[Retric]

Rotating a light bike like that takes a lot less energy than you might think. Kinetic energy is also V^2 and the out edge of that wheel is probably doing 30+mph as it's down hill, so IMO it's actually believable.

Granted, it would be much easier to get a motor to do the same thing.

[bch]

Rotating a light wheel that takes less energy, too, but that means too that wheel isn't a big power store. A light weight wheel is precisely the opposite of a flywheel; it should stop nearly immediately because it's _not_ a heavy rotating mass with lots of stored energy. If we're talking about Ryder Hesjedals video, he's still attached (right foot) to his bike, everything appears to be static (he's not sliding anymore) and his rear wheel appears to be pressed into the ground. Then when he unclips his foot, his bike starts spinning around. Weird[0].

[0] Conclusion based on watching https://www.youtube.com/watch?v=ynLMfzLTc8M at 0.25 speed.

[laxatives]

A light wheel with low resistance, like a highly tuned performance bike, can hold energy for a long time. My entry level road bike will spin for well over a minute with a light tug, maybe equivalent to 5-10 mph. He's going well over 30 mph in that video.

I'm not arguing whether this may or may not be an assisted bike, but what you're saying about a wheel is patently wrong.

[bch]

Are you talking about light resistance in the bearings?

I think you are (forgive me if I'm wrong), but that doesn't matter. That is not an example of the wheel "doing work". That wheel in the video looked stopped at the end of the crash, and even if it wasn't stopped, the video looks really awkward. Sure, his crash started at 30mph, but ended at 0, and his wheel looks to be firmly placed on the ground, immobile. For sake of understanding though, it'd be interesting to know what power could be stored in a theoretical wheel that could stand in for Ryder's.

Eg. a pair of Mavic Cosmic Carbone Ultimate[0] apparently weight 1185g. The rear will weight a bit more (because hub/drive/gears), although that weight will be at the center of the spin, so I _guess_ not contribute significantly to the velocity of the wheel (angular momentum). So, how many watts are stored in a ~700g wheel that was moving at 30mph? That will determine the ability to do work (like spin a bicycle on the pavement).

[0] http://www.bikyle.com/MavicRoadWheels.asp

edit: expand point, suggest exercise

[bch]

This is not my area of expertise, but to get things started, I went to a site[0] and looked up some formulas and specs, and came up with:

outside circumference of wheel ~2200mm (700c wheel w/ tire) 30 mph == 0.5 mpm (Miles per minute). 0.5 mile == 804500mm a 2200mm circumference will need to roll ~365.68 times to cover that distance. If that distance is covered in 1 minute, that means 365.68 rotations per minute.

This: Ef = 1/2 I ω2

wants I and ω.

I = m r squared.

m == 700g (this isn't properly distributed here, but it's a start)

r (at 2200mm circumference) == 350.14

0.7kg * ((350.14mm)(0.001m/mm))^2 == 0.7 * .122598019 == 0.085818613 kg/m2

1 rad/s = 9.55 r/min (rpm) 365rpm == 38.29 rad/s

so if this is at all correct:

Ef = 0.5 * 0.085818613 * 38.29^2 == 0.5 * 0.085818613 * 1466.1241 == 62.910368373 Joules

1J/s == 1watt. So with this fudgey math above (assuming it's even correct) we're working w/ ~60 watts(max, for an instance, then decreasing). I don't even know if that's enough to spin a bicycle around like was shown. I hope somebody that actually understands this field can chime in and fix my bad assumptions (which I think err to supporting this was strictly the spinning wheel (not a motor)) and what this means. I'll do practical tests later when I have my bike.

[0] http://www.engineeringtoolbox.com/flywheel-energy-d_945.html

[stouset]

Or simpler, you can just run the experiment yourself.

https://www.instagram.com/p/sh0fSPirPW/?modal=true

[bch]

Yeah, good find.

I saw that earlier too, and forgot about it. So the question is less about whether its even possible, but whether or not it was likely in Ryder Hesjedal's circumstance. And the stakes, circumstances, etc aren't exceptional enough to warrant a commission.

[laxatives]

60 watts is definitely sufficient to power a bike like that. I know I produce 50-80 pedaling at a "rest" pace, maybe 10-12 mph. Assuming your math is correct, it seems sensible that a wheel at 30 mph could move the bike. It seems like the biggest assumption here is that the wheel wasn't stopped in the crash and that it hovered above the ground, or sustained sufficient RPMs it was slowed.

[Retric]

Your assuming the wheel stopped rotating while it was lightly touching the side of the road. Visually you don't see the spokes on the low wheel, but you do on the vertical one, so it was still rotating.

If you think about it there was probably under 1 kg of force between the side of the wheel which has minimal grip and the road surface while he was attached for what 2-5 seconds?. Spin a wheel and try and slow it down from lightly touching the sides like that and it's going to spin for a while.

Further, while on the side the contact point of the wheel had a ~5 feet of leverage to the pivot point of the handlebars and the other contact point was a wheel. So, it would take very little force to get the bike rotating.

PS: I don't have a bike right now, but it should be easy to test if you do.