Techniques for braking on steep descents

[rareposter]

My bikes don't even HAVE rotors. Can I descend any hills safely? Or am I compelled to always use the "don't brake till you see the whites of their eyes" technique??

It's clearly unrideable and in fact you should offer your body to science for a study on how you've survived this long.

You are hereby instructed to visit a bike shop, and once they've overcome their amazement at your retro (and clearly unrideable) machine of death, they will sell you a more appropriate bike for twelvety thousand pounds. Be aware that it'll have a use by date of next month, after which it'll be obsolete and you'll be subjected to much derision and ridicule should you dare to be seen on it.

But for a month or so, you'll be king of all you descend.

[Psamathe]

I have heard the suggestion that if one goes very slowly indeed, braking all the time, overheating does not happen. But if I remember correctly, the speed at which that holds true is around 4mph. At that speed heating and cooling are balanced. ...

For a given hill the energy needing to dissipated will depend on the mass (rider, cycle, etc.), the potential energy. So my <60 Kg on an unladen carbon bike with BB7s will have a lot less energy to dissipate than a >100 Kg rider fully laden with camping gear, 2 days water riding a more than solid steel bike with BB7s.

Very different amounts of energy being dissipated through the same braking systems.

So I'd expect any steady state speeds to vary significantly.

Ian

Surely it will also vary with rotor size and material, caliper and pad design, ambient temperature, wind direction and speed... even direction of sun relative to rotors... Far too many factors to give one speed.

My understanding of physics (claiming no expertise) is that the amount of energy to be dissipated is just total mass and height (assuming gravity is constant). Things like rotor size, material, etc., wind, speed, etc. will affect how that energy is dissipated. eg 100 Kg dropping 1m altitude will lose 981 joules potential energy. Assuming start and end speeds the same, that means there is 981 joules will turn into heat irrespective of what sort of braking system you have.

Ian

[mattheus]

My bikes don't even HAVE rotors. Can I descend any hills safely? Or am I compelled to always use the "don't brake till you see the whites of their eyes" technique??

It's clearly unrideable and in fact you should offer your body to science for a study on how you've survived this long.

You are hereby instructed to visit a bike shop, and once they've overcome their amazement at your retro (and clearly unrideable) machine of death, they will sell you a more appropriate bike for twelvety thousand pounds. Be aware that it'll have a use by date of next month, after which it'll be obsolete and you'll be subjected to much derision and ridicule should you dare to be seen on it.

But for a month or so, you'll be king of all you descend.

Always good to be shown a Silver Lining! Bless you.

[drossall]

My understanding of physics (claiming no expertise) is that the amount of energy to be dissipated is just total mass and height (assuming gravity is constant). Things like rotor size, material, etc., wind, speed, etc. will affect how that energy is dissipated. eg 100 Kg dropping 1m altitude will lose 981 joules potential energy. Assuming start and end speeds the same, that means there is 981 joules will turn into heat irrespective of what sort of braking system you have.

I think the last few posts have been in jest. Obviously, the type of brake that you have does not affect the physics. The way that air resistance increases with the square of speed (and so, in some circumstances and on some hills, it might be effective to go faster in order to get more total effect from the air resistance) has already been covered.

I agree, I have no bikes with rotors either. This thread is an interesting example of some cyclists beginning to assume that discs are the only kind worth mentioning. I was looking at a bike in my LBS with rod brakes recently. That would give some an apoplectic fit, I imagine

Although I admit that it would not be my first choice for descending an alp.

[Bmblbzzz]

For a given hill the energy needing to dissipated will depend on the mass (rider, cycle, etc.), the potential energy. So my <60 Kg on an unladen carbon bike with BB7s will have a lot less energy to dissipate than a >100 Kg rider fully laden with camping gear, 2 days water riding a more than solid steel bike with BB7s.

Very different amounts of energy being dissipated through the same braking systems.

So I'd expect any steady state speeds to vary significantly.

Ian

Surely it will also vary with rotor size and material, caliper and pad design, ambient temperature, wind direction and speed... even direction of sun relative to rotors... Far too many factors to give one speed.

My understanding of physics (claiming no expertise) is that the amount of energy to be dissipated is just total mass and height (assuming gravity is constant). Things like rotor size, material, etc., wind, speed, etc. will affect how that energy is dissipated. eg 100 Kg dropping 1m altitude will lose 981 joules potential energy. Assuming start and end speeds the same, that means there is 981 joules will turn into heat irrespective of what sort of braking system you have.

Ian

Obviously rotor size etc doesn't alter the amount of energy to be dissipated. It alters the rate of conversion of that energy into heat and the rate of dissipation of that heat to the atmosphere.

[JohnR]

Assuming start and end speeds the same, that means there is 981 joules will turn into heat irrespective of what sort of braking system you have.

The aerodynamic drag is part of the energy conversion into heat although we don't notice it at cycling speed. It's another matter if re-entering the earth's atmosphere from space.

[Psamathe]

Assuming start and end speeds the same, that means there is 981 joules will turn into heat irrespective of what sort of braking system you have.

The aerodynamic drag is part of the energy conversion into heat although we don't notice it at cycling speed. It's another matter if re-entering the earth's atmosphere from space.

True, just as brakes, dragging feet, etc. but none of that affects the amount of energy, only the means of its conversion.

Aerodynamic drag is a form of braking system eg a parachute in your example of re-entering the Earth's atmosphere.

My response was in the context of

...
For a given hill the energy needing to dissipated will depend on the mass (rider, cycle, etc.), the potential energy. So my <60 Kg on an unladen carbon bike with BB7s will have a lot less energy to dissipate than a >100 Kg rider fully laden with camping gear, 2 days water riding a more than solid steel bike with BB7s.

Very different amounts of energy being dissipated through the same braking systems.

So I'd expect any steady state speeds to vary significantly.

Ian

Surely it will also vary with rotor size and material, caliper and pad design, ambient temperature, wind direction and speed... even direction of sun relative to rotors... Far too many factors to give one speed.

highlighting that the energy is from mass, height and gravity and has nothing to do with braking systems or wind resistance, etc. Brakes, wind resistance, etc. are just the means of conversion and don't affect the amount of energy.

(Think I'm going to give up repeating myself as people keep responding to my posts with unrelated stuff),

Ian

[JohnR]

highlighting that the energy is from mass, height and gravity and has nothing to do with braking systems or wind resistance, etc. Brakes, wind resistance, etc. are just the means of conversion and don't affect the amount of energy.

(Think I'm going to give up repeating myself as people keep responding to my posts with unrelated stuff)

I think we are both thinking the same. I just wanted to highlight that the wind drag is part of the energy conversion which, going back to the original question, can help to reduce the heating of the brakes.

[Brucey]

I have heard the suggestion that if one goes very slowly indeed, braking all the time, overheating does not happen. But if I remember correctly, the speed at which that holds true is around 4mph. At that speed heating and cooling are balanced. Any faster than that, and you get into the state where heating outpaces cooling while continuous braking is happening. The bottom line is that if you are willing to retard speed to a constant 4mph, you have a means of getting down a very steep hill without severe overheating. Has anyone here tested this? I haven't.

the point here is that this speed is reckoned to be very low; any hint of going faster than that would increase brake temps, unless you just let aerodynamic drag do all the work, which is much more fun....

[pwa]

I have heard the suggestion that if one goes very slowly indeed, braking all the time, overheating does not happen. But if I remember correctly, the speed at which that holds true is around 4mph. At that speed heating and cooling are balanced. Any faster than that, and you get into the state where heating outpaces cooling while continuous braking is happening. The bottom line is that if you are willing to retard speed to a constant 4mph, you have a means of getting down a very steep hill without severe overheating. Has anyone here tested this? I haven't.

the point here is that this speed is reckoned to be very low; any hint of going faster than that would increase brake temps, unless you just let aerodynamic drag do all the work, which is much more fun....

I heard about it from Thorn Cycles when I was buying a tandem from them, and it was cited as one tactic for getting safely down an extremely steep hill where just letting go is not an option. And it sounds like it should work. You heat the braking surfaces no quicker than they can be cooled. It was made clear that the speed above which constant braking would lead to problems is a very low speed indeed. And as you say, on hills where it is practical, just taking one's hands off the brakes and letting wind resistance limit the speed is always the fun option.

[aflook]

I might have missed a bit in this thread which I have been trying to follow, but has anyone mentioned noise? Surely kinetic energy is converted into noise as well as heat, which might suggest that noisy brakes are actually to be preferred. Also they help clear the way of pedestrians and other wildlife better than screaming “I can’t stop!!!!!”. How can I make my brakes noisier? Would it make any appreciable difference if I could?

[Carlton green]

I’m surprised about how this thread has run and run. It’s simple really in that all brakes have a safe working temperature limit after which you’re in trouble and all brakes vary in how much energy (by rate) they can scrub off. With my sidepull brakes the (relatively low) maximum scrub off rate is, as far as I can see, the limiting factor of their performance (I’m pretty much unconcerned about temperature), but with other brakes with higher energy scrub off rates heat dissipation rates become important.

Brakes have thermal mass and an ability to loose heat by radiation and conduction to the surrounding air, the rate of transfer goes up as the temperature rises plus relative air speed helps thermal transfer to the surrounding air. Try to scrub off too much energy too quickly and you’ll run out of capacity to absorb it in the disk (store via thermal mass) and to dump it to the surroundings by radiation and conduction (to the air) - you might also damage your brake shoes or pads. There’s a thermal balance here to be respected; know how hot your brakes can safely get and stay below that temperature by putting less energy into them (by rate) than they can dissipate (by rate) .

Historically folk knew about the importance of drag brakes and fitted them to Tandems (to steadily dump energy at a sustainable rate). Keep your brakes on lightly, don’t allow heat in them to build up and do so by going slow and stopping. It ain’t rocket science. Disk brakes are good at converting energy into heat, but that heat has got to be dumped to the surroundings - well that or you will have a problem.

I think it unwise to do so (that’s the polite version ) but of course you could just ignore using your brakes altogether, just letting your speed build up and loose energy to air drag - enjoy the thrill, speed and fun, etc. That’s maybe fine until there’s some issue like: a bend in the road; conflicting traffic; potholes; and debris on the road. Falling off of your bike at speed can be painful and even fatal … but I suppose that that’s all part of the ‘natural selection’ process. As for hitting someone else because you’re going too fast to be in proper control of your bike, well it’s best if I don’t pass particularly negative comments …

[531colin]

Drum brakes (tandem drag brakes) dissipate heat into the big heat sink which is the hub, you can carry on doing that for a while before stuff gets too hot.
Disc brakes stop you faster than the old tandem drag brakes, therefore produce a lot of heat in a short time. The way to stop overheating your disc brakes is NOT TO DRAG THEM, but to let them cool for example by alternating front/rear braking.

I think is what we established, pages ago

[Nearholmer]

just taking one's hands off the brakes and letting wind resistance limit the speed is always the fun option.

Have you calculated the speeds that can be obtained by doing that? They are truly impressive!

If there were no frictional losses, all of the potential (and any kinetic) energy at the top of the slope would be converted to kinetic energy, mgh = 0.5mv^2, irrespective of the angle of the slope.

Of course, in the real world there are all the frictional losses that have been discussed, but even on a creaky old bike, wearing flappy old clothes, and sitting bolt upright, in a fair headwind, the slope doesn’t have to be very great before things get “very exciting”.

[Carlton green]

…..
I think is what we established, pages ago

Without wishing to be controversial the many pages of this thread are a mix of personal experiences with a few bits of physics thrown in here and there. Though I respect the experiences of others I’d really like to see a decent scientific analysis that’s then proven by test results. Short of that I’ll be taking things steady and looking for feedback from whatever brakes I’m using at the time.