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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.

Chris H
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    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 06 '19 at 22:43
  • @Criggie yes, though in the end the detailed account didn't read as scarily as it might have done and it was a close decision. I've seen the effect on some people of telling the story. As for the levers, I recall squeezing really hard; I don't recall anything like floppiness followed by bottoming out, or compete lack of movement but I'm not sure how much to trust that recollection. I'm more confident about the lack of floppiness. The cables are intact – Chris H Jul 07 '19 at 06:59
  • I don't know enough about disk brakes to say much. What was the weather? Is there any chance of ice on the rotors? Probably not given you're in summer. I'd be getting your bike shop involved - don't just fix it yourself and hope. Also, you may choose to contact your insurer, and see if its worth making a claim. I'd check the forks, front wheel and bars very closely for crash damage. And the rest of the bike too. Don't leave insurance too late - they may have a time window for making a claim, which might be unrelated to any medical insurance claim. ... – Criggie Jul 07 '19 at 07:24
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    And get better too - don't rush the recovery part. Rebreaking things is scary-easy at first. – Criggie Jul 07 '19 at 07:25
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    @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 07 '19 at 07:48
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    ... Good point on recovery, but commuting will have to come in after a few weeks because all other options break childcare or are simply unaffordable (often both) – Chris H Jul 07 '19 at 07:52
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    My habit when picking my way down that sort of road is to use the back brake lightly and continuously to control speed That's absolutely the worst way to brake - it WILL overheat your brakes. The amount of energy that dissipates into brakes is literally enough to boil a significant part of any water bottle on any decently-long descent. Or the hydraulic fluid in hydraulic brakes. Or for rim brakes it can melt carbon rims or overheat and blow tubes and tires. – Andrew Henle Jul 07 '19 at 19:20
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    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 07 '19 at 19:26
  • @DanielRHicks this was on road, and that's what I've done in the past, but the fading light (I was an hour behind my plan) and surface conditions meant the bursts of braking almost joined up. But that's partly why I released them, only to find they did nothing at all when reapplied – Chris H Jul 07 '19 at 20:20
  • @Andrew I could have pulsed the brakes at 1-2Hz without going too fast for the surface and visibility. That wouldn't help much. What exact parts do you suggest can overheat in a cable disc system with ceramic pads? With clear lines of sight and road width availabIe I tend to break just before each bend, but the road in question was this one - very narrow, grass up the middle, potholes, loose gravel ,and damp. I have also descended steep twisty stuff like this before, either in the dark or stuck behind a car. Not ideal braking but it is normal – Chris H Jul 07 '19 at 20:29
  • ... the gradient when I crashed was about -16% – Chris H Jul 07 '19 at 20:44
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    One possible explanation for apparent simultaneous failure was that one brake - quite likely the rear - had already slowly degraded significantly without you noticing, then the other failed suddenly. – Argenti Apparatus Jul 07 '19 at 22:01
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    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 08 '19 at 00:17
  • @Andrew I can't rule it out completely, but I know what skidding the rear feels like (more often on the MTB, but I have done it on road too) and the steering didn't feel like it – Chris H Jul 08 '19 at 05:20
  • @ChrisH Sorry to hear about the crash - hope you heal up fast. Are you SURE the pads are ceramic? Can you post a link? I can't find any ceramic BB5 pads for sale in the UK. If they were in fact organic I may have an answer that makes sense – Andy P Jul 08 '19 at 08:39
  • @AndyP that's according to the bike shop that fitted them. I checked to make sure they hadn't fitted organics because I was concerned about wear given the distances I do in sometimes wet conditions. I'm not sure how I'd check myself. – Chris H Jul 08 '19 at 09:36
  • I've never done brake intensive work with steel cables (all my mountain rigs have been hydraulic). Don't steel cables expand/extend when they get hot? Seems like depending on how tightly adjusted they were to start, they could easily heat to past a workable length. – Deleted User Jul 08 '19 at 16:21
  • @DeletedUser you'd need to heat a significant length of cable for the expansion to be more than a tiny fraction of a millimetre. Not only that, you'd need to heat the inner and not the outer. Heating of the caliper/housing would be more likely. But even so, why would that be an issue this time, and not with the metal pads I've used before - I've had long descents such behind cars, when I've probably put more heat into the brakes faster than this time and for longer – Chris H Jul 08 '19 at 16:29
  • It is not really recommended to fiddle much with barrel adjusters as they can lower the active length over the calipers can move. Once a bit it does not matter, but if you did it several times, it could build up. One should adjust the screws on the sides instead when posible. – Vladimir F Героям слава Jul 15 '19 at 14:30
  • @VladimirF on this model I've found the fixed pad needs to be adjusted once for every twice the barrel adjuster is tightened. But there is no screw on the moving pad except the one that grips the cable, making the barrel adjuster absolutely necessary – Chris H Jul 15 '19 at 15:33
  • I would say that low-end disc brakes that take these piece-of-@#$, tiny "ping pong paddle" type pads are not suitable for descending from mountains. – Kaz Jul 18 '19 at 01:15
  • @Kaz they were great in the Alps last year, and they've done me well on a lot of comparable descents, but only ever with sintered pads. – Chris H Jul 18 '19 at 06:01
  • @ChrisH How's the healing? Its a year later now - are you fully recovered or still suffering niggles ? What did you end up doing with the brakes? – Criggie Jun 28 '20 at 22:19
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    @Criggie back to normal, thanks, apart from having to be careful with backpacks over the repaired collarbone. Not quite up to full distance yet, but that's more the restrictions we've been under. I replaced them with BB5s, mechanisms and rotors, but kept the (almost new) cables and levers. And I'm a bit paranoid about checking, cleaning and adjusting them. I'm still going very slow down really steep stuff but can enjoy fast descents up to about 10% in good conditions. – Chris H Jun 29 '20 at 06:17

4 Answers4

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Since recovering from my crash, I've put a bit more thought into this. I replaced the calipers, rotors and pads, with Avid BB5s - so very similar to the Promax ones that were essentially copies of the same design. The Avid ones make it slightly easier to tweak the fixed pad, and this turns out to be a good thing.

Fixed pad wear makes other issues worse: One thing I've since noticed with BB5-style brakes is that they're quite sensitive to the fixed pad position, and therefore to fixed pad and rotor wear. This makes other issues worse, and unlike getting them filthy, gets worse the more you brake. Riding in mucky conditions it's important to check and set the fixed pad clearance fairly close periodically. This certainly wasn't the only cause of my crash, as seen from the melted material in the rear rotor, but I think it goes some way towards explaining the uneven moving pad and rotor wear at the front. On brake with only one moving pad, the rotor is bent to contact the fixed pad. If the fixed pad and/or rotor (in my case, certainly both) are worn excessively, the necessary bend increases, meaning two things: more of your braking effort goes into bending steel; the contact between the moving pad and the rotor is concentrated nearer the axle, producing the uneven wear in the photo above. With slow wear this should be a small effect as the pad will take on a small taper, but if things wear too fast (or you somehow back the fixed pad off instead of driving it further in) this either won't happen fast enough, or it will wear down to the backing.

On the Promax brakes, adjusting the fixed pad with the wheel in was tricky - there was a knurled plastic part to turn by hand but the adjustment was too stiff (so I was used to thinking the adjustment had bottomed out when it hadn't), meaning using a less than ideal screw in an awkward place. The BB5s adjust in the same way but nicer. I'd checked and tightened earlier in the day, in daylight, but everything was wearing so fast (though I didn't know it) that I gave the fixed pads another tweak not long before the problematic descent, this time in the dark. At the front it still wasn't enough.

Conclusions:

  • I'm personally not going near "ceramic" pads again: this bad experience; they seem to wear rotors very fast; and the description is fundamentally not to be trusted if they can melt. For some people they might work.
  • Check the fixed pads frequently. If braking seems soft, stop and tighten, perhaps also cleaning the rotor (pre-injection isopropanol swabs are good for this).
  • If you run out of travel, fit new pads immediately. Wear is very dependent on riding (weather/surface) conditions as well as road surface and load, so you can run out fairly suddenly. Range of adjustment determines when the fixed pad wears out, so it can look like there's plenty of material when the pads are no good (much more than for the moving pad). I always carry a spare set (since an earlier issue which in hindsight offered hints about pad wear) and have changed them at the roadside, mostly recently coming back from a 2-day 200 km tour with rain, mud, steep hills, and gravel (and a record 4 punctures in 2 days).
Chris H
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  • Also my residual paranoia extends to riding companions. – Chris H Oct 22 '20 at 11:22
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    You check if your riding companions wear out and bring a spare set just in case? /duck – gschenk Oct 22 '20 at 16:23
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    @gschenk :) having only ridden twice with company (and one of those only a few km) in seven months, I'd be very glad of riding companions, spare or otherwise. – Chris H Oct 22 '20 at 16:25
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    Why did you stick with the BB5 design after the bad experience? Wouldn't most of those problems be less serious with brakes that move both pads, such as TRP Spyres? – gschenk Oct 22 '20 at 16:26
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    @gschenk I blamed the bad experience on the pads and the execution of the adjustment (by Promax), and to some extent still do. With what I know now, maybe I'd go with Spyres, but I've only learnt that by using the BB5s post crash and being alert to even minor issues. I had a stock of sintered pads, including 2 sets in my saddlebag when I crashed (10 minutes fitting those would have solved things if only I'd known) and no compelling reason to move to a different design. I nearly stuck with the Promax brakes, but would have wanted to strip them down and try to make them adjust more easily – Chris H Oct 22 '20 at 16:31
  • I think all the BBx brake series (certainly BB7) aren't self-centering...you need to move them in by tightening the positioning bolt. if you don't do that then the pads can be too far from the rotor (pad wear) and so not engage fully even when you yank them to their limits. – stevel Jan 09 '22 at 20:02
  • @stevel the positioning bolts don't need to be touched, but the fixed pad has its own adjustment bolt, that's discussed in my answer (last paragraph before "conclusions) – Chris H Jan 10 '22 at 09:12
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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.

Andy P
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    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 08 '19 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 08 '19 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 08 '19 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 08 '19 at 11:30
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    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 '19 at 12:37
  • I think I've figured out the uneven wear - caused by excessive wear of the fixed pad and rotor, meaning the rotor had to bend too much to make good contact with the moving pad (more details in the answer I've added) – Chris H Oct 22 '20 at 11:24
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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

chameleon-hider
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  • 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 07 '19 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 07 '19 at 20:26
  • I'd also only dropped about 60m from the top of the pass, so not a lot really, even though the potential energy is the same as in a single dead stop from >100km/h – Chris H Jul 07 '19 at 20:41
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    @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 - reinstate monica Jul 07 '19 at 20:59
  • @cmaster, yes, I've just run the numbers. It's quite a lot, but it's not a ridiculous amount. Bearing in mind this was a 16% descent, no pulsing technique would allow much time for cooling (though of course airflow would be greater at higher peak speeds). As I said above, this might not be ideal braking, but it is fairly normal, and a normal amount of heat. That's why cable discs have become popular on bikes that are likely to descend heavily laden. – Chris H Jul 07 '19 at 21:04
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    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 07 '19 at 21:07
  • @ChrisH Sorry for the need of painkillers, you have my full sympathies for your situation. Regarding braking technique, a 16% descent is nothing where you can hope to loose meaningful amounts of energy to air resistance if you are forced to slow down for turns every so often. At speeds as low as 35km/h gravity already provides you with about two horse powers driving you forward. Pretty much all this energy will go into your brakes. Thus, the only way to get your brake temperature down seems to be to stop and wait it out. – cmaster - reinstate monica Jul 07 '19 at 21:29
  • @cmaster if they're too hot in the first place, and the definition of "too hot" should be pretty generous with this setup. The total descent was due to be over 400m over a few km so that would be a lot of stopping and waiting. But the 35km/h figure was after the failure. I was aiming for much slower, no more than about 20km/h peak, and I'd got down to less than 10km/h in places. Going that slowly gives some time for heat to escape. But again, with other pads I've done comparable descents at a wide range of speeds and without issues – Chris H Jul 08 '19 at 05:28
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    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 08 '19 at 05:55
  • Even with extreme temps I would expect brake fade but not complete failure. – Dan K Jul 08 '19 at 07:54
  • The rotors are steel, specified for organic and metal pads. I hadn't looked into ceramics until the bike shop fitted them, and they'd worked for around 1000km including some hard braking on a more fun descent about an hour earlier – Chris H Jul 08 '19 at 09:38
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Late reply but I'd like to add another theory to this problem.

Mechanical brake (as I understand that this issue is about) needs constant adjustment. The fixed pad needs to be adjusted very close to the brake disk in order to give the moving pad support for applying pressure to the brake disc. Example of adjusting the fixed brake pad of a mechanical Promax brake can be found on YouTube, here is an example:

My guess is that the fixed pad was set too far away from the brake disc, and then the moving brake pad was worn out prematurely?

MagnusK
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    Your point is good (+1) but is at least partly addressed in my self-answer. I'd actually tightened up the fixed pad earlier that day but probably not enough. On Promax (and to a lesser extent the BB5s they copy) used in rough conditions the adjustment can get very stiff, bad enough in fact to damage the hex socket in the aluminium pad support. This can make it feel like you've bottomed out when you haven't. The ceramic pads also seemed to need adjusting more often than sintered metal because they wear as fast themselves, but wear away more rotor – Chris H Jan 05 '22 at 09:17
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    Ah, I tried to read all comments and answers but it seems I managed to skip that part... Well, I'll leave the reply here anyway as some sort of TLDR version. I hope you're fully healed now, anyways. – MagnusK Jan 05 '22 at 09:35
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    Thanks, yes, I'm back up to strength. I just have to be careful putting on backpacks over the plated bone and my backstroke is rather asymmetric (but who really needs to swim backstroke in their 40s?) – Chris H Jan 05 '22 at 09:38