Using PowerTap test to decide a new frame?

peterpen

Some places to check:

http://weightweenies.starbike.com/p...ight=tour+tests

and

http://fairwheelbikes.com/forum/viewtopic.php?t=2617

robkit

i reckon the only possibly useful test would be to have a powertap and srm on the bike.

then compare the relative difference in power between the two meters, on different frames.

if there were any measurable loss between the cranks and the hub due to flex, then maybe this would find it.

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I am my favorite rider.

HammerHead

I've got a 15 year old steel frame set up identically to a new (claimed to be) uber stiff carbon frameset. Same post, crank, bars, stem, pedals, seat, wheels - use the same PT SL. Bike position is set identical on both bikes: saddle hight and setback, bar drop and reach. (give or take a few mm)

Doing sprints in training, I've never come closer than 80 (peak) watts on the steel bike to what I do on the carbon bike. Could be coincidence, random varience, losses in frame. Who knows. But the numbers are consistantly higher by about 80 to 100 watts on the carbon bike.

I'm an engineer too.

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I'm only truly happy when I'm anaerobic.

ahaile

Unless the difference is due to pedaling ergonomics, not inefficiency in the mechanism.

Presumably, a flexy frame would have a higher max pedal velocity, and therefore a lower max torque (aren't pedal velocity v. torque charts roughly linear?) and potentially a lower power per stroke.

beerco

So do you really think that the frame is sucking up 80w? (seriously)

Is there something wrong with my gedanken experiment?

djconnel

This analysis concluded frame flex may reduce the peak power within a pedal stroke, but generally has no significant effect on average power over a full pedal stroke:
http://www.bikethink.com/Frameflex.htm

It will be interesting if you observe a difference.

Dan

beerco

Very cool link. I guess I was way off base. If one were to assume that all of the energy was lost in his model, that would be 6.5 whole watts in a 1300w sprint!

HammerHead

Sigh....

"Could be coincidence, random varience, losses in frame. Who knows."




I wouldn't dare consider your gedanken and how you experiment with it.

__________________
I'm only truly happy when I'm anaerobic.

djconnel

In particular, the plot on:
http://www.bikethink.com/Power_output.htm

shows no power loss evident to the resolution of the plot. So as you said, even 6.5 W/1300 W is an upper bound estimate.

That said, good sprinters seem to like stiff bottom brackets, not that a whippy Vitus Alumninum frame seemed to slow Sean Kelly, much....

Dan

jbvcoaching

How do the max 5s numbers compare?

vladav

Thanks a mil!
Dave

giannip

If you're looking at a 3-4k frame I seriously don't think that you should be that worried about frame flex.

If it's strong enough for the like of McEwen / Boonen........

Uhl

Even if formulas and experiments don't show a significant difference in efficiency of a "stiff" frame, there's nothing wrong with wanting a frame with that ride quality. There's something to be said for a bike that feels stiff and seems to rocket forward when you stand up and sprint...even if it's just a subjective observation.

You may even gain a small benefit from the placebo effect...if you think your frame is more efficient, you may go faster!

djconnel

Right -- feel can be important, I agree. I was commenting on the experiment to measure a tangible, quantitative effect.

There's been a big fuss made, for example, about crank length, with Lennard Zinn having published results showing it has little effect on power production. To me, this is another issue of the value of comfort -- I don't care if I am able to produce the same power, unless I can do so at least as comfortably. I doubt a rock in my shoe would much reduce my power production, either.

So I think frame stiffness is a matter of "feel" more than a fundamental biomechanical efficiency matter. But feel is still real.