Re: [CR] Frame Flex

(Example: Framebuilders:Masi)

Date: Fri, 30 Sep 2005 10:13:16 +0000
From: "Steve Maas" <stevem@mail.nonlintec.com>
Subject: Re: [CR] Frame Flex
In-reply-to: <75d04b48050929121851c3471d@mail.gmail.com>
To: Classic Rendezvous <classicrendezvous@bikelist.org>
References: <20050929174225.56920.qmail@web51111.mail.yahoo.com>


This will be my last post on the subject, unless there is something special I need to respond to. I've made my point, and I think that threads on this forum often last longer than their usefulness warrants.

Thanks to John for the link to the FEM frame analysis. Take a look at the curves on the page at http://www.bikethink.com/Power_output.htm. These show that the peak power stored in the frame is 0.5% of the pedaling power. Now, power is energy per time, energy is force times displacement (in this case, summed over the whole frame). The power is miniscule, the force isn't, so the displacement (AKA "flex") also is miniscule.

Now, you could say that this is just one case, a steel frame of the kind we all know and love, and the story might be different for different materials or sizes. That's true as far as it goes, but it seems unlikely that the difference, under all conditions, would be greater than a factor of two or three, which is still miniscule.

(Finite-element analysis is the gold standard for such calculations. It can be viewed as exact, within numerical limits.)

Frankly, I can't help but be fascinated by this. Bicycle manufacturers started focussing on the dual-triangle frame in the late 1800s, and since then, it has been optimized purely by trial and error. Now, we can look back at that process with modern analytical tools and see just how well it worked to perfect the frame: amazingly (to me, at least) stiff, strong, and light.

By the way, I've received a number of thoughtful comments off-line,from members of the list, and some not so thoughtful and comsiderably hotter. I'll be happy to continue this discussion off-line. However, if your point is just to vent your spleen for having cherished beliefs questioned, don't expect a response. I have better things to do.

Steve Maas Dublin, Ireland

Kurt Sperry wrote:
> Agreed. The best short take on the subject I've seen, seems based on sound
> first principles rather than anecdotes and unsupported assumptions.
> Worth the short time it takes to read.
> Kurt Sperry
> Bellingham WA
>
> On 9/29/05, John Clay <jmedclay@yahoo.com> wrote:
>
>>One of the things I learned in mechanical engineering school was that
>>structural analysis and testing aren't easy and the (correct) results are
>>frequently counter-intuitive. Adding an analysis of power delivery as it
>>relates to bicycle speed or efficiency and you've got a mighty imposing t
>
> ask
>
>>on your hands. Here is an article by an engineer who's put substantial
>>thought and effort into modeling a bicycle frame as it relates to power a
>
> nd
>
>>deflection. A quick read and his conclusions sound well grounded to me. H
>
> e
>
>>doesn't violate any big ticket "first principles" that I can see. The
>>analysis and conclusions make sense so I tend to think he's in the correc
>
> t
>
>>zip code.
>>
>>Short version: I wouldn't be too quick to utterly dismiss reasonable fram
>
> e
>
>>flex as an aid to propulsion.
>>
>>http://www.bikethink.com/Frameflex.htm
>>
>>fineartscrimshaw.com http://fineartscrimshaw.com
>
>
>
> _______________________________________________
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