Techniques for braking on steep descents

[mattheus]

What a frustrating thread - all the key points have been posted, but people keep repeating selected facts to suit their understanding.

- MATT!
- Wot?
- Never, repeat NEVER try to explain physics to non-physicists on the internet!

[rareposter]

What a frustrating thread - all the key points have been posted, but people keep repeating selected facts to suit their understanding.

- MATT!
- Wot?
- Never, repeat NEVER try to explain physics to non-physicists on the internet!

+1... and also, as with many many things on this forum, it is being wildly over-thought.

[Pebble]

What a frustrating thread - all the key points have been posted, but people keep repeating selected facts to suit their understanding.

- MATT!
- Wot?
- Never, repeat NEVER try to explain physics to non-physicists on the internet!

+1... and also, as with many many things on this forum, it is being wildly over-thought.

what else is there to do in this weather won't stop raining, and I have been wet twice this week already and its only tuesday

[JohnR]

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.

If, as I understand it, aerodynamic drag is proportional to velocity squared then the proportion of the time spent at above average speed will result in a bigger increase in drag loss than the reduction in drag loss while going at below average speed. Ignoring the other factors in the equation, 15 squared = 225, 25 squared = 625 so the average drag loss with varying speed = 425 whereas the average speed (20) squared = 400. That's only a 6% increase in drag loss in this example but if you are willing to speed up to 30mph (or even more) then the drag loss will be significantly increased. The more of the potential energy that's turned into drag loss then the less will end up heating the brakes. Putting on a big baggy jacket at the top of the hill will help.

Now that I've thought this through I know better how to handle the next long descent.

[Psamathe]

... but if you are willing to speed up to 30mph (or even more) then the drag loss will be significantly increased. ...

Remember that the faster your speed down the hill the larger and more extensive the potholes will be at the bottom.

Ian

[Chris Jeggo]

Here's my summary of the physics, for what it's worth.

If you go down the hill really slowly you can neglect rolling resistance and air resistance (because the main thing by far that's holding you back is the brakes) so all the potential energy you have at the top finishes up as heat generated in and dissipated from the braking system, whether rim, disc or drum. If you go slowly enough all this heat has plenty of time to dissipate so nothing overheats.

If you go down the hill as fast as you dare then much of your initial potential energy gets lost to air resistance, so less energy goes into the brakes, but it might be over a short enough period to allow overheating.

In between these extremes there might well be a pessimum, which might explain those alpine blowouts.

[JohnR]

... but if you are willing to speed up to 30mph (or even more) then the drag loss will be significantly increased. ...

Remember that the faster your speed down the hill the larger and more extensive the potholes will be at the bottom.

Sensibly one will only speed if a clear road can be seen. More important is that the kinetic energy and the braking distance also increase with the square of the speed so it will take four times as long at 30mph compared to 15mph to dump that kinetic energy assuming a constant braking force. And if the road is only wide enough for one vehicle then assume that something else is coming up the hill.

[Marc]

What a frustrating thread - all the key points have been posted, but people keep repeating selected facts to suit their understanding.

- MATT!
- Wot?
- Never, repeat NEVER try to explain physics to non-physicists on the internet!

Imagine trying to explain brake performance to people in forums, where the usual vehicles get really fast, really quick on descends (velomobiles). They blame their overheating and subsequently fading (drum) brakes on everybody (dealers, manufacturers) but not their own stupidity. The 'mountain experts' started using water cooled drum brakes in their velomobiles some years ago (short version: squirting water from a small (RC-model) water tank into the drums with a tube works really well!)

[rareposter]

Now that I've thought this through I know better how to handle the next long descent.

Go full gas from top to bottom, yes?

https://youtu.be/99wJn5QBvyg?si=bBY1rqlvSZswycPP

[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.

[Nearholmer]

^^^

If what you say holds true, that at 4mph heat generation and dissipation balance perfectly, and whether it does or not will depend on the design of the braking system, and the ambient conditions, among other things (edit: including those mentioned below), you will have no net heating of the braking system at all.

“Severe overheating” implies to me something quite a long way beyond no net heating at all; something which impedes braking performance might be one definition, I suppose.

Put politely, I don’t think it’s a very helpful rule of thumb.

[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

[Biospace]

Building up plenty of speed between braking periods allows for greater heat loss from the rims/discs and as mentioned by CJ, frictional losses account for some of the energy lost which otherwise would heat the brakes.

In reality, every situation is different and no hill is the same twice over.

[Bmblbzzz]

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.

[mattheus]

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 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??