Downhill braking, what should the balance be?

[Profpointy]

They don't, for the reason you state.

That's what I thought. phaeton seems to think different though

 

[RichardB]

That's what I thought. phaeton seems to think different though

I think he will be in a minority of one, though. I'd be (genuinely) interested to hear his reasoning.

 

[mjr]

[QUOTE 4629431, member: 9609"]I take it you are talking about disc brakes on a bike, is it possible to get them hot enough that they start to fade ? (well obviously it is possible but does it ever happen)
may be even rim brakes fade although I have never noticed it. I had brake failure on a bike so I'm always over cautious now, I keep my speed down and brake early so unlikely to cook them.[/QUOTE]
It depends on the pad compounds but you can get some rim brakes hot enough they go soft and wear very quickly even on some UK descents, including some in the south west.

My best brake failure was a lever coming away from the bars after something snapped in the clamp, leaving it bobbling around in the air, supported only by the cable. At least it was only a descent into Norwich from the south, so not too taxing to do on only the back brake.

 

[smutchin]

That might apply to overheating brakes, but there's your rims to think about too

I don't know what @Pale Rider was thinking of in the post I was replying to, but I was thinking about both rims and brake pads, all being part of the same equation.

I have since witnessed someone exploding his front tube by braking too much on a shortish, steep hill near Stroud in very mild British weather conditions.

Interesting. Are you sure it was caused by an overheated rim?

In any case, it's not a problem I've ever had myself, since I prefer short bursts of hard braking rather than continuous gentle braking.

 

[Pale Rider]

I don't know what @Pale Rider was thinking of in the post I was replying to, but I was thinking about both rims and brake pads, all being part of the same equation.



Interesting. Are you sure it was caused by an overheated rim?

In any case, it's not a problem I've ever had myself, since I prefer short bursts of hard braking rather than continuous gentle braking.

Smutch, I think you're confusing me with @Donger - neither of us are road whippets although he's the slimmer one.

As you say, short bursts of hard braking work best, provided the road surface is suitable.

 

[smutchin]

It depends on the pad compounds but you can get some rim brakes hot enough they go soft and wear very quickly even on some UK descents, including some in the south west.

But could they get the wheel rim hot enough to cause a tube explosion? It sounds unlikely to me but I'm open to being convinced otherwise.

There was one descent in Devon I did in the summer that was fairly short but steep enough for me to hit 82kmh. Perhaps on a descent like that if you're applying the brakes to the point that you're going very slowly, the descent might take long enough to cause serious heat build-up in the rim...

 

[smutchin]

Smutch, I think you're confusing me with @Donger - neither of us are road whippets although he's the slimmer one.

I was referring back to my post that was in reply to your post about brake fade and overheating. You didn't specify whether you meant overheating in the brake pads or wheel rim, which is what @Donger was picking up on in his reply to me.

 

[Dogtrousers]

I think there's a degree of over-thinking going on here.

If you read the road ahead, don't go faster than your visible stopping distance*, keep alert for potential hazards, remove your speed in advance (using whichever brakes you feel like) you should be reet.

* That doesn't make sense, but you know what I mean.

 

[Pale Rider]

I was referring back to my post that was in reply to your post about brake fade and overheating. You didn't specify whether you meant overheating in the brake pads or wheel rim, which is what @Donger was picking up on in his reply to me.

Ah, with it now.

I was just generalising, as in short bursts of sharp braking are better for whatever type of brakes you have, rim or disc.

Lots of heat in a disc is less likely to cause a problem than lots of heat in a rim, which I'm told can cause the tyre to blow out.

As doggers says, there's a danger of over-thinking.

I expect few of us do descents which are long enough and require enough braking to cause overheating problems, no matter how the brakes are operated.

 

[ColinJ]

Lots of heat in a disc is less likely to cause a problem than lots of heat in a rim, which I'm told can cause the tyre to blow out.

As doggers says, there's a danger of over-thinking.

I expect few of us do descents which are long enough and require enough braking to cause overheating problems, no matter how the brakes are operated.

Actually, I regularly used to have thermal problems with the old Hope C2 disk brakes on my mountain bike. They have sealed hydraulic systems which are very sensitive to the expansion of the fluid when it gets hot. Each brake lever has a rotary control which can be operated by a thumb to 'back off' the brake if it starts to overheat. I could usually just about get down the local descents with the range of control available but sometimes the brakes would virtually lock on by the bottom. When that happened they would get even hotter and I would slow to a crawl or even have to stop.

Maybe I had too much fluid in the brakes? I left them as they were supplied to me so I don't know how much fluid was in each.

PS A scary consequence of this problem was that I would occasionally continue the ride with hot brakes and forget to adjust the controls back to the original positions as the hydraulic fluid cooled. A few times I tried to do an emergency stop, pulled the levers to the bars, but got no braking!

 

[smutchin]

[QUOTE 4629877, member: 9609"]a slow descent would give more time to cool, same amount of energy being converted into the brakes as heat but longer to cool.[/quote]

Good point.

I have always wished for something on a bike that reduced speed that was in addition to the brakes, such as gears in a car or retarder / Jake brake on a lorry.

Fixed gear? On a very steep descent, it's impossible to use your legs to actually stop the bike, but I find you can at least use it in combination with the brakes to help slow the bike down a bit.

 

[ColinJ]

[QUOTE 4629924, member: 9609"]I wish they showed exactlt where the bike was, but interesting all the same
[/QUOTE]
I discovered that disk rotors can get very hot when I accidentally branded a leg with one after a steep local descent!

 

[Will Spin]

I'm not sure what the theory is, but I always brake early to get the speed off. Also, having done few thousand miles recently I've gone through one or two expensive rims, so I always try to do most of my braking with the front as it seems to wear less than the back.

 

[Pale Rider]

Actually, I regularly used to have thermal problems with the old Hope C2 disk brakes on my mountain bike. They have sealed hydraulic systems which are very sensitive to the expansion of the fluid when it gets hot. Each brake lever has a rotary control which can be operated by a thumb to 'back off' the brake if it starts to overheat. I could usually just about get down the local descents with the range of control available but sometimes the brakes would virtually lock on by the bottom. When that happened they would get even hotter and I would slow to a crawl or even have to stop.

Maybe I had too much fluid in the brakes? I left them as they were supplied to me so I don't know how much fluid was in each.

PS A scary consequence of this problem was that I would occasionally continue the ride with hot brakes and forget to adjust the controls back to the original positions as the hydraulic fluid cooled. A few times I tried to do an emergency stop, pulled the levers to the bars, but got no braking!

Not heard of that design before.

Presumably, the venting adjustment was done by the knurled wheel on top of the reservoir.

All sounds - and looks - a bit clonky by today's standards.

I doubt you'd have any problems with a modern hydraulic disc.

 

[lpretro1]

The old Hope system was a 'closed' system - that's why the dials were there otherwise when heat was generated the pistons would move and lock on the rotor. Giant had a similar set up with their MPH3 brakes ( perhaps Hope sold them the technology). The problem with the adjustable thumbwheels was always that they'd seize up very quickly and they are a stupid shape to get hold off. The Giant ones were so poor that if you left your bike in the sum you'd come back and find the brakes locked on! If the thumbwheel was stuck then the only option would be to let a little fluid out of the system - but you can't do that too often obviously as there isn't a lot of fluid in there in the first place. Modern systems have a rubber diaphragm which allows for this expansion of fluid thereby avoiding this problem