How Do Brakes Work? The Science of Stopping

I *do* like the very subtle “shimano baaaaad, SRAM gooooood’ message 🤣

Mineral oil (Shimano) is for noobs who don’t ride HARD and who don’t maintain their bikes. Dot (SRAM, in this instance) is for REAL riders, on REAL terrain, riding REAL bikes, like REAL tigers! Grrrr, tiger!

I should point out that my 3 main current bikes have SRAM (Dot), Shimano (Mineral) and Hope(Dot). I like all of them 🙂

The Enduro Mag dyno tests alluded to in the video are here, the summary makes interesting if slightly inconclusive reading.

The best MTB disc brake you can buy

“Pick a Brake Fluid and be a dick about it…”

How do brakes work, i believe the coefficient of friction, specifically kinetic friction and braking provide the answers.

As for that Enduro Mag test, what an absolute waste of time and effort, to basically just give some ratings without much thought, or realism.

Doesn’t explain the basics of how breaks work, nor how hydraulics work (bigger master cylinder etc). Just an advert for Sram justifying their design choices. 🤷🏻‍♀️

If memory isn’t failing too badly, all you’re basically doing in braking is turning kinetic energy into heat, via friction, that then provides you with some level of deceleration against a given criteria. The functioning of the brake is basically depressing the lever and that will then push more oil into the hose to function the pistons and provide the pressure to provide the required level of contact and force with the pad.

The SRAM video is just a bit too complicated and talking about the extremes more than the actual parameters for MTB brakes, so yes, all good knowing mineral oil operates in a temperature range, but not really relevant when mapped against a MTB brake operating temp range, same with rubbers, pretty sure you can use viton with both mineral and synthetics.

As for that Enduro test, honestly, it’s like those science papers you would read and wonder whether they were bored that week and just came up with a test, logged the numbers and then added stars at the end, they don’t even give a breakdown of the ratings, how do they rate each part, modulation is a rating with no mention, there is no balancing of the brakes with the same pad materials or providing the rig set up and comparison.

Some beginners don’t realise that friction between the pad and rotor is a good thing, some even apply oil to quiet them down.

Some Shimano brakes even had a built in system to spray fluid onto the pads.

Some Shimano brakes even had a built in system to spray fluid onto the pads.

🙂

No mention of how the boiling point of DOT fluid plummets as it absorbs water, but of course all SRAM brake users have a regular brake fluid flush scheduled because “Grrrr, tiger!”

1. It doesn’t “plummet”. It decreases gradually. It has to absorb a lot of water to be affected to a major degree.

2. MTB brakes are sealed systems. Unless you do something silly like jetwash your brakes, water can’t get in. If water could get in, fluid could get out. If there is no source of water for DOT fluid to absorb, there is no problem.

3. DOT fluid absorbs water and it disperses throughout the system. Mineral oil does not absorb it. If water does get into the system, it will pool at the lowest point in a mineral oil system, which is the caliper. The caliper is also the most likely part of the system to be immersed in water if you ride through a water crossing, etc. The caliper is the part of the brake system most exposed to heat under heavy braking so any water in the caliper is going to reduce the boiling point to 100 degrees C. When you have the brakes applied, the system is under high pressure and the boiling point will be increased. When you release the brakes, the system drops back to atmospheric pressure and the boiling point drops back to 100 degrees (or lower if you are at altitude). So, the fluid will boil after you release the brakes and you won’t have any brakes for the next stop.

Of course, this is highly unlikely to happen in real life. The point is that the “absorbs water” thing is not actually a problem in real life but, if water does get in, it’s actually a bigger problem for mineral oil.

4. Mineral oil is toxic too. This is not an advantage of mineral oil over DOT fluid.

Saying that, you can store mineral oil for much longer than DOT.

You can store DOT fluid indefinitely in a sealed container. Water can’t magically diffuse though the bottle.

As a bit of a numpty mechanic, when I moved to hydraulic disc brakes I went for mineral oil as I tend to spill stuff and was under the impression that DOT fluid was somewhat corrosive – is this wrong?

3. DOT fluid absorbs water and it disperses throughout the system. Mineral oil does not absorb it. If water does get into the system, it will pool at the lowest point in a mineral oil system, which is the caliper.

yeah, this was my understanding too. DOT is used BECAUSE its hydrophilic, not inspite of it. your brakes get progressivly worse and worse, rather than just throw up all of a sudden. Maybe an issue in Cars on the road, but not in reality on push bikes.
the seal material thing seems like a massive red herring to me. Have you ever heard of anyone cooking thier shimano brakes and causing a leak due to heat? rubbish.

For me the main advantage of DOT fluid is that you can go into any Halfords and pick some up.

Im not saying its “the same”, or even advisable, but i started using Citroen LMH 15 years ago, following some advice on here and have had no problems at all. I suspect its more viscous which will have a minor effect on lever feel, but ive got brakes that have had it in for years and never had an issue.

https://www.halfords.com/motoring/engine-oils-and-fluids/brake-clutch-fluid/comma-l.h.m.-plus-1l-124424.html

Im not saying its “the same”, or even advisable, but i started using Citroen LMH 15 years ago, following some advice on here and have had no problems at all.

Good to know, thanks!

The messaging is slightly mixed up…You can leave mineral oil brakes sitting for aaages and they’re fine, but if they’re left in the cold and then heated, they may leak.

In reality, this might make a lot of sense as to why Shimano callipers fail with what we call microleaks which cannot be traced. The bikes are left, sat in the cold for long periods, then suddenly get used and the seal doesn’t adapt properly, a tiny amount of fluid leaks, contaminates the pads and the brakes are damaged. It might also explain the wandering bite point. if the seal leaks fluid, it can also allow a small amount of air/water into the brake which would alter the bite point. It doesn’t take much.

In a MTB braking system, what oil you use is not going to be the weak point

Pretty much. Unless you’re racing DH, it’s not going to make any functional difference. I use both. Fluid type has never been a consideration in buying brakes.

was under the impression that DOT fluid was somewhat corrosive – is this wrong?

Kinda. It destroys paintwork and leaves the metal underneath exposed. The metal then corrodes. Brake systems are made of steel, aluminium, brass, etc. DOT fluid does not corrode those.

if you open it it’s no longer a sealed container.

And then when you screw the cap back on it reverts to being a sealed container. Honestly, the whole “absorbs water” thing is massively exaggerated. Sure, if you live in Singapore or somewhere where it’s always humid and you leave a container of DOT fluid with the top open, it will absorb moisture. But nobody does that. You put the cap back on. If you’re really worried about it, put it in a ziplock bag.

And then when you screw the cap back on it reverts to being a sealed container.

Not really.

Taking precautions against moisture absorption probably isn’t necessary unless your brakes are running at the design limit so for most of us it’s not an issue most of the time.

Also, even though your brake system is sealed, it still absorbs moisture. Have you ever found a bike that’s been siting for a while (over a year) and found the brakes are locked up? The fluid has expanded. I don’t know what could have caused it to expand if not moisture.

Moisture absorption is an easy thing to avoid (it might cost you £5 over the year) so why not avoid it?

For this to be a real thing, the system would have to be overfilled to the point where there is no space in the reservoir for the fluid to expand into. Brakes do not absorb fluid in the way you imagine, certainly not enough to pressurize the system and cause the wheels to lock.

I found a bike on someone’s garage (ok fine, it was my parent’s garage where I left a Juicy 3 equipped Inbred for a few years) where the pads were in contact with the disc and the reservoir cap was corroded due to brake fluid weeping out.

What is more likely is that the rotors corroded and stuck to the pads.

If the pads weren’t in contact with the rotor then how would they have corroded in place without the pads closing on the disc?

The pads or the discs would have had to have grown a layer of corrosion that bridged the gap and caused the two surfaces to become connected. That seems less likely than fluid expansion.

If the garage is poorly ventilated and poorly insulated, the rubber components on a bike will degrade if you leave it there for years.

After a bleed everything was fine so the rubber components seemed to be OK.

If the brake system is overfilled, it might pressurize in hot weather due to thermal expansion.

I think thermal expansion is even less likely than corrosion, tbh. Afterall, brakes are exposed to an awful lot of thermal conditions by their nature.

Brake fluid in a sealed system is not going to absorb moisture from the air sufficient to cause the brakes to lock on.

I think you’re wrong. All DOT fluid brake systems absorb moisture at a certain rate. The numbers I’ve seen are 3% after 18 months. I don’t know what the rate of absorption is over longer periods but 5% doesn’t seem unreasonable.

If the volume of fluid in the system is increased by 5% I don’t think it’s crazy to think that could bring the pads into contact with the discs.

I probably used the wrong phrasing earlier when I said locked up. What I meant was that the pads were dragging significantly on the disc rather than rigidly clamped.

The problem here is due to the bike being stored in a poor environment, not the type of brake fluid.

It wouldn’t surprise me if the system absorbed more moisture if it was being kept in a garage compared to indoors, yes.

It wouldn’t surprise me if the system absorbed more moisture if it was being kept in a garage compared to indoors, yes.

They don’t absorb water the way you think they do.

I think you’re wrong. All DOT fluid brake systems absorb moisture at a certain rate. The numbers I’ve seen are 3% after 18 months.

That’s for an open container of fluid. It’s not for a sealed system like on a mountain bike.

Do cars have a diaphragm on brake fluid caps like bikes do? I don’t recall seeing one.

No, that’s for cars.

Many cars don’t have a sealed system like MTBs. Because they are never turned upside down, they just have a vent in the cap. That’s the same as leaving fluid in an open container.

Brake systems are highly pressurized when you clamp on the brakes. The fluid on the inside is at much greater pressure than the air on the outside. If moisture could pass through the joints and seals to get into the system when there is no pressure differential, fluid would squirt out when you applied the brakes. This doesn’t happen because the system is sealed.