Asymmetric chainrings.

[rfryer]

if an oval chainring is oriented so that the leg speed raised on the downstroke vs a round ring (i.e. 'the biopace way') then the argument is that this increases the kinetic energy and inertia available to carry your legs through the dead spot.

I see what you're saying, but don't think Biopace is the solution. The "problem" that both asymmetrics are trying to solve is the lumpy power delivery inherent through using our legs as pistons. This has a tendency to accelerate the bike through the powerful part of the stroke, then slow during the dead spot.

Both types of asymmetric chainring aim to smooth out this power delivery.

Biopace's solution is to use some of that spare additional energy during the power stroke to store rotational kinetic energy in your feet, that will help in pedalling through the dead spot. I'm not convinced by this; my feet aren't massive enough to store the kinetic energy to get me through the elongated dead spot. YMMV

The more "normal" asymmetrics smooth out the power delivery by allowing your feet to move more slowly during the power stroke (when you can support a higher gear), and then more quickly through the dead spot (when you need a lower gearing). By shortening the dead spot, you smooth the power delivery, and also reduce the time spent in the more fatiguing part of the pedal stroke. More efficient, less tiring, win all round. I should be writing marketing speil for these guys

[Brucey]

nonetheless, it is both the foot and the leg that are going downwards and they are pretty much stopped at the bottom of the stroke. It may not seem like it, but there is quite a lot of kinetic energy stored in ~5kg of lower leg/foot. Provided the ankle joint isn't flexed towards the bottom of the pedal stroke, then this interia can be (and is) turned into useful work on the pedals. Either that or everyone I have seen measured on a pedal force machine pedals 'stupidly' i.e. pushes hardest at the wrong time.

One counter-argument to 'conventional' oval chainrings (and especially those used by Sky which have an abrupt change in radius; the rings pretty much look like round rings with two flats on them in fact) is that if the vertical speed changes more suddenly than normal, this makes for a larger force of shorter duration; this might increase the chances of the ankle joint flexing, thus losing some of the otherwise 'free' force on the pedals that the vertical deceleration generates.

cheers

[rfryer]

I don't think that magnitude of the pedal force (which I agree is surprisingly high toward the bottom of the pedal stroke) is of much use in this context. What we care about is the force perpendicular to the crank, pushing the chainwheel round, and this is generally accepted to be at a maximum around the 3 o'clock position (I've seen this in literature, and also on the display of a WattBike that I was pedalling carelessly).

So I think that I'd argue with the proposition that "this interia can be (and is) turned into useful work on the pedals", either in the normal case (see evidence above), and with Biopace. I can't see how the major, vertical component of the inertia can add much to the pedal stroke within about 40% of bottom dead centre, when we really start to want that extra power.

[ANTONISH]

10% ??
10% of what?

Exactly- one hears lots of claims about performance enhancing products and as Brucey says if the improvement is that good everyone would be using them.
IMO if such a device gave a 1% improvement (say about 18seconds on a 30min climb) all professionals would use them. The other thing I notice is that Froome seems to have a markedly high cadence and I would have thought there was less advantage for him than say Rodriguez who seems to have a much lower cadence.
I believe that Sir Bradley Wiggins has reverted to circular rings .
However there does seem to be some scientific thought in these things unlike the highly amusing L shaped cranks.

[Brucey]

I don't think that magnitude of the pedal force (which I agree is surprisingly high toward the bottom of the pedal stroke) is of much use in this context. .......... I can't see how the major, vertical component of the inertia can add much to the pedal stroke within about 40% of bottom dead centre, when we really start to want that extra power.

well, it does do, because until the vertical velocity compenent is zero, it always has a tangential component. If this energy it wasn't used you would lose significant chunks of power, i.e. rider effort for no gain.

What happens mid pedal stroke is always about the same, but happens more or less often depending on the chainring design.

Arguably what happens when the cranks are more nearly vertical is the big difference between chainrings. Because it is a human being, not a machine attached, what goes on is complicated and will vary from one rider to another....

cheers

[[XAP]Bob]

Whilst there is a small tangential component the majority of the force is applied in a radial direction (and does no useful work)

I think we *can* agree that it's not entirely obvious, and there may be different effects people are trying to use...

[Brucey]

Whilst there is a small tangential component the majority of the force is applied in a radial direction (and does no useful work)

...and the radial compenent therefore costs you nothing if it is an inertially generated force...?

BTW at the 5 o-clock crank position, a vertical force of 10kg still has a tangential component of ~5kg, which is a lot better than a poke in the eye with a sharp stick, especially if it comes 'cheap' metabolically speaking...

I think we *can* agree that it's not entirely obvious, and there may be different effects people are trying to use...

Absolutely....

cheers

[ukdodger]

Froome beat Contador by a few seconds yesterday because of aerodynamics; in the final few kilometres Froomy stayed down in position on his aero bike and Contador rode out of the saddle on his roadbike; no contest.

If Froomy were packing another 10% with super chainrings then

a) he'd have won by a lot more and

b) everyone else would be using them too....

cheers

Surely it would be easy to set up a mechanical test rig to prove the increased efficiency of it if it exists.

[[XAP]Bob]

Not really, the efficiency is biological, not purely mechanical.

[ukdodger]

Not really, the efficiency is biological, not purely mechanical.

How's that exactly?

I thought about this last night. If you were to arrange both the elliptical and round chainwheel with the same chain around both and with their respective cranks at 3pm and 9am (assuming the apex of the elliptical chainwheel is at 12noon when it's crank is at 3pm). Then letting go of each crank simultaneously should, if the mechanical advantage of the elliptical chainwheel is greater than that of the round chainwheel, result in the elliptical chainwheel winning the tug of war with the round chainwheel. Assuming too that all else is equal.

[Mick F]

Not really, the efficiency is biological, not purely mechanical.

What about psychological too?

[ukdodger]

Not really, the efficiency is biological, not purely mechanical.

What about psychological too?

Or even plasibo perhaps.

[Mick F]

I agree that if asymmetric rings work, they must be able to be demonstrated to work though trials and tests on a test-rig in lab conditions.

We'll be getting oval rear wheels next ...........

[Brucey]

the problem with this is that (as I have mentioned before...) the effects of rider habituation cannot be isolated in any such lab tests. Riders will pedal differently on different chainrings, and will become habituated to one chainring type potentially at the expense of efficiency with other chainring types. Wait long enough for habituation to another chainring type, and the the rider's athletic potential may have changed in the meantime.

Devising a set of tests that convincingly overcomes these problems is somewhat tricky, I'd say.

cheers

[ukdodger]

the problem with this is that (as I have mentioned before...) the effects of rider habituation cannot be isolated in any such lab tests. Riders will pedal differently on different chainrings, and will become habituated to one chainring type potentially at the expense of efficiency with other chainring types. Wait long enough for habituation to another chainring type, and the the rider's athletic potential may have changed in the meantime.

Devising a set of tests that convincingly overcomes these problems is somewhat tricky, I'd say.

cheers

True but the question is does it work or doesnt it. If it cant be demonstrated that it does you have to assume that it probably doesnt and it's use has more to do with hope than anything else.