Energy lost into frame flex never reaches the chain. That is why it is lost.

The question is whether that energy is returned to the chain when the BB "unflexes" in the direction of the foot that's not exerting force on the pedal.. I don't know the answer to that and people who seem to know a lot more about the subject than I do disagree.

It is lost.... until it is returned. That is how springs work.;)

It’s hard to argue with that. But is it a significant loss?

You could attempt to measure the loss across stiff vs less stiff frames using pedal power meters and a rear hub power meter, swapping those specific power meters between the bikes. The bikes would need to have the same drivetrain in the same condition, same chain lube etc. If you could consistently measure a higher differential between pedal and hub power on less stiff frames then we would have something to discuss here. But I have not seen any attempt to measure power loss due to frame flex objectively.

On a more pragmatic level I’m pretty sure none of the bikes I would personally consider riding competitively would exhibit a significant difference in frame stiffness in terms of power transfer. The only thing I would be likely to notice in terms of stiffness is their vertical compliance and thus their ability to absorb road vibration.

it is definitely not obvious. You could potentially model it in FEA, but even that would be an estimate at best. Modern frames are certainly modelled in FEA, but probably not to the degree of attempting to quantify anything more than lateral, vertical and torsional flex and resultant stress distribution across the frame. Frames like the Specialized Aethos have probably been subject to the most analysis in this regard.

It certainly is not obvious, and it definitely is not as simple as "metal springs are pretty lossless, therefore no energy is lost to frame flex." Even extensive FEA analysis of a frame won't give you the answer, unless you have a way to include the rider in the analysis.

I obviate the problem by not generating enough power to flex even the slightest frame.

I'm going with "hell if I know" since my FEA machine seems to be on the blink and I don't want to risk stepping into the thing.

You guys are good sports, I appreciate reading your stuff even if I have no idea what you're talking about.

People keep talking about data, some of those people have offered anecdata.

I don't see how this question really can be answered satisfactorily with analysis. Any real life testing is going to be polluted horribly with confirmation bias. Jan Heine and his crew are going to be faster on bikes with the stiffness of a playground bouncy toy, and the people that think bikes need to be stiff are going to go faster on stiffer bikes.

And everyone seems to ignore the fact there is a hinge with very weak restoring force right in the middle of bike frames. That has got to influence the feeling of stiffness that a rider gets. This is masked quite a bit because most bikes have very similar geometry

I’ve ignored most of the posts as I imagine this is how life will be in a retirement home.

But buried the the sea of letters and numbers was this and I had to comment on it. Legs are the finest example of springs/suspension there is. And not just using an example of a hardtail, but in everyday life. The number of cycles we put our legs through absorbing impacts is phenomenal.

John

Absorbing impacts is not what defines a good spring. If it was, the crushable liner in your helmet would be called a good spring.

No, legs really are terrible springs. In fact, they are barely springs at all.

Sprung versus non-sprung (rigid) pogo stick? Running along a very long trampoline? My head hurts.

Someone upthread suggested performing tests using bikes with power meters in both the rear hub and cranks or pedals. If there's anyone out there who happens to have access to both types of power meter (and to both ultra-flexy and ultra-stiff bikes), please help put the rest of us out of our misery.

It's easy to make that measurement, but hard to make it in a way that produces useful information.

Nah. Now kangaroo legs are great springs. Did you know that they actually have a lower heart rate at top speed because they're just boinging along?

It has been done in the recumbent world.

Besides me, there was another recumbent rider on this thread. Lateral flex is not a fast climber in our world. The frames are much longer and the effect is quite exaggerated

The lateral movement of the BB on a flexible frame puts force onto the tires increasing hysteresis losses.

Undoubtedly, some of the stored energy in the deflected frame is returned to the chain. However, if this causes articulation of the chain on the drive side of the chain (top), the frictional losses increase in the chain.

I don't think that a power meter on the rear hub and cranks is going to fully answer the question of efficiency and power delivery. (thought it will answer some of the more specific questions, here). It will let you know the efficiency between the pedals and the wheel, but it does not tell you what is happening on the rider's side of the pedals.

It would measure power loss from the pedals through the bike frame and drivetrain. At the pedals you are measuring the rider's torque and cadence to give their power output. I guess you are asking if the frame is flexing enough to directly affect the rider's pedalling efficiency and power output? I suppose it could in the extreme, but that really is going to be very subjective. My own power limitation comes from my engine! Pedal forces are relatively low.

I stand corrected :o

I think you could have a good go. You need 2 test frames with different lateral stiffness, a pair of power meter pedals and a rear wheel with a hub power meter. You might as well throw in some crank power meters too for good measure. Calibration is not that critical as long as they are consistent, which good quality meters are.
Then you just ride the first bike through a set power cycle, swap the meters over and repeat on the other bike. Swap back and forth a few times for repeatability. Then it's just a matter of comparing pedal/crank vs hub power loss between the 2 bikes. If there is any power worth actually saving it should show up in such a test.

Someone mentioned confirmation bias, but that would be irrelevant in the above test as you are not asking anything subjective of the rider. All they have to do is ride to the specified power targets.

It sounds pretty simple, but I don't think it is. I suspect Mr. Chung could shed a lot of light on the issue, but I don't know if he's on this forum.

I think it is pretty simple, but needs careful attention to detail. We are looking for significant differences in drivetrain/frame power loss. Not absolute values. If these "frame flexing" losses are in the order of a few percent then they should be easy enough to measure in a controlled test cycle. Like it's not too difficult to measure a 3-4% loss in a drivetrain.

I also think one of the reasons you don't see this kind of test performed is simply because there are enough other legitimate reasons to make the BB and frame laterally and torsionally stiff. Tuning vertical compliance is probably of more interest to frame designers. Frames that absorb more of the energy/vibration imparted by bumpy road surfaces rather than transmitting it through to the rider reduce fatigue on long rides. Being primarily a century rider in my 50s this is certainly what I look for in a frame and modern carbon bikes are F'ing stiff at the BB anyway.

But I did notice that Vielo claim a 30% increase in BB lateral stiffness with a frame dedicated to using a 1x drivetrain:-

https://www.vielo.cc/pages/r1-road-bike

Does it matter? Probably not.

Then you should do it.