Missing the point. Weight on the upstroke is balanced exactly by weight on the downstroke from the other leg. Now, tell me how the actual motion and forces work out to force the leg around in a circle. And I know this is tough because computers and all that, but please draw it out for us, or at least for yourself. This is a math problem, not a philosophical exercise in logic.

Make light of it all you want.

It demonstrates you don't know what you're talking about. Were you claiming to be a bike mechanic, you would have just confused an inner tube with a brake lever.

It kinda ruins your credibility.

^^^
This is true. A practitioner skilled in an art rarely mistakes a screwdriver for a wrench. This is more than just word play going on here.

http://cdn.memegenerator.net/instanc...0/36761303.jpg

http://www.bikeforums.net/showthread...faster-Discuss
:thumb:

/end

Well, I will say that certain frame dynamics have sped me up in my move from a steel racing bike to a stiff CF bike, but all of that has to do with handling characteristics.

Evidently not the end. We all know where you stand, but I think this discussion has made it further than the last. Care to join?

Evidently, he has as much evidence as everyone else arguing in this thread...

The difference probably isn't that much at all.

But to claim it's zero flies in the face of physics and the 2nd Law of Thermodynamics. Good luck with that.

It's kind of telling that no one's replied to my question about why you can't push on a string....

I'm arguing basic physics - multiple transfers of energy simply CAN NOT be as efficient as directly transmitting that energy to the designed target.

Period.

No evidence necessary.

And quite a bit of evidence would be required to support any claim to the contrary.

OK. Perhaps I should have said data. Because, as you acknowledged, it probably doesn't make much difference. If the difference in performance between modern frames is less than 1w who cares?

Given bike manufacturers propensity for touting the performance benefits of even the most marginal design changes, it's hard to believe that someone wouldn't have quantified this improvement by now. I suspect as far as power transmission goes, frames reached the 'stiff enough' point many years ago and it is a non-issue now.

But carry on if you want to try and convince everyone that there is substantial (or even measurable) power lost due to frame 'distortion'.

edit: I just re-read your post and I'm not sure what you mean by 'multiple transfers of energy'. I believe all frames flex.

True, but beyond a certain point it doesn't really matter. On a bike you've got aero losses, rolling resistance, drive train losses. Once you have the frame stiff enough that its losses are negligible the stiffness simply doesn't matter. Imagine subtracting 1g from from a bike, you'll be faster, but by such a small amount that its irrelevant.

Consider the example I stated above. k=2e5 N/m, x=1mm, The total stored energy is 0.1 J per crank. Storing that much energy at 80 RPM = 1.33 Hz (two cranks per cycle). Total Power = 2*0.1J*(1.33 Hz) = 0.3 W.

The maximum you could possibly lose between a modern frame and an INFINITELY stiff frame is 0.3 W. Most of the aero concerns are ten times that value. Drive train losses are typically 2-4% (depending on gearing). You likely lose more power from a sub-optimal gear choice than to frame flex.

Of course you can push a string, it just isn't very satisfying.

You're an idiot. GTFO.

If it matters a little, it matters. All else is cost/benefit. Some are willing to pay for any benefit. I lost a race by possibly this amount this weekend.

Yes, but this is why the marketing is XX% stiffer, rather than quoting a real number. Quoting a real number is difficult and likely to completely underwhelming.

Okay.

I'm not missing anything.

Let's keep with a requirement of 100N of force to lift the leg on the upstroke. If you are pedaling down with 200N of force, that includes the force of gravity acting on the leg on the downstroke.

If I am not pulling up at all -- if my leg is dead weight on the upstroke -- then 100N of the downward force must be used to push up my leg.

If I am pulling up on the upstroke with 75N, now I only need to use 25N from the downstroke to help lift my leg. 175N of the downstroke is now going to the pedals.

And if something is pushing my foot up on the upstroke with an extra 24N, then that frees up another 24N to go to the drivetrain.

Right?


'kay

http://www.trainright.com/assets/new...ockdiagram.jpg

The goal is to pedal in circles. The reality is that the force vectors are almost always down, cyclists do not applying consistent amounts of force to the drivetrain throughout the rotation, and power peaks at the 3:00 (90º) position. When you are at the 12:00 and 6:00 positions, the force is straight down, and thus no force is going to the drivetrain (hence the "dead spot.")

Naturally, this brings us to your favorite chart....

http://members.home.nl/vd.kraats/lig..._Power_Tot.jpg

(And yes, it looks like it's the same basic process regardless of whether you're climbing or sprinting.)

The data on force vectors during pedaling are determined by direct observation. Unlike just about anything else in this thread so far. ;)

Sigh... The force vectors pointing away from tangential to the pedal circle do not turn the pedals. Only the components of force which are directed tangential to the pedal arc contribute to powering the bike. Please, draw the problem. Stop saying the same thing over and over and cut and pasting pictures others have quoted. We are talking about forces applied to the drivetrain from sources other than the legs (i.e. energy stored in the bottom bracket). What you have posted is a word and picture salad.

http://imgs.xkcd.com/comics/science.jpg

You know, I've been giving this some thought since the first time I heard of this whole "stiffer bikes are better for sprinting" issue.

I don't believe that it's at all a given that any energy spent in deflecting the frame ends up back providing useful work to propel the bike forward.

I think that as some have touched on, the mental issues may well be important. When you are really hammering on the thing. Rocking the frame it seems entirely possible that the more flex the frame has the more likely you are to back off a bit unconsciously.

I find the scarcity of empirical evidence, given the money and interest, to suggest that while there may be an effect that you can measure between some 1970s Schwinn varsity and a brand new carbon fiber superbike, the magnitude of the effect between bikes in a given class is probably very small.

Torque-induced precession.

I think your stance is entirely correct.

Umm, yeah you are.

The energy used to lift your leg on the backstroke is pretty much directly offset by the energy returned to the pedals by the falling leg on the front of the stroke.

Look up "conservative force".

You do know what a conservative force is, right?
Really? How can anyone determine the forces placed on the pedals by observation - looking at them.

Direct measurement, I can believe. Observation? No way.

"Hmm, that looks like 172.6 N at 68 degrees past vertical."

Riiight.

Once again, the use of incorrect terminology is telling.

Brian: I've explained it to the best of my ability, and linked to a site which explains it in more technical terms. If you don't get it, that's not my fault.

I made that point earlier as the reason that mfr's don't tout stiffness nos. Not only would it discourage buying up within a given brand because say the difference between a Tarmac Comp and a SL4 would not be that significant, but it would turn into liar's poker from brand to brand. Plus as discussed because of diminishing return and average guys not requiring an uber stiff bike, some buyers would use this as a discriminator against buying up. Inevitably the conversation would turn to correlating frame stiffness with rider's strength. A 120 lb 16 y.o. doesn't need the frame stiffness of a 190 lb CAT 3 racer...because a light weak guy won't flex the frame much. Don't believe mfr's want to go there.

It isn't Brian. Most here rebut what you wrote. We get it, you don't.