- This topic has 13 replies, 10 voices, and was last updated 8 years ago by molgrips.
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Brake fluid boiling
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molgripsFree Member
I question whether or not this is possible at all on a mountain bike, to start with. If the caliper is hot enough for even wet dot4 to boil, 150C or so, I’d imagine you’d see your hose melting and failing.
But anyway – internet wisdom says that your lever/pedal will go spongy and go to the bar/floor – but from a physics point of view that doesn’t make sense.
For the lever to come to the bars, the total volume in the system would have to decrease. If the fluid boiled,the volume would INCREASE, quite a lot. So you’d feel the lever pusing back against your fingers in a spongy way, and further braking would feel spongy. But if you were able to squeeze hard enough the extra pressure in the system would force the vapourised fluid to tend to condense again. In other words, if you continue to squeeze hard enough you’d end up with the same pressure at the pad as if you were squeezing that hard with cold fluid.
The petrol head world seems to be full of inaccurate ‘folk’ wisdom – is this another example?
bigjimFull MemberWell, I don’t know where to begin. Have you been watching too many Pluto is fake videos?
scruff9252Full MemberUnless the heat caused the chamber to increase in volume; ie caliper expanding due to heat or the hose ballooning…
pdwFree MemberIt is possible, I speak from experience.
You’re right, that in theory you’d feel the lever pushing back, although as the gas is compressible and the piston small, you won’t notice it that much. The brake feels fine until you release the lever, at which point the gas in the caliper can expand and pushes fluid up into the reservoir. The next time you pull the lever and close the system you’re pressing against a mixture of liquid and gas. The gas compresses and the lever will pull to the bar with minimal force.
That’s why it’s so nasty: the brakes feel fine on one application, and then completely fail the next time.
As for the temperatures, I think you’ll find the brake components will handle a lot more heat than that. Even a modest descent can get your discs and calipers hot enough to instantly boil water.
tomdFree MemberI question whether or not this is possible at all on a mountain bike, to start with. If the caliper is hot enough for even wet dot4 to boil, 150C or so, I’d imagine you’d see your hose melting and failing.
In reality the whole caliper will never reach that high a temperature. All it takes is for the fluid at the piston, which is closest to the back of the bad to boil at that the surface.
But if you were able to squeeze hard enough the extra pressure in the system would force the vapourised fluid to tend to condense again. In other words, if you continue to squeeze hard enough you’d end up with the same pressure at the pad as if you were squeezing that hard with cold fluid.
That may happen but also as soon as you did you would get more heat input and it would boil again, also if you reduced the applied pressure slightly it would boil. The brakes wouldn’t work as far as the user is concerned.
Three_FishFree MemberThe petrol head world seems to be full of inaccurate ‘folk’ wisdom – is this another example?
Your post.
globaltiFree MemberYou’ve missed the point completely.
Brake fluid doesn’t boil; what boils is the water that’s dissolved in the brake fluid if it’s old. Brake fluid is hygroscopic, meaning that it absorbs atmospheric moisture readily. That’s why you should never use brake fluid from a container that’s been opened for a while.
The water boils and creates bubbles of steam, which compress, absorbing the lever effort.
molgripsFree MemberWell, I don’t know where to begin.
Well you could start by thinking about it, instead of resorting to piss taking straight away, but I know that’s not cool these days 🙂
It is possible, I speak from experience.
I know that the lever can suddenly go to the bars, I have had that happen, I was wondering if fluid boiling is really the cause.
The gas compresses and the lever will pull to the bar with minimal force.
Provided enough fluid has boiled to create more volume than the master cylinder can move… But yes that makes sense. The lever would fail to work after you’ve released it and re-braked. Although reports are not very well worded – I was questioning the idea that the lever would come to the bars as you still have the brake applied.
Brake fluid doesn’t boil; what boils is the water that’s dissolved in the brake fluid if it’s old.
Not sure that’s correct…? Wiki describes glycol based brake fluid as maintaining single phase characteristics when wet.
RorschachFree MemberIt’s aliens….it’s always alien….or immigrants.
Obvious really.molgripsFree MemberYour post.
The post is a question, not a statement of wisdom, so no 🙂
MarkBrewerFree MemberI’d imagine you’d see your hose melting and failing
I’m not sure what standard hoses are rated at temperature wise but if you’ve got braided hoses they’ve got a ptfe inner so the fluid would need to be up around 250c (which is beyond the wet boiling point of nearly all racing brake fluids anyway) before you’d need to worry about them melting.
Its simple anyway 😉 Braking systems rely on fluids, fluids are incompressable. Brake fluid absorbs water and when water boils it turns into a gas. Gases can be compressed hence the pedal/lever having more travel.
pdwFree MemberProvided enough fluid has boiled to create more volume than the master cylinder can move… But yes that makes sense. The lever would fail to work after you’ve released it and re-braked. Although reports are not very well worded – I was questioning the idea that the lever would come to the bars as you still have the brake applied.
My experience of it was exactly as I describe – fine one application, gone the next. I think the volume of gas will easily exceed the master cylinder capacity, as the gas has a much larger volume than the corresponding liquid, you only need a tiny bit of it boil.
I think that for as long as you have the lever pulled, it won’t pull to the bars. It’s only when you release the lever and fluid is pushed into the reservoir that you have a problem.
The time it happened to me I was on the very last bit of ride which was a road descent of about 20 hairpins. Half way through, the front brake started to eat the spring clip that holds the pads apart and making a nasty clicking noise. Not knowing what the problem was, I decided that I’d just mince down the rest of the hill to my car on the back brake and sort it out later. That worked fine, the road eventually levelled out, I rolled up to my car pulled the back brake and it went straight to the bars. Obviously I’m now older and wiser on a number of points 🙂
molgripsFree MemberNot whilst you are still braking, only after you release and reapply as pdw says.
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