Stiff vs Flexing bike frames

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marc2131
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Postby marc2131 » 25 Nov 2014, 09:25

Came across this guy Jan Heine, a US-based bicycle journalist.
He claims that stiff bikes are not necessarily good. Says that an appropriate flex in a frame is often desirable. Heine refers to this as 'planing'. A term taken from boating. Have a listen to this podcast interview.

http://community.terrybicycles.com/medi ... laning.mp3

Heine claims (it is) "clear that many riders perform better on flexible frames, apparently because it allows them to apply their power more efficiently. Many riders and builders extol the virtues of a “lively” frame made from flexible tubing. When we tested different frame tubing in a double-blind test (Bicycle Quarterly Vol. 6, No. 4), we found that two of three riders preferred the most flexible frame both for constant efforts and for all-out sprints. (The third rider could not tell the – very small – differences between the frames in our test.)
Of course, the real story is more complex. There is more to bicycle performance than overall frame stiffness. Frames can be too flexible for a given rider and application. Some riders may even prefer very stiff frames. However, it is clear that the old mantra of stiffer = more performance is not true for most riders." (END QUOTE)

Heine's assumption is that for 'normal' riders, stiffness isn't necessary in a bicycle. A rider with limited power output is not optimised by a stiff frame. A flexible frame wouldn't work for a strong rider, but will work with a weaker one. Flexible frame 'returns' power back to the drivetrain. It is just a matter of choosing a frame with the right amount of flex for your power output.

http://janheine.wordpress.com/2011/10/0 ... stiffness/

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weiyun
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Postby weiyun » 25 Nov 2014, 09:48

"Prefer" is highly subjective and may have no relevance to objective mechanical performance. Better define "performance" first.

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mikesbytes
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Postby mikesbytes » 25 Nov 2014, 12:57

It depends on where the flexing is and how its applied. The flex helps deliver the power from the pedals to the wheel. If there was zero flex it would cause a skipping effect

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weiyun
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Postby weiyun » 25 Nov 2014, 13:22

There's flex in the tyres as a minimum. Given the subject, I assume it's frame specific.

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Dougie
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Postby Dougie » 25 Nov 2014, 14:27

how much "flex" is there in a rider. does the Femur "flex" under load? does a taller rider flex more than a shorter rider?

andrewb
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Postby andrewb » 25 Nov 2014, 19:42

marc2131 wrote:... Says that an appropriate flex in a frame is often desirable. Heine refers to this as 'planing'. A term taken from boating.

Ugggh! What he is describing has no relation to planing in a boating sense (or a woodworking sense).

marc2131 wrote:Heine claims (it is) "clear that many riders perform better on flexible frames, apparently because it allows them to apply their power more efficiently.
...
Frames can be too flexible for a given rider and application. Some riders may even prefer very stiff frames.
...
It is just a matter of choosing a frame with the right amount of flex for your power output.

Sounds like impedence matching to me (in the physics/engineering sense). (Still a bit dubious that really what's going on, but the observations are not inconsistent with it being the case.)
Andrew

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marc2131
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Postby marc2131 » 27 Nov 2014, 15:49

I am not an engineer, so please feel free to comment.
There appears to be little scientific-based research on the topic re. the effectiveness of the stiff or flexi frames (correct me if I am wrong). Lots of discussion on this if you google, but largely based on 'feel' and intuition. On one hand you have the big CF bike industry repeating the mantra of stiff=more power. On the other hand, you find small bike frame builders and some long time users who claim this is not totally true and that flex in the frame can be used to a riders' benefit, particularly if they are not that strong.

Another 'expert' made the following observations. He claims:

"If the bottom bracket is too stiff for the rider something very interesting happens. We've all ridden with someone whose rear tire makes lots of scuffing noise when they are out of the saddle. What does that noise come from? When the rider pushes down (and unavoidably sideways) and the frame can't flex and store the energy, something has to give. So the tire loses hold of the pavement, resulting in a sideways scrub. In this case energy from the pedal stroke is lost as the tire slides sideways on the road, neither propelling the bike forward, nor returning energy into the next pedal stroke. So it's easy to see why a frame that is too stiff would be slower. "
(Source: http://www.kirkframeworks.com/Flex.htm)

Any comments on this?

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mikesbytes
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Postby mikesbytes » 27 Nov 2014, 21:36

It is probably highlighting that different frame flexibilities suit different riders. A rider who delivers most of the power over a small portion of the pedal stroke would suit a frame that spreads that power pulse more, where a rider who produces that power over a larger portion of the power stroke would have less need for the pulse to be spread.

Probably more relevant for many rides is the power in balance between the legs. If for example a rider uses a lot of quad and little hamstring then its likely the pedals are being pushed outwards on the down stroke. This rocking motion, where does it go? if the bottom bracket flexes then that could be taking some of it, but what about the rest of it? if we are standing, we can rock the entire bike left and right or when seated the riders body could be rocking. If nothing flexes or rocks, then it may be distributed by the bike moving left and right (as distinct from rocking) and therefor tyre skip. (I need to explain this better, that's hard to understand)

Should point out that there's a number of reasons for flex and so far we are talking about power distribution;
- ride comfort, though this tends to be more about the wheels
- cornering. A frame with no flex gives no feeling resulting in a lack of confidence. Conversely too much or the wrong flex reduces handling
- braking. Perhaps but probably more about feel.


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