Active Spokes?

"When you see the simple concept in action you’ll probably do the same dummy slap I did and wonder why you didn’t think of it."

Only if I can slap the same person he did - for being enough of a dummy to fall for it.

"And since climbing a hill on a bike is essentially a series of alternating decelerations and accelerations with every pedal stroke, we certainly don’t want heavy rims when climbing a hill."

Wrong, wrong, wrong. If you do accelerate and decelerate with each pedal stroke, then you actually want as heavy a rim as possible to minimise the speed variation (though personally I'll just take as light a rim as possible when hill climbing - preferably without extra weights on the spokes either).

Mr Friel just lost a lot of credibility with that article, though I'll be generous and assume he does know what he's talking about with training, just not with physics.

There's a lot of commonly accepted baloney about the physics of bicycles and cycling.

I don't claim to have thought through the wheel thing completely - but bear in mind that "rotating mass" just means that when the top edge of the wheel - the 12 o'clock bit - is travelling twice as fast as the rest of the bike, the 6 o'clock bit isn't moving at all. The bit at 3 o'clock which is being lifted through your sweat-soaked effort is also balanced by the bit at 9 o'clock which is falling under its own weight, saving you that exact same amount of effort.

The fact that rotation is constant acceleration towards the centre doesn't mean you're effectively constantly using energy to rotate that weight - planets wouldn't spin if that were the case.

I haven't yet heard a good case to explain why weight in the wheels is more detrimental than anywhere else. Light tyres are often better because a thinner tread grips the road better and offers less resistance to undulations in the surface, which is more significant than the actual weight advantage - this might give the impression that it's a more significant weight saving than the same number of grammes saved on a stem or seatpost.


Weight saving in general is a good thing, and the argument that you can use weight because what you lose on the ups is regained on the downs doesn't hold water, since simply put you lose more than you gain. The faster you go down a hill the more your gain is diminished by wind resistance, whereas you gain nothing from the smaller wind resistance when you go up a hill. Also while going down the hill you don't get any of the energy back that you put into getting up it, therefore whatever extra effort was put into humping extra weight is simply lost. Keeping your speed range as narrow as possible is a good way to minimise the losses you get when you're going fast to make up for having gone slow. This is before even considering the mundane matter that the faster you go down a hill the more likely you are to need to brake round bends, further minimising any regaining of what you've lost on the way up.


" you actually want as heavy a rim as possible to minimise the speed variation" - why do you want to minimise speed variation? If the legs were electric motors it would be most efficient for them to produce constant speed, but they work alternately pushing downwards, or at least that's the most powerful part of the stroke. Trying to make an artificial constant power from that is irrelevant. The loss you incur from putting energy into a theoretical flywheel won't all be regained once that flywheel is turning. Muscles work more efficiently when they produce movement - pushing against something that doesn't want to move is not good for muscles, hence we use low gears to get up hills, gears at which we can get the pedals turning at a reasonable rate even though the bike might be moving slower than that overtaking snail. The acceleration/deceleration doesn't matter because that's how legs work. For the bike it's a slightly different matter in that the stresses on it are greater at the peaks of acceleration, but then that's just tough, man and machine have to get on somehow.



PS - tell yer wot, get a motor to drive a generator, connect the two so the generator powers the motor, put them both on your bike and hey presto, smile sweetly as you overtake everybody up that hill without even pedalling!

Raph wrote:

" you actually want as heavy a rim as possible to minimise the speed variation" - why do you want to minimise speed variation?

Because wind resistance is non-linear, so keeping a constant speed results in less work than doing the same average, but with variation in speed. Of course the difference is negligible, but the point is that keeping the speed constant and getting rid of the micro-accelerations isn't actually a bad thing.

simbil1 wrote:

At constant speed it is negligible, but when climbing or accelerating it plays a small part.

No only accelerating, not climbing (for that only the static weight matters). I'm sure you knew that and just made a mistake, but worth pointing out. We actually all seem to be just quibbling over the detail here - the point is that adding weights to your wheels and so making their static weight more, and undoubtedly not helping the aero, is a bad thing.

See - all those years ago those Spokey Dokeys that came free in your breakfast cereal would have been ideal for my dad's bike on his club runs, but he always said they were rubbish

I seem to recall such an idea being tried before, and in fact being banned by the UCI.

So not a single person who thinks they're a good idea? We got this discussion all wrong - would have been so much more fun to have got somebody explaining why they help you go faster.