Techniques for braking on steep descents

General cycling advice ( NOT technical ! )
Nearholmer
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Re: Techniques for braking on steep descents

Post by Nearholmer »

Yep. Me.

Locking the wheel doesn’t produce heat at the brake, it produces heat at the tyre-road interface.
Pebble
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Re: Techniques for braking on steep descents

Post by Pebble »

I was trying to make some caclulations of how much energy is involved and seem to be getting a higher fig than I guessed. Hopefully someone will critique my adding up

100kg bike and rider, 100m descent in 1km at 10m/s (22mph)

kg x gravity x height = 100,000 Joules in 100 seconds = 1000w (? seems high)
with a CRR of 0.004 = 100*9.81*0.004) = 4 Newtons at 10m/s = 40 watts

air resistance Don't know (they used to say air resistance equalled rolling resistance on a wagon between 50 and 60mph (don't know if that is why the limiter is at 56, somehow doubt it) )
But a bike will have much higher air resistance to weight ratio
so lets guess at 10m/s air resistance is double rolling, therefor in this case 80w

So would that gave us brakes having to dissipate 880w of heat to hold at 10m/s 440watts per wheel - seems very high - where have I went wrong?
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531colin
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Re: Techniques for braking on steep descents

Post by 531colin »

Biospace wrote: 18 Apr 2024, 12:13pm
Introducing 'efficiency' here can lead to all sorts of confusion and misunderstanding, I qualified how Colin531 was appearing to use the term as "ability to transform energy".
Lost me. In normal everyday English, a brake which is effective or efficient or just plain old good slows or stops you properly or well or quickly: and produces heat.
A brake which stops you quickly produces more heat than one which only slows you down a bit.
Jdsk
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Re: Techniques for braking on steep descents

Post by Jdsk »

Pebble wrote: 18 Apr 2024, 5:04pm I was trying to make some caclulations of how much energy is involved and seem to be getting a higher fig than I guessed. Hopefully someone will critique my adding up

100kg bike and rider, 100m descent in 1km at 10m/s (22mph)

kg x gravity x height = 100,000 Joules in 100 seconds = 1000w (? seems high)
with a CRR of 0.004 = 100*9.81*0.004) = 4 Newtons at 10m/s = 40 watts

air resistance Don't know (they used to say air resistance equalled rolling resistance on a wagon between 50 and 60mph (don't know if that is why the limiter is at 56, somehow doubt it) )
But a bike will have much higher air resistance to weight ratio
so lets guess at 10m/s air resistance is double rolling, therefor in this case 80w

So would that gave us brakes having to dissipate 880w of heat to hold at 10m/s 440watts per wheel - seems very high - where have I went wrong?
Values for aerodynamic drag and rolling resistance are given in Bicycling Science. I'll look them up when I get home.

...

Thanks for trying to quantify this. It would be a lot easier to follow the calculation if you used the conventional terms (eg mass, force, energy, power) rather than the names of units in which those can be measured.

Jonathan
Last edited by Jdsk on 18 Apr 2024, 5:18pm, edited 1 time in total.
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531colin
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Re: Techniques for braking on steep descents

Post by 531colin »

Nearholmer wrote: 18 Apr 2024, 5:03pm Yep. Me.

Locking the wheel doesn’t produce heat at the brake, it produces heat at the tyre-road interface.
One way to cool the brake on an Alpine decent I guess!
Pebble
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Re: Techniques for braking on steep descents

Post by Pebble »

531colin wrote: 18 Apr 2024, 4:55pm A brake is a brake.
A tandem drag brake won’t lock the wheel.
A disc brake will lock the wheel.
Therefore a disc brake can produce more heat than a drag brake.
I expect somebody will misinterpret that; you can misinterpret anything if you try.
Why are they called 'drag brakes' is it more like some sort of retarder that you switch on and it gives a set amount of resistance that goes up with speed? ie it will hold you at a certain speed?

Obviously you could not do this with a derailleur, but with hub gears can you lock the freewheel (turn them into a fixie) and would that help on long descents, sort of turning yourself into an engine brake? (I have never road a fixie so don't know what it is like braking with our legs but fixie riders seem to do it very well)
Jdsk
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Re: Techniques for braking on steep descents

Post by Jdsk »

Pebble wrote: 18 Apr 2024, 5:53pm
531colin wrote: 18 Apr 2024, 4:55pm A brake is a brake.
A tandem drag brake won’t lock the wheel.
A disc brake will lock the wheel.
Therefore a disc brake can produce more heat than a drag brake.
I expect somebody will misinterpret that; you can misinterpret anything if you try.
Why are they called 'drag brakes' is it more like some sort of retarder that you switch on and it gives a set amount of resistance that goes up with speed? ie it will hold you at a certain speed?
...
I don't think that the name is connected to any special technology, only that they can be used for a long time as a drag, and with the understanding that that leaves the other brakes ready for shorter harder braking. They are sometimes actuated with a friction lever so that you don't have to continuously apply force with your fingers, but I've never ridden an HPV with one of those.

As it happens an air-cooled rear drum brake was found to be suitable for this purpose on tandems.

Jonathan
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531colin
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Re: Techniques for braking on steep descents

Post by 531colin »

Tandem drag brake; usually a rear hub ( or drum or internal expanding) brake. Several ways to work it, maybe a friction/ ratchet gear lever worked by pilot or stoker, or a lever worked with the pilots heel, or a a normal handlebar lever worked by pilot or stoker.
Pretty simple mechanical brake, just to “ hold it back a bit” adding in other brakes as required to adjust speed to road conditions, bends etc. I can’t think of a way you might be able to control a brake to hold speed steady?
drossall
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Re: Techniques for braking on steep descents

Post by drossall »

Anyone else feel that this thread is going downhill rapidly?
JohnR
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Re: Techniques for braking on steep descents

Post by JohnR »

If worried about overheating the brake rotors then fit these or similar https://www.merlincycles.com/shimano-xt ... 80696.html along with pads that have fins.
Usually riding a Spa Cycles Aubisque or a Rohloff-equipped Spa Cycles Elan Ti
Pebble
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Re: Techniques for braking on steep descents

Post by Pebble »

drossall wrote: 18 Apr 2024, 6:29pm Anyone else feel that this thread is going downhill rapidly?
but hopefully with less heat
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plancashire
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Re: Techniques for braking on steep descents

Post by plancashire »

JohnR wrote: 18 Apr 2024, 6:31pm If worried about overheating the brake rotors then fit these or similar https://www.merlincycles.com/shimano-xt ... 80696.html along with pads that have fins.
I looked for some measurements of the real difference these "Ice Tec" rotors make. I couldn't find any.

From the physics I doubt that you could do much other than make the rotor thicker (more heat capacity), make it bigger (more radiative/conductive surface) or add some fins to improve heat dissipation to the air, which would of course increase wind resistance. Adding some material with a high heat capacity might help a tiny bit but the amount of energy would quickly overwhelm this small effect. Making the whole system of rotor and pads work at higher temperatures could help by increasing the rate of heat loss to the air.

Brucey's sponge with boiling water works but is hard to implement. Ice has nothing to do with it.
I am NOT a cyclist. I enjoy riding a bike for utility, commuting, fitness and touring on tout terrain Rohloff, Brompton M3 and Wester Ross 354 plus a Burley Travoy trailer.
rareposter
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Re: Techniques for braking on steep descents

Post by rareposter »

plancashire wrote: 19 Apr 2024, 11:51am I looked for some measurements of the real difference these "Ice Tec" rotors make. I couldn't find any.
There have, over the years been various attempts by magazines, journos etc to run back to back test of Ice-Tec vs normal rotors although there's a limit to how repeatable you can make such tests in the real world.

The general consensus seems to be that Ice-Tec tend to fade a bit less than regular but it's only noticeable when you're really pushing the bike hard, probably not worth it for regular trail riding.

Anyway, I've just come back from a week of road riding in Spain and I'm happy to report that my normal Shimano hydraulic brakes worked perfectly all the time, didn't fade or boil or spontaneously combust and I didn't even attempt to work out heat dissipation nor did I need to apply water, sponges, or magic potions to them at any point.
Bmblbzzz
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Re: Techniques for braking on steep descents

Post by Bmblbzzz »

531colin wrote: 18 Apr 2024, 4:33pm
Bmblbzzz wrote: 18 Apr 2024, 2:26pm Sorry, incorrect term. Perhaps effective would be a better term? Although I think there probably is an element of efficiency in there: disc brakes produce more retardation (and therefore more heat) for a given pressure on the lever.
Sorry, should I have defined the term “brake”?
As above a brake is a device to retard (in this case) a bicycle. Do I need to define bicycle?
As the function of a brake is to retard the bicycle, a brake which is effective or efficient or just plain old good ; will be good at retarding a bicycle.
And as earlier in the thread, heat will be produced…
As has “ more heat than light” on several occasions.
The amount of heat is proportional to the amount of retardation and independent of the type of brake.
I think you have perhaps misinterpreted me. "Incorrect term" was referring to my own use of "efficient" rather than "effective" (or just plain old good!), which was then criticised by others as in "efficiency should be a ratio".
Pebble
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Re: Techniques for braking on steep descents

Post by Pebble »

rather a mad test


anyone ever get them to glow red whilst riding?
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