last spring I experienced what I can only call brake fade after descending a 200m steep slope at speed. towards the bottom of the slope (actually a fireroad) I hauled on the front brake (only to test the power with my 16stone bulk onboard)and the brake shrieked a bit and slowed me but the lever felt rather loose and spongy and could touch the bars ..(when normally it's rock solid). I was suprised as it was only one steep slope not an alpine descent!
the brakes are older style xt's btw with a 180 up front.
I presumed the fluid needed replacement and at the same time fitted new pads ..which initially stopped it happening again but........
today I was descending a very steep and long devonshire road at speed and experienced the same thing.
Is this normal and if not what's going on ?
any suggestions to sort (preferably not new brakes or to lose weight) would be much appreciated as it's really bugging me!
thanks in advance
Brakes fade by two mechanisms:
1) Pad temperature. Above a certain temperature, the friction coefficient of the pad falls.
2) Boiling fluid, which creates gas in the system which is compressible, and therefore stops your power getting to the pads.
I suspect that you're experiencing number 1, and that your front brake could be dragging a little (or you're dragging your brakes), meaning it's already warm when you go to use it.
On long descents, it's better to brake in bursts then roll back up to speed, or to switch between front and rear brake if you're dragging them. Or to just buy big rotors / high temp pads so it's no longer a problem.
It sounds more like he's getting fade from expanding gas in the brake fluid as he says the lever feels spongy and compresses all the way to the bar, which is classic of having boiled the brakes.
It's pretty unusual to get brake fade in the UK; we just don't have descents long enough to make it easy to induce. It's obviously not impossible though.
Your experience might be exasperated by not having bled the brakes as well they could be, i.e. you're still getting a lot of air in the system, which will mean they are more prone to fade, even under moderate use.
If that's the case, then having your local bike shop do a good quality bleed should cure the problem. Some brakes are easier to bleed than others (Formula for example are notoriously diffiuclt, whereas Shimano are supposed to be very easy) so you shouldn't feel bad if it's down to poor mechnical skills.
Are those brakes using DOT brake fluid or mineral oil?
if its oil, I doubt that its you're problem - change of pads is probably easiest first move.
GT....surely if I hadn't bled the brakes well enough I wouldn't experience a fairly rock solid feeling at the lever (when not going downhill at speed!)
Is it possible to have a solid lever feel but still have air in the
system?
Tron ...if it were the simply the pads losing friction then surely the lever wouldn't feel spongy /loose?
Mineral Oil
..again ..surely pad problems wouln't affect the feeling at the lever?
I've only ever managed to get car brakes to fade, not bike ones. I've found that when the pads are merrily smoking away, the pedal is soft. That's not to say that you can't have both types of fade at the same time!
With Shimano brakes, you have mineral oil as the fluid. It won't mix with water, and water in the caliper should sit in the bottom of caliper. Once the fluid in the caliper reaches somewhere > 100°C, you can get boiling water, which means steam, which is effectively the same as having air in the caliper. Once everything's cool, it condenses again and everything feels fine.
My advice would be that if the bleed nipple is at the top of the brake (normally are to make removing air simple), then bleed the brakes once with the nipple at the bottom to remove water, then again to make sure there's no air in there.
However, I've no idea how water would get into a Shimano brake, which makes that theory rather shaky. The car brake fluid used by most other brakes actively attracts and absorbs water. Mineral oil doesn't, which is why people don't have to bleed them every 15 minutes.
I've worked alot in the past in the motorsport industry primarily in rally cars and what you've experienced isn't brake fade.
Brake fade only occurs when you've been on the brakes for a long period of time, it's purely because you've overheated the rotor and the pad itself.
If you've experienced brake fade the lever will go rock solid and give no braking performance what so ever, normally leaving the brakes sticking to high heaven and your rotor a shade of blue.
from what you described as their being a spongy lever with very little feel you must have some air trapped in the system somewhere, only exacerbated by the raised temperature making the air expand. Boiling the fluid isn't brake fade as it gives completely different symptoms.
sounds good tron I'll give it a go ....are you in the trade?
thanks Pinch ...all food for thought!
I would be tempted to give your brakes a proper bleeding. I run XTs on mine with the servo levers and they are really odd at times. I've had them freshly bled and set up perfectly, then on the trail they will just disappear, lever to the bar and no actual braking force. I've not got to the bottom of it yet though (although i've not really tried anything other than a bottom up bleed so far)
are you in the trade?
Nope, but I've bled a few car brakes in my time. The pushbike ones are much the same, just smaller and more nicely made 😆
I'll definitely bow to Pinches' experience though - sounds like I had problems with the hydraulic and friction side of things. It's been a while since I had a car with brakes that bad.
Just to clarify kaiser yes you can have air in the system and still have a solid feeling lever pull when the brakes are cool only to find that with moderate use you get that spongy feel. It's also possible to spend five hours trying to get a decent bleed but fail miserably give up and then find that the next day they've magically sorted themselves out.
I used to have similar problems as tron describes, my own car left foot braking, i actually had to fit significantly harder compound brakes to stop them overheating so quickly as the brakes would fade massively then boil the fluid as a consequence of having to brake harder and harder as the brakes became less effective!