Largely has to do with the sampling rate of a power meter to create an average power per unit time aka per second. If a higher average power sampling were taken, the algorithm with a power meter would have to be more sophisticated to interpolate the change in stain due to power change. At a cadence of 90 RPM, this would mean 1.5 revolutions of the crank per second and likely differential power recorded between PM on the crank and PM on rear wheel would be missed. So the difference between BB and rear wheel is probably mostly how a PM reports power. That coupled with likely very little power loss anyway.


So much can and has been written on the subject of stiffness and power transfer and I believe Abe is right that it is perhaps one of the more beguiling conversations and not perfectly understood, in part because there is no right answer. Idealized stiffness and where to put it in a frame varies based upon rider strength. Just like some may feel a carbon handlebar is whimpy when sprinting and I don't. A 1500 watt sprinter can pretzel a given handlebar and I can't flex it. The stiffness equation can of course elude custom frame makers sitting down with their customers and of course big bike brand manufacturers change stiffness...move it around on pretty much every model change which suggests they are searching as more is learned with the passage of time as technology advances. Specialized which is a heavy R&D company is a notable example with the evolution of both their Tarmac and Roubaix. Every model was either slightly or dramatically changed in flex and frames ended up looking nothing alike model to model every 2-3 years apart.
Just like the past 5 years was a watershed for Al as lessons were learned about carbon which were applied to Al because Al could be formed like molded carbon, I believe the next 5-10 years will continue to be further refinement for carbon and Al as more is learned about stiffness. Steel and Ti are of course more constrained by inability to change frame section shapes due to limiting manufacturing practices and of course weight constrains stiffness of steel compared to lighter carbon because a frame as stiff as carbon made of steel would be much heavier.


The podcast...thanks to Abe for the reference, was perhaps one of the best discussions I have heard or read about stiffness. It buttresses in fact what many of us know intuitively as many of us have owned that bike which just seems to create more effortless speed than others. And it may not be the stiffest bike just like in the test.


When the Specialized SL2 Tarmac came out, professional riders clamored for a stiffer bike. So Spesh released the SL3 which rode like a cattle truck and pros loved the speed of the bike but hated the ride quality. Then Spesh decoupled stiffness and created the SL4 which was much more vertically compliant but if anything was more laterally stiff. This was a period when leaps were starting to be made with differential section modulus and frame design was really starting to improve....not very long ago either. The podcast said the SL4 Tarmac is one of the stiffest frames ever created...and yet it had now a good ride quality. Then Spesh came out with the 'rider first' SL5...a slogan that suggested greater parity was created throughout different frame sizes. This is when working with McClaren they strain gauged a bunch of different frames and studied what riders of different sizes preferred. What did Spesh do? They softened the flex for the SL5 compared to the SL4. And now there is the new aero SL6 just coming out which no doubt will be preferred overall as further refinement is achieved in the stiffness equation.


The podcast revealed that mfr's now based upon data mining and continuous testing coupled with computer modeling, maybe zoning in on an idealized bottom bracket stiffness....a sweetspot...not too stiff, but not too soft.
Cannondale has identified a number for this force/deflection as a target.


Take away from the podcast is...because pedal stroke force isn't linear...there is a power zone when force is higher as the foot crests over the crank center and pushes down, a bike has a pulse or a resonance. And of course most objects based upon their moment of inertia have a resonant frequency. There maybe a relationship between stored and applied energy that can be used as a synergy to propel a bicycle. This was implied in the pod cast and why the less stiff bike was faster for some strong amateur riders tested...which belies the stiffer is better mantra. The problem for mfr's is where to place this sweetspot which maybe be different for Peter Sagan than it is for me with probably less than 1/2 his power. Stiffness is based upon force. An object doesn't deflect without a force and a large force will deflect an object more. So an idealized stiffness is most likely different for different riders which creates a great challenge for mfr's which perhaps contributes to what some believe that stiffness doesn't affect speed. To them based upon their experience, it doesn't. But to a pro who cruises at 300 watts based upon a given frame stiffness sold to the public, stiffness may matter because they flex a frame more than the average rider.


My further thoughts.