Frame Flex on GCN

[etu]

Interesting new video on GCN with a nice illustration on how frame flex might or might not make a frame better.

https://www.youtube.com/watch?v=BH_AL4rxrp8

[Kontact]

I've been saying the same thing for years. Bikes are springs, and springs are actually very efficient.

Too stiff a frame and you are going to have friction losses because the tires are going to scrub to an extent instead of conforming to the road surface because of the uneven loading that pedaling causes.

[ultraman6970]

Look back in the day was saying something similar and designed the KG86 to spring back helping the rider in the next pedaling, basically creating harmonics in the BB area, ask lemond he used that thing to win a tour

There is limit for everything, you have a stiff bike put stiff wheels or you get nowhere, stiff wheels in a no stiff frame and you have loss too IMO.

[Kontact]

While there is obviously a minimum and maximum stiffness that you'd want in a drivetrain, springy materials like those used in bikes are so efficient at storing and re-transmitting force that you can bend them nearly 90° and get enormous return:



Can you imagine any other unpowered device that would allow a man to jump 20 feet vertically with nothing more than the energy of a running start? The poles are just fiberglass, like a Look KG56.

[ergott]

A spring is only a benefit if its return to original position is propelling the rider and bike forward. It also assumes your legs are rigid enough to too. If your legs simply absorb that energy it won't matter.

I haven't seen any complete studies of this that are very conclusive either way.

Sent from my SM-G950U using Tapatalk

[Kontact]

Originally Posted by ergott

A spring is only a benefit if it's return to original position is propelling the rider and bike forward. It also assumes your legs are rigid enough to too. If your legs simply absorb that energy it won't matter.

I haven't seen any complete studies of this that are very conclusive either way.

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If your legs aren't rigid enough, you couldn't pedal with them.

If the drivetrain returning to normal isn't in line with propelling the bike forward, it will never be in line to propel the bike.

[Look585]

Francois Pervis, Flying 200m on a NJS steel track frame.

I don't know if this clarifies anything about energy recapture from flexy frames, but it does show what a guy that can do a 9.3s 200M does to a steel frame. (I'm pretty sure his Look did not flex as much.)

[ultraman6970]

Cant see the darn video but since long time ago the the guys do pretty much the same times they are doing now but in steel frames. And besides being light, a funny bike from the 80s probably is as fast as the best aero pinarello TT machine.

Marketing do magnificent things to sell stuff sometimes.

There is a video from trek with LA, they were saying that he was getting a bunch of frames custom made in the R&D stage of the products and who knows what else. happened now that in his cast for the tour he said stragith forward that his bikes were the same cr_P you can buy all over the place but with another paint job. Great news for the ones that bought those 7000 bucks replicas

[Peter P.]

All this discussion is what the term "planing" is all about.

If the spring-like characteristics of the frame match the power output and pedaling style of the rider, then wonderful things happen with regard to speed and bike feel.

Stiffness is over-rated.

[Kontact]

Originally Posted by Peter P.

All this discussion is what the term "planing" is all about.

If the spring-like characteristics of the frame match the power output and pedaling style of the rider, then wonderful things happen with regard to speed and bike feel.

Stiffness is over-rated.

Planing may or may not be any old flex - I imagine there is a certain type of flex and frequency that feels especially ergonomic and in tune with pedaling. Flex that is of greater than ideal magnitude for "the plane" will be felt, but the question is if it is inefficient even if it is annoying.

[Bantamben]

I had a xl Merlin titanium bike that I could sprint up 2-3 minute climbs like no other bike I ever had it just seemed like a spring. But it was terrifying descending down the other side of the hill anything over 35mph was speed wobble city

[andrewsuzuki]

No mention of Jan Heine! or BQ!

[andrewsuzuki]

I know flexy frames aren't less efficient, but I still don't understand how flex enables more power (besides flexy bikes subjectively feeling smoother and thus enticing the rider to put more power into them).

Though today I was riding some 38mm tires and noticed they squirmed quite a bit when I did hard out-of-saddle efforts up a hill at relatively low cadence. First, when a tire "squirms", is is a relatively large hysteretic loss (as in > 5 watts)? Second, is it possible that lateral frame flex could store enough energy on each hard pedal stroke to reduce the flex in the tire?

Another theory I have is that since the rear wheel sees a smoother power input, and therefore the entire bike-rider system accelerates less on each pedal stroke. How big are these micro-accelerations between pedal strokes and does it amount to any nontrivial loss?

I'm most skeptical of the claim that flex lengthens the power stroke / reduces the dead spot at the top/bottom of the crank. How would this be any different than elliptical chainrings? Last I checked, there's really no consensus on elliptical chainrings enabling more power, and certainly no claims that they enable 12% more power on a hard effort.

[Kontact]

Originally Posted by andrewsuzuki

I know flexy frames aren't less efficient, but I still don't understand how flex enables more power (besides flexy bikes subjectively feeling smoother and thus enticing the rider to put more power into them).

Though today I was riding some 38mm tires and noticed they squirmed quite a bit when I did hard out-of-saddle efforts up a hill at relatively low cadence. First, when a tire "squirms", is is a relatively large hysteretic loss (as in > 5 watts)? Second, is it possible that lateral frame flex could store enough energy on each hard pedal stroke to reduce the flex in the tire?

Another theory I have is that since the rear wheel sees a smoother power input, and therefore the entire bike-rider system accelerates less on each pedal stroke. How big are these micro-accelerations between pedal strokes and does it amount to any nontrivial loss?

I'm most skeptical of the claim that flex lengthens the power stroke / reduces the dead spot at the top/bottom of the crank. How would this be any different than elliptical chainrings? Last I checked, there's really no consensus on elliptical chainrings enabling more power, and certainly no claims that they enable 12% more power on a hard effort.

I don't think this is any different than elliptical chainrings. Q rings have increased gearing on the power stroke, Biopace had decreased gearing on the power stroke, and round rings are right between. All of them seem to work about the same once you get used to them, despite all doing different things to the way power is transmitted, yet all having claims about the way they increase output through greater efficiency of the pedal stroke.

And they may be pretty much like too stiff, too springy and just right.

[93KgBike]

Originally Posted by etu

Interesting new video on GCN with a nice illustration on how frame flex might or might not make a frame better.

https://www.youtube.com/watch?v=BH_AL4rxrp8

Retrogrouches everywhere are grumbling, "No sh*t."

So will the price of steel go up or down now?