Tailwind and speed difference

[TheDarkLord]

You're both right in a way, depending on whether you're comparing force or power required. In the equations I've seen, rolling resistance (force) is usually considered as a constant factor x weight, independent of speed. But the power needed to "overcome" rolling resistance is linear with speed, because power is always force x velocity.

EG, at a typical coefficient of rolling resistance (CRR) of .006, and a total weight (bike plus rider) of 90 kg, drag force due to tires would be .006 x 90 x 9.8 = 5.3 newtons Since power is just force x velocity, at a speed of say 10 m/sec, power needed for RR is 53 n-m/sec, or 53 watts. Halving the speed would of course halve the watts.

If you think about gearing, it's easy to see why the power needed to overcome a constant drag doubles when the speed doubles. Even though the force needed at the rear wheel doesn't change, we have to either pedal twice as fast at the same force, or use "twice" the gear at the same cadence, meaning we have to push twice as hard on the pedals. Sadly, either way, power output of the rider has to double

Yes, but is CRR itself a function of velocity? According to this website, at least for motor vehicles, that seems to be the case. If correct, then it is probably true for bicycle tires as well.

 

[Alex Simmons]

You're both right in a way, depending on whether you're comparing force or power required.

Yes, my bad - I meant power required to overcome resistance, not the resistance value itself.

 

[Alex Simmons]

Yes, but is CRR itself a function of velocity? According to this website, at least for motor vehicles, that seems to be the case. If correct, then it is probably true for bicycle tires as well.

I was aware it can change with temperature, although increased speed would alter the tyre temperature and resistance accordingly.

 

[dhk2]

Yes, but is CRR itself a function of velocity? According to this website, at least for motor vehicles, that seems to be the case. If correct, then it is probably true for bicycle tires as well.

The equation in the link which has CRR varying with speed was new to me....but then a lot of things continue to be

www.biketechreview.com published results last year of a "home" test to determine CRR of many common race tires. The data indicate a big variation in CRR and power requirements. However, the tests were done on a set of small (79 mm dia) rollers, which I think might exaggerate differences vs riding on the road.

It's noted on the test results that CRR may increase by 50-100% on actual road surfaces. The final note states that tire temps rose 20-40* F during testing vs an estimated 4-8 *F on a flat road surface.