While it may not mean much in terms of energy loss, that doesnt mean a flexible bike doesnt matter. I dont want a bike where I ghost shift when I stand and climb. I dont want a bike where the wheels are visibly twisting and rubbing the brake pads.
The phrase 'laterally stiff and vertically compliant' is rightfully made fun of, given how often it is used in marketing, but its legit.
I have never heard someone say 'man, I wish this bike twisted more when I pedal' because that isnt a thing.

In general, the benefit of "stiffness" in a bicycle increases with rider weight and decreases with road/trail roughness. A heavy rider on smooth pavement gets the most benefit from a stiff bicycle; a light rider on rough pavement, the least.

Experienced plenty of, um, noise / vibration / harshness when we were growing up, riding in the fields. More vibration when barreling down the bluffs to the jump we'd made at the bottom ... but more harshness when landing over the tracks and in the cactus patch on the other side. (Heavy or light didn't seem to matter, picking ourselves up from the cactus patch. But, boy! were we stiff ... )

OH I am sure , BUTT , a little play on the word used before the *---* that had I used instead of *believe* most likely would have gotten me in trouble.

r.e. -- droopy , might also need to consider a possible Peyronie's disease syndrome when extrapolating.

I know it's invited by the OP, but could we just avoid the obvious juvenile double entendres for once?

It’s pretty easy…

The more flexible you are, the less flexible your bike needs to be.

The less flexible you are, the more flexible your bike needs to be.

Some mornings I wish I had a Vitus.

John

Oh, and here I was thinking about what the little blue pill is for - ALEVE for stiffness after a ride!

Not even close to the same thing. A suspension system (front or rear) on a bicycle is intended to take the energy of an impact and use it to generate heat elsewhere. An undesired side effect is that the force of pedaling also acts on the suspension, so some of the force gets turned into heat.

The OP is speaking of force which is played into an undamped spring and which is overwhelmingly played back.

I look forward to OP's new bike made of pad thai

You don't think it'll be all noodly?

I call BS. Frame flex matters a great deal for my recumbents. My stiffest bent is also the best climber of the bunch, likewise my flexiest one is the worst climber. (Climbing relies less on aerodynamics and more on power input.) Even if we're talking about traditional 'double diamond' frames, there is a difference. Power lost deforming the frame is never recovered for propulsion.

Not their money, no, but they’ll keep us wasting ours as long as people on the Internet tell other people on the Internet that suspension forks are “inefficient”

That's not what she said.

So really, this is just another “my saddle makes my junk go numb” thread.

There are a few suggestions to add to the quality here.

-Sildenafil has shown promise among high altitude mountaineers and theoretically could be helpful as a performance enhancing drug for endurance athletes.

-Has anyone ever experienced the unexpected diamond cutter when waking the nethers back up? My current seats don’t cause this but I have experienced it in the past.

Carry on

Seriously, I'm no physics expert, but it's only logical that if the exertion you're putting into moving your bike across the ground is also causing some flex in the frame, then part of your effort is wasted in bending the frame back & forth. Or, the frame is absorbing some of the force instead of putting it fully into the pedals. I'm sure someone might show some science to prove me wrong, so have at it...

Can't say I've ever see watts measured at the pedals and also at the pavement at the same time to disprove anything. Or to prove anything.

Where does the energy go? Does the frame flex and stay in that position? No, it flexes back. Does the frame get hot? No. Pretty much all the energy that causes the frame to flex is returned.

i suppose it would be possible with two frames, indoors, same weather, same rider, same wheels and tires and cassette, same chain, brand new matching cranks, add some weights to match up frame weights, same power pedals, ride at a predetermined cadence and power and carefully measure speed of the bicycle.

it would be pretty surprising if the difference between a noodly frame and a worlds-stiffest would be measurable, or above the margin of error of the experiment. but it would probably feel totally different to the rider, which is what’s actually important for most of us.

Here’s a good video covering stiffness and Young’s Modulus, he shows some good modeling and decent figures also mentions some studies.


Further reading about bike engineering, and materials analysis in some studies about frame types, geometries, and materials specifically referencing FEA (which is method for numerically solving differential equations arising in engineering and mathematical modeling)

Enjoy.


Derek Covilla, 2014. “Parametric finite element analysis of bicycle frame geometries”, the conference of the International Sports Engineering Association 2014, Elsevier Procedia Engineering 72 (2014) 441 – 446

Bharati A. Tayade, 2015. “A study on structural health of bicycle frame using Finite Element Analysis”, International Journal of Innovative and Emerging Research in Engineering Volume 2, Issue 4

Joachim Vanwalleghema, 2014. “Development of a multi- directional rating test method for bicycle stiffness”, the conference of the International Sports Engineering Association, 2014, Elsevier Procedia Engineering 72 (2014) 321 – 326

M.A. Maleque, 2010. “Materials Selection Of A Bicycle Frame Using Cost Per Unit Property And Digital Logic Methods”, International Journal Of Mechanical And Materials Engineering (Ijmme), Vol.5, No. 1, 95-100

M.V.Pazare, 2014. “Stress analysis of bicycle frame”, International Journal of Engineering Science and Technology (IJEST), ISSN: 0975=5462, vol. 6, No. 6

Aparna Deshpande, 2016. “Design and Optimization of Bicycle Frame for the Cyclist's Comfort”, International Journal on Recent and Innovation Trends in Computing and Communication, ISSN: 2321-8169, Volume: 4 Issue: 5 220 – 224

You must be new here.

Well, a bicycle that wiggles on turn-in has a weak frame but the poster allowed for that.

Now, aluminum that flexs then cracks. So aluminum is used in larger cross-sections than steel. Allowing for the larger cross-sections then the weight advantage of aluminum over steel is only about 5%. However, current aluminum frames have smaller cross-sections than the original aluminum frames. Now the forks can be carbon-fiber and that helps.

Presently, carbon-fiber frames often have seat or handlebar compliance systems.

I thought the current theory is that a flexy frame doesn't keep the rear wheel inline with the frame during hammer time, and that is a source of energy waste.

I had the exact same thought ...

Even if all the energy that goes into flexing the frame is returned, it may not be returned in a way that propels the bike forward. You have to look at the whole system and where the losses are.