Frame flex and spring/planing effect

Discuss light weight issues concerning road bikes & parts.
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jeanjacques
Posts: 123
Joined: Thu Apr 14, 2016 11:01 am
Location: France

by jeanjacques

Hello,

A recent video from GCN show this spring effect that some people on steel frame suggest:



In relation, the work of Jan Heine and his blind test with 4 steel frames: https://janheine.wordpress.com/2018/01/ ... re-faster/
Jan Heine wrote: I hypothesized that relatively flexible frames were easier to pedal, because they did not resist our pedal strokes. We could get in sync with the frame, pushing down harder during the power stroke. The frame stored the excess energy as it flexed, and returned it to the drivetrain during the “dead spots” at the top and bottom of the stroke.
https://janheine.wordpress.com/2011/02/ ... stiffness/
https://janheine.wordpress.com/2011/10/ ... stiffness/

by Weenie


Marin
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Location: Vienna Austria

by Marin

Yes.

bremerradkurier
Posts: 259
Joined: Mon Jun 13, 2016 4:18 pm

by bremerradkurier

Powertap hub to quantify the effect?

jeanjacques
Posts: 123
Joined: Thu Apr 14, 2016 11:01 am
Location: France

by jeanjacques

bremerradkurier wrote:
Wed May 02, 2018 2:39 pm
Powertap hub to quantify the effect?
Good idea. If the first statement (stored energy) is right, the curve of power input have to be more linear. And it will be even better with also a crank power meter to compare both curves.

jorryt
Posts: 87
Joined: Tue Jul 11, 2017 10:37 am

by jorryt

The power you put in the frame lets say 200N will always be returned in the same amount but it may be in different forms. By example heat. So you will never get the full 200N back to the drivetrain.

I think, not a scientist by any means.

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Rick
Posts: 2035
Joined: Sat Aug 29, 2009 4:30 pm

by Rick

spring/planing effect was debated TO DEATH here sometime last year.
SEARCH for the hilarity.

FreaK
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Joined: Sat Aug 27, 2005 5:57 am
Location: mOntreal

by FreaK

Search under what terms? I think the idea that the frame could even in theory return the force on axis is bonkers.
it's actually possible to come to the conclusion even before realising it makes no sense at all
-
tymon_tm

Zakalwe
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Joined: Sun Jun 17, 2012 3:15 pm

by Zakalwe

I think you search for “batshit theories”

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Rick
Posts: 2035
Joined: Sat Aug 29, 2009 4:30 pm

by Rick

If you think of it as the frame "returning" energy, it sounds like a "BS theory".
But if it is not "returned", then the alternative would be that it is "lost", which is also a BS theory.
(Yes, some energy is "lost" when any material flexes, but it is so minescule as to be irrelevant. You are not heating up your frame or crank by flexing it a little)
So, the truth is that the energy IS "returned" if you just keep pressure on the pedals throughout the stroke. Your leg is resisting the torque, both as you apply it and as the crank/frame tries to spring back.
So I think the real question is whether the springiness really ever actually HELPS anyone, by making the pedal stroke "more efficient."

My personal belief is that that is also a BS theory, from and energy/thermodytnamics point of view.
BUT....if you have been riding a stiff bike that is beating you up as you cross every little pebble for 4 hours, you may feel fresher and be psychologically willing to put out more effort if you are on a bike with a little bit springier, softer, ride. I think that is why some people feel that the bike is "returning some energy", because it sort of FEELS that way. It feels smoother and more efficient than a super stiff bike.

Kurets
Posts: 148
Joined: Thu Feb 25, 2016 9:55 pm

by Kurets

One importang point which is missed is that even if the frame flex would spring back in a useful way for propelling tha bike forward, your muscles still have to provide the counteracting force to actually have it go into the drive train. And, as we all know, even static loads tire your muscles. Therefore, springyness will be less efficient from an "muscular energy expenditure" point of view. But, just as the poster above writes, any increase in comfort might help the rider feel fresher at the end of a race.

None the less, the amount of deflection in angles which are relevant for propelling the bike forward are likely minimal. And any side to side or lateral deflection cannot be returned to the bike in any productive way.

FreaK
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Joined: Sat Aug 27, 2005 5:57 am
Location: mOntreal

by FreaK

Kurets wrote:
Sun May 06, 2018 8:37 pm
One importang point which is missed is that even if the frame flex would spring back in a useful way for propelling tha bike forward, your muscles still have to provide the counteracting force to actually have it go into the drive train. And, as we all know, even static loads tire your muscles. Therefore, springyness will be less efficient from an "muscular energy expenditure" point of view. But, just as the poster above writes, any increase in comfort might help the rider feel fresher at the end of a race.

None the less, the amount of deflection in angles which are relevant for propelling the bike forward are likely minimal. And any side to side or lateral deflection cannot be returned to the bike in any productive way.
I know it's a bit unnecessary to quote a comment directly above mine, but this right here is the crux of the absurdity. If this nonsense is even possible, it still requires resistance from your leg on the return, and most of the deflection is lateral.
it's actually possible to come to the conclusion even before realising it makes no sense at all
-
tymon_tm

by Weenie


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