Active Spokes?

bahzob

New spin on an old contradiction that best wheels going up may not be so good going down.

http://www2.trainingbible.com/joesblog/2008/01/active-spokes.html

Not to my taste but interesting to see if it takes off.

aracer

"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.

andrewgturnbull

Hi there.

Is it April 1st already?

How do people expect that they can get something for nothing?

In order to move the weights outwards you are using energy, both to physically move the mass and to work against the retaining springs. Yes, you might save some energy later as you'll have more momentum - but you put that energy into the system in the first place!

I'd like to see some power test data from a lab test without the human variable...

Cheers, Andy

ps apologies if you've read this already - I posted this on the timetrialling forum yesterday.

Raph

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

PPPS - another way of looking at the flywheel thing - if a wheel is on a static hub, the energy comparison between spinning and not spinning is obvious. The everage mass of the entire wheel is still static so the momentum is nil. When it's moving at the rate so that the lower edge is static and the top is moving twice as fast, the bottom half negates what the top half does, so the energy is the same as if it were moving at that rate without spinning. The average momentum of the entire wheel is still that of the centre of the wheel. If you then stop that wheel spinning but keep it moving at the same speed, you have to slow down the top edge to half its speed and speed up the bottom half to that same speed, and everything inbetween proportionally... so the momentum remains the same.

...I think!

aracer

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

Raph,

I agree with most of that apart from where you seem to suggest that the rotational weight doesn't matter at all. It's a tiny effect next to wind resistance and not as big a drag as overall weight, but rotational weight makes a difference especially when spinning up the wheels.
It's all down to angular momentum as you probably know. A torque is needed to create the angular momentum of the wheel, the torque is the fork pushing the hub whilst the wheel is anchored to the road. A larger torque is required to achieve the same speed of rotation in a heavier rotating mass. So more force is required to spin up a heavier wheel.
At constant speed it is negligible, but when climbing or accelerating it plays a small part.

As for that 'invention', what a waste of time :P

aracer

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.

Raph

The torque aspect of accelerating the wheel makes me rethink this (for the n'th time!)

If it stands, then the pumping aspect of climbing would mean that rotational weight of a heavier wheel plays a part, since you'd have torque issues with every speeding up/slowing down.

I still can't get to grips with the rotation issue though - when you set off, if you were setting off in mid-air, no contact between front wheel and road (never mind how you're moving - let's concentrate on front wheel/road for the moment) - you accelerate the mass of the front wheel with the rest of yourself and bike, nothing's rotating. Now the same action with front wheel on the road - hmmmm. The road, being static, pulls the 6 o'clock bit back as you move off, The average mass is still the same but.... damn! I think you're right. you have extra momentum in the rotation. (thinks...) So there is more momentum in a spinning wheel even though the average momentum is the same... Oh dear - school physics was a long time ago!

So I think from that, subject to checking it over a few more times... I have to reverse my stance for the last 20 years that weight on wheels makes no more difference than elsewhere. Sh1t!!

As for the invention though - at the point when the weights spring outwards, that's when you have to put a bit more energy into the wheel - spin an umbrella until it opens itself centrifugally, and it will slow down at that point, since the speed of the spokes when closed will represent many turns per second, but as they move outwards that same velocity represents fewer turns - I assume the same would be true of the weights, as they slide outwards on the spokes, that's when you need to put more energy in to increase their velocity so as to keep the rotaion of the wheel from reducing.

There's no such thing as a free lunch!

stevec205gti

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

andrewgturnbull

Hi there.

Raph, I'm glad you cleared that up for us...

Cheers, Andy

Smokin Joe

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

wildmoustache

the thing about these discussions - and i'm not disagreeing with what's written above - is that a) they tend to ignore the fact that people attack on hills ... meaning that a light rim is important in road racing b) they tend to regard human power output and fatigue as proporitionate ... when in fact the harder accelerations needed with a heavy rim are ultimately more tiring even though you do "get it back" in speed as the wheel holds momentum.

aracer

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.

Smokin Joe

I can see the point of them. As far as I recall they were developed for use in a disc wheel and were intended for time trialing where the ability to climb or accelerate quickly is not really an issue. The theory being that once the wheel was rolling and the weights moved to the outside it would have a flywheel effect and make momentum easier to retain. The idea seems sound in theory, who knows how it works in practice.

Though I can't remember the exact details, as I said before I am pretty sure the idea of movable wheel weights are banned by the UCI, which makes one think there may be something in it as that organisation would like to see everybody back on the bikes they used in 1955.