fishtailing. I didn't notice a different feeling in the levers, but I had bigger problems on my mind.

What could have happened? Could they really jam in the off position, perhaps due to heat? With one specific set of pads? I'm happy to replace the brake mechanisms when I can ride again, but want to understand the cause.

  • 1
    What was the brake lever pressure when the brakes were not working? Were they floppy and loose, or moving like normal, or hard to move ? – Criggie Jul 6 at 22:43
  • 2
    And get better too - don't rush the recovery part. Rebreaking things is scary-easy at first. – Criggie Jul 7 at 7:25
  • 1
    @Criggie it's only insured against theft, and anyway the costs are a wheel build +spokes, probably brakes, and possibly forks, so not too much. It's all steel, so tough and not prone to hidden cracks. Main bars are fine but I'll need new aero bars once I get up to 300km+ days again. Medical bills are covered by taxes (including Ireland, where it happened, by EU reciprocal arrangements). – Chris H Jul 7 at 7:48
  • 1
    Never done any mountain biking (and don't use disks) but my habit on long road descents has always been, rather than continuous light braking, to alternate bursts of moderately hard braking between front and rear, feeling for any loss of braking power. If I ever felt a loss of braking I'd immediately try to come to a complete stop. – Daniel R Hicks Jul 7 at 19:26
  • 1
    I am thinking it’s most likely that the rear was still working and may have locked up your rear wheel, but on that sort of grade that did nothing really to stop you much. A failure just of the front brake seems much more likely than simultaneous failure of both. – Andrew Jul 8 at 0:17
3

This is purely a hypothesis

Firstly, i'm going to work under the assumption that the pads are either organic, or some sort of organic/ceramic hybrid compound. I can't actually find pure ceramic BB5 pads for sale in the UK (which is where the OP lives). And anything I can find with a red backing plate is organic.

Making the above assumption, I think its likely the rear brake was either cooked, or completely worn down by dragging the brake. Organic pads are great for stopping, but can wear out very quickly. This, combined with the fact that a rear brake will have very little effect on a steep(>10%) downhill would explain the sensations of no braking from the rear.

At the front, the wear pattern on the moving pad suggests some serious issue. I think its possible the moving pad was not securely held by the retaining clip and was in fact moving within the caliper. The wear pattern shows that only half the pad was in contact with the rotor, and significant contact has been made with an area with no pad material. If this was the case, then its also possible there was not enough force available to push the rotor into the non-moving pad. Hence a complete failure of the front brake.

  • 1
    I should note that whilst its far from ideal practice, the OP should not feel bad about dragging the rear brake. Steep narrow descents with all sorts of holes/debris are very common here in the UK, and dragging the rear brake is the only realistic way to manage speed in many circumstances. – Andy P Jul 8 at 10:32
  • I've had a front pad become loose but the brakes still worked pretty well, if anything I would have though the OP would have felt the vibrations from a loose pad, mine felt like ABS when it happened, that's how I knew something wasn't right with the pads. – Dan K Jul 8 at 10:34
  • @DanK Perhaps depends on the design of the specific caliper? Or maybe the retaining clip was bent? The only thing we can be sure of, is somehow the pad was misaligned with the rotor by 5-10mm, otherwise how could the backing plate be worn like that? – Andy P Jul 8 at 11:03
  • That's certainly interesting if there's no pure ceramic pad, because I was treating them as I would (essentially heat-proof) metal pads. The front clip isn't mangled, but I've no way of being sure it was properly in place, and I guess the performance could be OK until enough pad had worn away that the backing was what hit the rotor. – Chris H Jul 8 at 11:30
  • 1
    Curiosity got the better of me (I'm bored recovering from surgery on the injury) and I had a better look at the pads. Cleaning them up they turn out to be Gigapower GB-837, which are 70% ceramic. I assume the rest to be resin. The rear pads are worn fairly evenly. I could believe I've found some melted material on the rear rotor and around the fixed pad, but there's also a bit of mud around after the crash. Both rotors show a huge amount of wear (and on the front, uneven wear to match the wear on the pads) – Chris H Jul 14 at 12:37
3

I believe this could have happened due to brakes overheating.

I am fairly certain that by continuously dragging the rear brakes you could have overheated them. You should investigate the rear brake pads for extensive wear. If you find this prediction correct, it's likely the front brakes suffered a similar fate.

It is more difficult to say what exactly happened to the front brakes. The moving front pad being worn into a wedge shape right down to the metal is suspicious.

It could be that in-between the braking, the moving brake pad did not stop touching the disc. Leading edge of the pad could have been pulled in by the friction with the disc. Maybe the brake callipers went out of alignment and started rubbing the disc.

Another possibility might be disc warp due to heat. Then a warped disc would produce even more heat until total failure.

To sum up: continuously rubbing brakes -> overheat -> brake wear & failure

  • The materials in question are steel and ceramic. Overheating shouldn't affect wear unlike with organic pads, and there's no fluid to boil. The sudden failure didn't feel like wear either. This is by no means the first time I've had to descend very slowly for reasons out of my control. I can't inspect the back until I've recovered from surgery on the injury, because I can't lift it onto my work stand. – Chris H Jul 7 at 20:25
  • ... The bike in question is a touring bike - exactly the sort of thing on which slow, steady descents are sometimes necessary. In this case I was lightly loaded – Chris H Jul 7 at 20:26
  • 2
    @ChrisH A drop of 60m is very roughly the amount of energy needed to get a glass of water boiling (very roughly = could easily be off by a factor of two, but it's the order of magnitude). Of course, your brakes had quite a bit of time to dissipate that energy. I cannot estimate how much energy they did dissipate, but the amount of energy you dumped into the brakes is probably higher than you think. Nevertheless, I'd still question the conclusion of this answer: The two brakes were used to dump quite different amounts of energy, so simultaneous overheating is unlikely. – cmaster Jul 7 at 20:59
  • 2
    Also I'm late for a dose of painkillers, which combined with autocorrect acting up may have led to a bit of unintended impatience in my recent comments – Chris H Jul 7 at 21:07
  • 1
    Could there be any problems with brake disc and brake pad material incompatibility? If things are as you say, then the sudden brake failure does seem strange. – chameleon-hider Jul 8 at 5:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.