How could hydraulic disc brakes make sense on MTBs and gravel bikes?

Question

This How To Clean Hydraulic Brake Pistons video (Park Tool's Tech Tuesday #104) shows how dust particles can interfere with the proper functioning of hydraulic pistons.

Disc brakes have been adopted so often, one could almost suspect rim brakes will vanish. Likewise, it is becoming increasingly common to see hydraulic disc brakes thoughout line-ups of MTBs and gravel bikes.

How do hydraulic disc brakes make sense on these styles of bikes, given the tendency of pistons to "stick" when dust makes its way inside, as it inevitably will on trails?

In other words, cyclists who hear gritty noise from otherwise well-tuned rim or mechanical-disc brakes can simply squeeze the levers a couple of times to crush the sand particles and keep going, but with hydraulic pistons it seems that getting rid of sand/dirt is an intricate operation, and it may be necessary to put up with extended squealing from brake pads that became stuck near the rotor; is that accurate?

Answer 1

Maybe this operation is complicated, but the first question to ask yourself when seeing this kind of video is "how often do you need to carry out this operation?" instead of thinking to "it doesn't make sense, pistons will stick if I'm riding on gravel". Hydraulic disc brakes are so widely adopted for these riding styles because they are the most suited. The low requirements in maintenance is also one of the reasons (but when there is maintenance, it can be more complicated it's true).

Also, your argument of having to "squeeze the levers a couple of times to crush the sand particles and keep going" misses an important aspect of disc brakes: because they are located much further from the ground, they are actually less exposed to dust/mud/water than rim brakes.

To take my personal example: I have an hybrid bike with the basic Shimano hydraulic disc brakes (MT200). I don't know how much I've ridden with this bike, but certainly more than 10000 km, mostly on gravel trails and light MTB (in a dusty environment). I've changed the brake pads 4 or 5 times (I clean the head of the pistons with a simple brush then), and changed the discs once. I still didn't have to do any maintenance on the brakes circuit itself, that works still as well as when it was new. At some point I thought that I would have to bleed them (because they were still 'mushy' after having changed the pads), the actual reason was: the rotors were worn out.

With rim brakes, I would have to adjust to the brake pads several times to compensate for the wear, and it's likely that I would have to replace the cables and casing because dust infiltration would have made them less responsive. And of course, I wouldn't have had the nice "feeling" that hydraulic disc brakes give.

Answer 2

The question, as it is formulated, assumes that there is a fully rational explanation for the observed situation. In reality, acceptance and widespread of any technology depends on many factors, not all of them being rational, and even not all depending on the merits of the said technology in isolation:

1. Engineering compromises.
2. Economical considerations.
3. Fashion and cultural aspects.

Of these three factors, your arguments only slightly touch the first one. Let me go through them.

1. In real-world engineering, using any technology is always a game of compromises. Rim brakes have their own pros and cons. Same applies for hydraulic actuation vs mechanical actuation, or using rotors as braking surfaces. The issue of sticky pistons is only a single tiny maintenance aspect of that. I had used maybe all types of brakes on my bikes over the years; and it always was something about them that I did not like. All of them had to be maintained one way or another. But all of them did their job (except when they suddenly didn't, but that is another story).

2. A technology does not exist in isolation. You would need tools and materials to manufacture the product. It is critical that you would not need to invent them as well. If an objectively superior technology costs too much to have, it won't become widespread simply because the "environment" is not ready for it. Many innovations in bicycling were not realistic until the world around had developed matching manufacturing, logistics etc. capabilities which had made their application in bicycling feasible.

3. A technology won't be given a chance if it is not "cool", whatever that means. Similarly, a tech won't be used if it is banned by an institution for any reason. This aspect is impossible to truly rationalize. See e.g. the history of UCI's stance on disc brakes in professional road cycling. The waves of fashion affecting bicycle construction can easily be seen in e.g off-road bike geometry evolution. Certain things became "cool", then back to uncool, then cool again (see e.g. 29-inch wheels). We often omit the importance of such irrational factors affecting the technology, because it is often easy to rationalize them in hindsight. While in reality, when the thing was originally "hot", there was much less rational reasoning behind why it was considered good. For my own example of 29-inch wheels, one can argue that their original decline was caused by lack in supporting technology of tires, and inferior frame geometries used back in the day, i.e. reduce the fashion argument to the economical argument.

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So, does it make sense to have hydraulic disc brakes on MTB? Yes. Does it have to have disc or rim brakes on road or gravel bikes? Also yes. Does it make sense to have no brakes at all? Strangely, yes (see BMX freestyle).

Answer 3

There's a distinction the video doesn't really make clear between things one does to prevent piston stickiness issues as a matter of having good technique, and things one does to solve the problem when it happens.

Cleaning the pistons with alcohol and a rag or q-tip before resetting them goes a long way to keeping things on track. So does centering the pistons correctly from the beginning and keeping them that way. Most brakes don't really have major centering problems if you do those things. Of the times when it happens anyway, the vast majority of cases it's because you made the mistake of buying a SRAM brake despite overwhelming evidence they don't know how to make brakes that aren't prone to these issues, and are content with their hubris of calling it all "proven" in glossy ads. Of the remaining times, it's often a broken Shimano ceramic piston, which is frustrating but they're good about warrantying them. Uncooperative pistons where none of the above is the cause is not a common problem; this entire conversation is mostly about issues that result from either user error or failure to vote with your dollars.

This video also has a highly optimistic take on the odds of success of lubricating and "exercising" the pistons back into a state of symmetrical motion. That does work sometimes but the fail rate is considerable. Needing rebuilt, replaced, or accepting a compromise is also common.

Overall the possibility of this issue is not a dealbreaker for most because getting rid of cables is worth it in messy, braking-intensive applications and leads to a lower overall maintenance burden. If you decide you have an application where the calculus is different and you want to keep it all more simple and serviceable, that is fine and reasonable.

Answer 4

How do hydraulic disc brakes make sense on these styles of bikes, given the tendency of pistons to "stick" when dust makes its way inside

MTB brakes don't actually have much of this tendency. I've had my brakes covered pretty gruesomely in mud, and spraying with water was always enough to get them working again. Not great – they'll certainly go a bit shrieky – but not sticky. Anyway,

as it inevitably will on trails?

– well it won't, “inevitably”. As long as you maintain a bit of speed, centrifugal force will generally keep the dirt away from the brakes, very much unlike with rim brakes. The times I got my discs muddy was only when I got completely stuck in really bad bog holes, so the mud would drip from the top of the wheel onto the cassette and brake. Yuk. Surely you'll agree that it's best to avoid these situations entirely. But what is quite inevitable is getting dirt on the rim surfaces, when riding in any wet conditions. So that's where the biggest advantage of discs over rim brakes lies.

In dry conditions, you'll certainly get some fine dust on the disc brakes, but they can deal with that no problem.

All the dirt considerations aside, hydraulic discs are just the type of brake that works best. They have excellent modulation, plenty of power when you need it, they seldom need any adjustment, and wear the pads more evenly than mechanical brakes... what's not to like?

Sure, every kind of brake will degrade when handled badly. Regularly dunking in mud is certainly not the smartest thing to do with discs, not to mention getting any sort of fats on the surfaces. The only brakes immune to those issues are caster brakes (enclosed in the hub), however those are no good for prolonged use at all, nor are they compatible with derailleurs or usable on the front or allow back-pedalling. In short, not at all an option for MTB.

The only way I could see this changing is if somebody finally got a good electric hub motor design out that can double as an anti-locking regenerative brake. That really would be a game-changer. But unfortunately the e-bike market is all on the mid-engine ship, with no consideration for regenerative braking.