Techniques for braking on steep descents

[Vorpal]

why would periodic braking help ?
On a long steady slope
a) holding brakes at constant pressure so as speed remains at a constant 20mph
b) repeatedly letting it go up to 25 then braking down to 15
c) alternate harder braking from front to rear brakes but keep speed at constant 20

It is the same amount of kinetic energy being transferred into heat.

Yes, but if you brake for a couple seconds, then cool for a few seconds, you can dissipate the heat more readily. If you drag the brakes, they are constantly generating heat, and have less surface areas to dissipate it.

I do b), though my speed probably varies a bit more than that. If know the descent, I tend to freewheel where I have a long straight section, then brake hard enough before a bend, both for keeping my speed reasonable, and taking the bend. I probably go a bit more slowly through bends than some folks would do, but I am not a fast descender.

[TrevA]

As a heavy (110kg) rider who sometimes rides in the Peak District, there’s a few things I do to mitigate heavy braking. Pick gentler descents - after a death- defying, white knuckle descent of Curbar Edge, I now go down either the main Chesterfield road into Baslow, or down Froggatt Edge or carry on to Owler Bar and drop into Hathersage. A long, swooping descent is more fun than constantly having to be on the brakes on a steep one. Instead of dropping down through Bonsall to Cromford, I carry on down to the Hollybush and drop-down the Via Gelia instead. Yes, there are more cars, but you are going faster so you more able to keep up with traffic.

Also, don’t gather too much speed and then try and slam on the brakes at the bottom. Like an oil tanker, you take a lot of stopping!

[Psamathe]

...
Yes, but if you brake for a couple seconds, then cool for a few seocnds, you can dissipate the heat more readily. If you drag the brakes, they are constantly generating heat, and have less surface areas to dissipate it.
...

I'm no physicist or engineer but whilst I can see that the brief periods where pads are released exposes more disk surface area to air, how much of the heat is conducted through the pads into the calliper which has a significant surface area. (unlike rim brake blocks disk pads are pretty thin).

I have read of concerns with some (poorer design?) hydraulic disk brakes with the brake fluid boiling so I assume some heat must be conducted through the pads.

Just asking the question as I have no idea how impotent different heat dissipation paths might be.

Ian

[Pebble]

...
Yes, but if you brake for a couple seconds, then cool for a few seocnds, you can dissipate the heat more readily. If you drag the brakes, they are constantly generating heat, and have less surface areas to dissipate it.
...

I'm no physicist or engineer but whilst I can see that the brief periods where pads are released exposes more disk surface area to air, how much of the heat is conducted through the pads into the calliper which has a significant surface area. (unlike rim brake blocks disk pads are pretty thin).

I have read of concerns with some (poorer design?) hydraulic disk brakes with the brake fluid boiling so I assume some heat must be conducted through the pads.

Just asking the question as I have no idea how impotent different heat dissipation paths might be.

Ian

yes, in between braking the pads will be within a fraction of a mm from the rotor so no real increase in cooling surface, and the heat transfer into the calipers will be lost, and yes the calipers do get very hot, hence the need of specialist brake fluids to any stop water that may form or appear near the caliper boiling and causing the brakes to go spongy.

(dot brake fluids absorb water, where as any water can sink to the bottom of hydraulic fluid, and the bottom of the system is usually where the brakes are)

[mattheus]

I would *hypothesize* that rim-brake designs naturally give better air-flow to the hot bits*, which would help avoid problems from hot brakes.
If so, that's another reason why 70's tourists in the Alps didn't often complain about brake-fade and the like. And it was rare when the racers first used the same roads, many decades earlier.

(and at the lower speeds common in UK off-roading, air-flow isn't as important as on Alpine tarmac descents.)

*one of which is a disc of ~27" diameter, remember!

[rareposter]

I have read of concerns with some (poorer design?) hydraulic disk brakes with the brake fluid boiling so I assume some heat must be conducted through the pads.

Just asking the question as I have no idea how impotent different heat dissipation paths might be.

Yes but the brake fluid itself is designed as a heat sink. You'll lose some heat through the rotor itself, some from the pads and backing material, some will go into the fluid. How much that happens depends a lot on riding style, speed, air temperature and the rotor material and backing pad material.

If you're on the brakes constantly, the fluid can boil but you'd really have to be trying quite hard to do that - modern braking systems it's rarely an issue. You'd need a prolonged descent and a heavy rider/bike combination being ridden quite poorly to get close to that on current disc brake systems.

I would *hypothesize* that rim-brake designs naturally give better air-flow to the hot bits, which would help avoid problems from hot brakes.
If so, that's another reason why 70's tourists in the Alps didn't often complain about brake-fade and the like. And it was rare when the racers first used the same roads, many decades earlier.

I would *hypothesize* that speeds in the 70's were a lot lower than road bikes of today will do!
That said, I've seen several instances of burst tyres on long Alpine descents when the heat from the rim brake has gone into the aluminium of the rim and blown the tyre. Also used to be a problem on tubulars back in the day cos the heat would melt the glue that held them on. Carbon rims, you can get delamination where the resin bonding within the rim basically melts.

[Nearholmer]

I can’t help thinking that the best answer to all this is the wimp’s strategy: enter the top of the hill at a very low speed, and never allow speed to build-up significantly.

By doing that, using alternate front and rear “scrubs” to keep the speed low while avoiding the “glazing” that comes from dragging one brake, the braking system is never asked to dissipate a great amount of energy in a short time, so is very unlikely to heat up to problematic extent. Remember always that the kinetic energy to be shed is proportionate to the square of the velocity - velocity, speed, is the thing to avoid.

Of course, if you have good nerves, get a high from speed and/or risk taking, or have an overly developed competitive streak (racing even with yourself), this will seem exceedingly dull. Me? I’m a Grade A Wimp.

[mattheus]

...

I would *hypothesize* that rim-brake designs naturally give better air-flow to the hot bits, which would help avoid problems from hot brakes.
If so, that's another reason why 70's tourists in the Alps didn't often complain about brake-fade and the like. And it was rare when the racers first used the same roads, many decades earlier.

I would *hypothesize* that speeds in the 70's were a lot lower than road bikes of today will do!
That said, I've seen several instances of burst tyres on long Alpine descents when the heat from the rim brake has gone into the aluminium of the rim and blown the tyre. Also used to be a problem on tubulars back in the day cos the heat would melt the glue that held them on. Carbon rims, you can get delamination where the resin bonding within the rim basically melts.

Really? I thoughts tents (and luggage racks) were getting lighter?!?

Honestly, I haven't seen enough of these failures* - and have never ridden tubs OR carbon rims - to comment. I was really only commenting on the exposed-to-air nature of a rim-brak design vs more enclosed/smaller caliper+rotor. There are of course many other factors :-)

*I've seen one - a Sportive rider braking for a hairpin on the Galibier, as I plodded upwards against the traffic. I believe he survived!

[cyclop]

Descending the steep cobbled road into Alston,one handed with large pastie in the other,rear brake only,on the drops saw me at the limit....never lost the pastie though.Should be an integral park of cycling profficiency test

[Pebble]

getting back to pulse braking - Change in velocity requires energy, so would varying between 25 and 15mph (average 20mph) use more energy than maintaining a steady 20mph ? (newtons laws of motion)

So would that generate more heat in a braking system or is that energy coming from gravity - I can't quite get my head round this.

I think if you rode a bike at a steady 20mph would be easier than pedalling up to 25 then freewheeling down to 15 (but av 20) - Or maybe not?

[drossall]

More physics: if you move slowly the descent takes longer and heat has more time to dissipate.

Interesting, because I once read an argument that over-caution, leading to over-braking and hence additional heat, can be more dangerous. I suspect there's some truth in both, but your argument makes perfect sense.

Good news for me because, the older I get, the more cautious.

I was thinking about this overnight. I suppose the argument would be this. Let's assume that cautious and exuberant riders both take bends in a similar manner, on a switchback downhill. However, the cautious riders brake down the straights, whereas the exuberant allow themselves to run free, relying on air resistance to moderate their speeds. Given decent straights, they'll quite quickly reach a terminal velocity and stay at it. So the cautious are doing braking where the exuberant do not. Then, the question becomes whether the kinetic energy dissipated as heat by the cautious on the straights exceeds the extra braking needed by the exuberant to scrub off maybe 20-30 mph into corners.

I can see how it might, however much I might instinctively be cautious.

[Psamathe]

getting back to pulse braking - Change in velocity requires energy, so would varying between 25 and 15mph (average 20mph) use more energy than maintaining a steady 20mph ? (newtons laws of motion)

So would that generate more heat in a braking system or is that energy coming from gravity - I can't quite get my head round this.

I think if you rode a bike at a steady 20mph would be easier than pedalling up to 25 then freewheeling down to 15 (but av 20) - Or maybe not?

My understanding is, assuming you have the same speed at the top and at the bottom and that you are not pedalling then the only energy input is your loss of potential energy (the height you are losing and your total mass (cycle, rider, luggage, etc.)).

Forces slowing your descent will be friction and for this discussion friction from brakes and other friction (wind resistance, tyres on road surface, bearings, etc.).

So the issue becomes how efficiently you lose the heat generated from friction and the time over which you lose that heat. eg crawl down the descent at 0.1 mph taking 2 days and whilst you'll still be losing the same amount of energy it will be over such a log time it will dissipate readily without you noticing anything getting warm. But race down at 30 mph and the same amount of energy will need to be dissipated but over a much shorter time so things will get hotter (and as pointed out, friction wind resistance will be higher) but total energy needing to be lost will be the same.

My understanding (maybe wrong?).

Ian

[pwa]

The classic approach to bicycle braking with rim brakes (I know we are talking about discs) is to keep off the brakes some of the time, allowing surfaces to cool, then compensate with heavy braking when necessary. This is thought to lead to less overheating than constant feathering of the brakes.

In the mid 1990s we went to the French Alps for a fortnight and I tackled big climbs and descents on my Campag bejewelled race bike. But on two descents I had a rear tyre blowout, where the rear inner tube went bang and the tyre came off the rim and wrapped itself around the mech. Which was interesting. A local bike mechanic told my wife that visitors had this problem all the time and it was down to poor braking technique and the resultant overheating. Brake less often and brake harder when you do, was his advice. When I got home I looked this up and it was a widely given piece of advice.

So when I returned to the Alps and other mountain ranges I adopted this new approach (brake late and brake hard) and found that I no longer had blowouts. Alternating between front and rear brakes also gives each a rest, or so the theory goes.

All this was in the days of rim brakes, but it must still be true for disc brakes if you are trying to avoid heat build-up. In UK situations, though, it isn't always possible to just let the brakes go for twenty seconds. Some very steep and twisty hills don't give you that much time off the brakes. So there are times when you might want to just pull over and admire the scenery for a minute or two, while things cool down.

[Nearholmer]

can't quite get my head round this.

Sticking with coming down a hill, assuming for simplicity that you are starting from a stop to look at the view at the top, and that you need to stop at the bottom for a road junction, and imagine that it is a continuous, single, gradient:

- the amount of energy that has to be dissipated is the potential energy resulting from height difference, (mass x gravity x height);

- that energy gets dissipated in various resistances (air drag, bearing losses, tyre hysteresis etc), and in the brakes;

- if you freewheel (no pedalling for the moment please, because that just complicates matters), build-up lots of speed and lots of kinetic energy, then brake from a high speed to a stop at the junction, all the energy being dealt with by the brakes has to be dissipated in a short time, heating-up the braking system as it struggles to shed energy at the rate asked of it;

- if you use the brakes gently, all the way down, speed, and therefore kinetic energy, doesn’t build-up, the potential energy that you had at the start is dissipated at a low rate, dribbled out into the atmosphere by a braking system that can easily shed it so doesn’t heat-up much in the process.

- if you make yourself as “non-aero” as you can, a greater proportion of the energy is dissipated in air drag, so the brakes don’t have to dissipate so much.

- if you are a speed-merchant, and decide not only to defer braking for as long as possible, but to select a high gear and pedal down the hill, you are adding more energy, from your personal reserves, on top of the potential energy resulting from height difference, so giving the brakes yet more to dissipate.

- if you ride over the crest of the hill, rather than starting from a break to look at the scenery, the kinetic energy you have at that moment also has to be got rid of before you can stop at the bottom, so the faster you’re going when you start the descent, the more energy the brakes have to dissipate.

It is all very case/hill dependant though, because there are doubtless some really steep hills where even with the slow-speed, near-continuous gentle braking technique the rate of energy dissipation required is enough to tax some braking systems. Our local short, sharp descent is 65m in about 500m, ending at a T-junction, which isn’t all that evil by some standards, but a quick calculation suggests that coming down it stop-to-stop dissipates enough energy in what feels like next to no time to boil a cup of water.

[Pebble]

getting back to pulse braking - Change in velocity requires energy, so would varying between 25 and 15mph (average 20mph) use more energy than maintaining a steady 20mph ? (newtons laws of motion)

So would that generate more heat in a braking system or is that energy coming from gravity - I can't quite get my head round this.

I think if you rode a bike at a steady 20mph would be easier than pedalling up to 25 then freewheeling down to 15 (but av 20) - Or maybe not?

My understanding is, assuming you have the same speed at the top and at the bottom and that you are not pedalling then the only energy input is your loss of potential energy (the height you are losing and your total mass (cycle, rider, luggage, etc.)).

Forces slowing your descent will be friction and for this discussion friction from brakes and other friction (wind resistance, tyres on road surface, bearings, etc.).

So the issue becomes how efficiently you lose the heat generated from friction and the time over which you lose that heat. eg crawl down the descent at 0.1 mph taking 2 days and whilst you'll still be losing the same amount of energy it will be over such a log time it will dissipate readily without you noticing anything getting warm. But race down at 30 mph and the same amount of energy will need to be dissipated but over a much shorter time so things will get hotter (and as pointed out, friction wind resistance will be higher) but total energy needing to be lost will be the same.

My understanding (maybe wrong?).

Ian

I can quite understand all that, but that is not what I was asking. I was going a little deeper into the laws of physics and was wondering if the energy losses through acceleration / deceleration would be apparent in heat generated through braking. (everything else being equal appart from steady speed of 20mph as opposed to varying between 25 and 15 but averaging same speed.
Newtons laws of motion and all that.