disc brakes

[Profpointy]

You only need localised heat enough to form bubbles inside the caliper to bugger up brakes to near zero. You don't have to boil the whole lot. Bear in mind motorcycle brake discs can get far too hot to touch - above boiling water easiliy from a a couple of quite modest stops and I dare say the weight of caliper is in proportion with the extra weight of bike, so not wholly an invalid comparison. I dare say it's not a huge issue on motorbike nor pedal bike if used reasonably, but I'd not dismiss it quite so glibly

Pale-riders got me curious now. Got to do some sums.
Lets say heat capacity of water is 4J per K per cc. Let's also say you're riding down an alpine descent at 30C in the Summer, so 70 C to go before water boils. Proper brake fluid will boil rather higher temp, but will probably also need less heat per degree temp rise.... Let's say there's 5cc of fluid within the caliper that heats up together, so to boil a caliper's worth of fluid requires 70 x 4 x 5 Joules = 1400 Joules. Add the capacity of the caliper itself - aluminium 1 joules per gramme per K - maybe 100g for a caliper, so that's another 700 Joules needed. The iron disc itself .5 J per g per K - say another 100g item, so that's 350 more .

So total energy required is are 2500J to get a modest volume of fluid to boiling temperature.

Energy to be dissipated per meter descended = mgh so for a 120 kg bike & rider & luggage (big lad touring say) that's 1200 Joules to be used for each metre descended.

Now there are frictional losses in the above - so maybe 600J only to be lost via the brakes by braking - for each metre of descent.

And the brakes themselves are designed to lose heat as fast as possible, so if we brake gently we'll be fine.

Still - the heat in and too much heat figures are not that wide apart - so I'd say if you braked like a gorilla down a step hill in hot weather, boiling the fluid might not be out of the question.

That was fun wasnt' it?

PS there could easily be a howler in the above calc somewhere.

 

[Pale Rider]

Still - the heat in and too much heat figures are not that wide apart - so I'd say if you braked like a gorilla down a step hill in hot weather, boiling the fluid might not be out of the question.

PS there could easily be a howler in the above calc somewhere.

If there is a howler, I couldn't spot it, but I think your conclusion is fair.

I wonder if cause and effect are not being confused.

The effect is the brakes stop working at the bottom of a significant descent, but what is the cause of that?

I've not seen any evidence to say it is boiling fluid.

Can pads lose their friction when they get very hot?

Ditto disc rotors?

I've had brake fade on a car, I had to push the pedal harder at the bottom of the hill than at the top.

I don't believe the fade was caused by overheating fluid, but I don't know what did cause it.

 

[Profpointy]

If there is a howler, I couldn't spot it, but I think your conclusion is fair.

I wonder if cause and effect are not being confused.

The effect is the brakes stop working at the bottom of a significant descent, but what is the cause of that?

I've not seen any evidence to say it is boiling fluid.

Can pads lose their friction when they get very hot?

Ditto disc rotors?

I've had brake fade on a car, I had to push the pedal harder at the bottom of the hill than at the top.

I don't believe the fade was caused by overheating fluid, but I don't know what did cause it.

I think I had boiling fluid brake failure on my car once. Symptoms were pedal-to-floor nothing there. I was being a bit of a nob playing about on empty snowy roads. Brakes recovered once I stopped being a nob. Never had the issue with anything remotely like normal driving. Fade is (I think) slightly different in that it's brakes gradually working worse and worse as the discs and pads get hotter than they are supposed to get. I gather "race" brakes don't work proper until they get very hot on the other hand.

I'm not naysaying fluid brakes for bikes though - I was just playing with the numbers to see if was nonsense or possible, or plausible anyway. I don't think I'd bother with fluid for a road bike though, even if the debate has almost put discs onto my fantasy bespoke audax spec now.

 

[mr messy]

Old brake fluid will boil quite easily. You have to bring the temps down then will work again. Fade iirc is due to hot gases building up between pad and disc

 

[Pale Rider]

I think I had boiling fluid brake failure on my car once. Symptoms were pedal-to-floor nothing there. I was being a bit of a nob playing about on empty snowy roads. Brakes recovered once I stopped being a nob. Never had the issue with anything remotely like normal driving. Fade is (I think) slightly different in that it's brakes gradually working worse and worse as the discs and pads get hotter than they are supposed to get. I gather "race" brakes don't work proper until they get very hot on the other hand.

I'm not naysaying fluid brakes for bikes though - I was just playing with the numbers to see if was nonsense or possible, or plausible anyway. I don't think I'd bother with fluid for a road bike though, even if the debate has almost put discs onto my fantasy bespoke audax spec now.

Hydraulic discs work a treat - I have them on three bikes.

Good performance in all conditions I use them in, and next to no maintenance.

Cable discs can need a bit of fettling, not least because in most systems only one pad moves, so the position of the static pad in particular is critical.

If you accept, for whatever reason, hydraulics are unlikely to fail on you, it would be a pity dismiss them from your dream audaxer.

 

[w00hoo_kent]

There was a journo when I was first doing research was positive his brake failure was from overheating and his case held water. He'd weight weenied the brakes, including going for a smaller rotor and was panic braking (dragging) on an unexpected long descent, some way down his brakes gave up. It sounded like an atypical failure, was near the beginning of road bikes with discs (all the tech was mountain bike I believe) and he could have dealt with it by changing his braking style.

Or that wasn't what happened.

Race cars/bikes having to warm up brakes is all about the components, ceramic discs and the matching pads need to be hotter to work properly, cold they have very little friction. I believe that's a different thing.

Hydraulic brakes need to keep the brake fluid from boiling, different fluids have different properties, most car systems use synthetic fluid which gathers water over time, this is generally what boils in older fluid and it's the air bubbles that stop the fluid working. In its normal form it's not very squishy (see how scientific I'm being) which is good, with air in it's squishness goes up a lot and it stops working. The system on the Sirrus uses mineral oil which is supposed to get round that. I presume the factor in what to use is largely cost.

The big issue with cycle brake systems and heat vs car and motorbike is that cycle systems are made to be as light and streamlined as possible and less weight means less metal which means less options for heat sinking. More streamlined means less presence in the air flow which means less options for cooling the brakes. Couple that with very small amounts of fluid in the system (even compared to motorbikes) and that is where the worry can occur. But having said all of that, it's not a big thing, I've only heard of it happening in the early days and not very much and it's only likely to happen on things like Alpine descents (agreed, not the best place to experience brake failure) and even then only if the technique used is sub-optimal. Then again, I've had similar brake failure on a Ford Transit motorhome coming down from the GroB Glockner glacier, so it's not just a bicycle thing.

As it has been documented in what seemed a reasonable and believable way, I felt it worth mentioning, but I'm not losing sleep over it. The day I descend Mt Ventoux, I'll add it to my worries, somewhere on the list under dying riding up it...