Where are the numbers relating stiffness to speed or power?
 

As many of you may have seen, there is another Ti vs. CF discussion active in a current thread. Of course, one of the main themes is the stiffness/comfort combination achievable with CF and its relationship to attainable speed. For the moment taking no side on this question, I am wondering if there are numbers to support the contention that an appropriately stiffer frame (e.g. at the BB) facilitates power transfer from the rider to the wheels. Everyone talks about the feeling of power transmission, but I haven't seen any experimental results.

This shouldn't be so hard to measure, at least not for a well equipped corporate (bike manufacturer) or academic laboratory. It could be as simple as measuring power at the rear wheel (Powertap wheel) and/or rear wheel speed at a set resistance on a trainer stand for any given frame. Or power applied to the crank could be compared to power at the rear wheel. Different pedaling cadences, in and out of the saddle, acceleration bursts, etc. could be examined. The results could then be correlated (or not) to frame stiffness measurements. One aspect I am not so sure of is could this experiment be done with a human pedaling the bike, or would a pedaling machine be needed to give reproducible results. For sure it is important that the pedaling motion reflect the normal idiosyncrasies of a human rider.

Does anyone know whether this has been done?

I've never seen the slightest evidence that it noticeably affects anything except subjective feel.

I've never seen any data on this, and for what it's worth, Saint Sheldon called it bunk back in the day.

As far as I can tell, no such studies exist.

Stiffness does appear to improve control in certain situations, notably on the descents, and probably during sprints. Otherwise, I'm not really aware of any reason why frame stiffness would genuinely improve power transfer.

Imagine my surprise when I saw Cervelo's engineering page stating that stiffness has an actual % gain in power transfer.

"Power transfer is one aspect of BB stiffness, but the loss is at most 0.3% to 4% depending on if you’re cruising along or a world class sprinter."

Wow. Yeah. That's definitive. :D

I don't suppose you found a copy of the actual research they used? What bikes were used as a comparison? Is this a linear increase? Do increases past a certain point offer diminishing returns? How much wattage do you need to pour onto the pedals before BB stiffness matters? What about the rest of the frame?

Considering how wide-spread the belief is, the data seems pretty thin on the ground. But, I have an open mind. If you've got the data, I'd be happy to see it.

Some very knowledgeable and experienced posters here made really good arguments why there's no correlation.

The one thing though is pretty much all the pro sprinters want a stiff as bike as they can get. Maybe it's just in their heads but it seems like all out sprints need max stiffness

I wonder if on a noodly frame the excess scrub from the rear tire would have an additional loss compared to a stiff frame that doesn't have the rear wander as much.

Perhaps any relation to force vectors as well from the rear being out of plane from the front end?

Here's the thing: I'm tired of wondering. Why is no one measuring all this and publishing the results?

And that's very important. You only approach your potential if you don't feel like you're taking excessive risks at it. Riding a bike, like many things, has a big mental component.

It may well give them better power transfer under high wattages. But no one appears to released any data, or quantified the difference.

I would also guess that if frame flex does make a difference, most new race bikes are all in the same range of stiffness. There could be a huge leap from an 80s era straight top tube frame to a Giant Propel, and very little difference between a CAAD8 and a Propel.

I'd also point out that sprinters were sprinting long before they had today's über-stiff frames. ;) I have no idea of the differences in speed or tactics due to frame stiffness, but obviously they could do it.

Sure, I have no qualms with that. But if you're going to specifically claim that "stiffness increases power transfer," I'd like to see some numbers.

It's added to the pile of excuses for us to upgrade our bikes along with several dozen others, including my personal favorite--its SO much lighter than my current bike! :roflmao2:

Of course stiffness helps power transfer. If nothing else, then it is because the load path is shortest with a stiff frame, and the shortest load path is the most efficient as a first principle.

One thing to ponder when people start trying to reason their way around the need for frame stiffness... in a "whippy" frame, there is no reason why all that energy stored in the frame (by bending it) has to come out of the frame to the road. There are a myriad of loss mechanisms once you start lengthening the load path, not the least of which are in the legs themselves by affecting the pedaling motion (feedback). In a flexy frame of carbon fiber, it might be worse as it is likely the energy is absorbed in the frame material itself (ever wonder why carbon frames "dampen" vibrations? That's the frame absorbing energy).

FWIW, I went from a CAAD8 to a non Evo SS and I felt a difference. Both in comfort (from the carbon) and my sprinting efforts. Now, this was mainly a notice in feel, but I truly think I the bike accelerated faster than my old bike. Could be mental, could be real...... Who knows.

Too many variables to conduct a meaningful design of experiment. In a real world road situation, we feel and hear system stiffness in FD scrubbing noise, break drag, wheel flex. That system is a combination of wheels, tires, crank, pedal, seat post, seat post extension, stem, and bars. How do you come up with meaningful numbers that guarantee a meaningful trend over all combinations of variables?

IMO, stiffness is largely subjective, and may cause things that lessen someone's enjoyment of the experience. I had a BMC SL02, with full 105, and I got FD rub on every aggressive climb. I returned it, because what I considered flex was annoying. Now I primarily ride a Felt steel single speed, that is no where near as stiff as the BMC, but I get no noise no matter how aggressively I climb (because there is no FD :D). Of course the BMC is stiffer, but the combination of variables made an intolerable condition for me.

There's nothing definitive published because it's all subjective, as has been stated. Look at it this way - any force that forces the BB sideways isn't going into moving your bike forward anyway - it's the use of superposition of forces when looking at a structure. I used to have a link that showed it, but can't find it now. There are basically three energy losses in riding a bike - aerodynamic drag, rolling resistance of the tires, and drivetrain friction. Since the frame (any material) flexes within the elastic range of the material, there is essentially no loss. Maybe in a sprint there could be losses from chain rub, tire scrub, etc, but they are secondary effects, not loss from the frame flexing.

A few years ago, one of the British cycling mags published an article claiming to have quantified the losses between a modern CF bike and a 1983 Pinarello. There were so many errors I found when I read it that their conclusions can be disproved.

Good luck in your search for something - I doubt anybody has published a reliable, scientific paper showing any correlation between stiffness and efficiency.,

You use the same wheels, the same tires, the same FD, etc, to hold as many of the variables constant as possible. Then you do analysis to find the contributions from the other variables and eliminate them. We've mapped the human genome, we do mtDNA studies to determine ancient migration patterns, we build metal things that fly across oceans, and even do hard things like testing bicycles in wind tunnels. I don't think it's beyond our capability to know whether frame stiffness has a meaningful effect on bike speed or not.

I agree that the specific issue of "what are the effects of frame stiffness on power transfer" is eminently quantifiable. It'd take a bit of work, but it should be possible.

Once you've figured that out, you can determine whether it's worth the trade-off with some other condition, or if it's applicable to one's own riding style.

Of course, I'm still amused by how much dudes get revved up by stiffness. It is the age of Viagra, after all.... :roflmao2:

I don't mean any offense, but life must be tough for you. Did you need maps and calculations before you believed the earth was round too?

perhaps...no one really cares...or maybe it's a Freemason conspiracy...

Yeah.. that's how that whole argument was ended...

I did not say it was beyond our capability, I said the variables would make the end data meaningless, IMO.

The 41 missing the point, as usual. Continue with the over thinking. . .

No, you're missing the point.

There is nothing wrong with wanting to see some sort of quantitative backing to support a claim.