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What we need is a Man of Science to work out the energy generated by a spirited descent of Ventoux by a laden tandem.
Right then
The major component of this would be kinetic energy (rather than the component of gravitational potential energy you are resisting by going down a slope), so lets estimate this.
Assume m = 175kg (tandem, two riders, loaded)
At thirty five miles an hour, v = 15m/s approx.
Kinetic energy = 1/2 m * v squared
= circa 19,700 joules (or Watt-seconds)If you wanted to stop all of this dead in ten seconds, you'd need to apply 1970 watts.
Or five seconds, 3940 watts.
Olly398-
Assume m = 175kg (two riders)
That sounds about right for my tandem. A loaded touring tandem could easily be 50kg over the bare rider mass without even considering camping.
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What we need is a Man of Science to work out the energy generated by a spirited descent of Ventoux by a laden tandem.
Right thenThe major component of this would be kinetic energy (rather than the component of gravitational potential energy you are resisting by going down a slope), so lets estimate this.
Assume m = 175kg (tandem, two riders, loaded)
At thirty five miles an hour, v = 15m/s approx.
Kinetic energy = 1/2 m * v squared
= circa 19,700 joules (or Watt-seconds)If you wanted to stop all of this dead in ten seconds, you'd need to apply 1970 watts.
Or five seconds, 3940 watts.My understanding* is that peak loads are not a problem per-se, disc brakes are pretty good at dealing with big decelerations (albeit, not again and again and again), it's the desire to have a brake constantly applied to keep speeds within a certain range where discs start to run out of answers as they generate more heat than the system can radiate away.
At which point the fluid can start to boil and goodbye to the brake until things cool down again.
So we're not looking for a dead stop, we're looking at applying (say) 500 watts opposed to the direction of travel, as it were.
Or to put it another way, venting 500 watts worth of kinetic energy as heat.
*Which may be wrong
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The major component of this would be kinetic energy (rather than the component of gravitational potential energy you are resisting by going down a slope), so lets estimate this.
Good science but let's try and account for GPE (neglecting air resistance first, but then trying to approximate that).
175kg going down a 10% hill at 35mph (v=15m/s), so they are losing elevation at a rate of 1.5m/s.
So the GPE difference for 1.5m elevation difference is 175 * 1.5 * 9.8 = 2572J, since this is per second it's a healthy 2572W.
Of course, some of the energy 'gained' from GPE is eaten up by air resistance and rolling resistance.
A quick play with http://www.kreuzotter.de/english/espeed.htm shows that you'd need 520W to power a tandem with a total weight of 175kg along at 54kph (15m/s), so that is a reasonable approximation as to how much power will be sapped by air resistance and rolling resistance at 15m/s.
This leaves ~2000W that needs to be constantly dissipated (without overheating) in order to keep the speed at a constant 35mph down a 10% hill for a 175kg tandem. Oof.
gbj_tester
Dammit
Greenbank
The other option might be to get some discs laser cut that have a much larger surface area and can radiate the heat out more ably than a standard 203mm disc.
What we need is a Man of Science to work out the energy generated by a spirited descent of Ventoux by a laden tandem.
On my cycle to Excel last night I'd been imagining how we might use a magnetic drag brake as seen in (for e.g.) turbo trainers - they seem to have no issue generating 300 watts of retardation for an hour without cooking themselves. Lot of mass there, of course.