Re: [CR]frame flex

(Example: Framebuilding:Restoration)

From: <>
Date: Thu, 29 Sep 2005 18:34:33 EDT
Subject: Re: [CR]frame flex

In a message dated 9/29/05 3:49:09 PM, writes:

> Good question, but there's a simple answer to it: there are a lot more
> variables than you've identified. The bearing, spindle, crank, and so on. Are
> these perfectly stiff? Why are they outside of consideration?
> It's obvious that the top edge of the chainring moves when you put force on
> the crank; all my bikes do this, modern and old. What isn't obvious, however,
> is what's moving. I suspect that most of this is the spindle taking up
> inevitable looseness in the bearing, multiplied by the much larger radius of the
> chainring. Some deformation of the crank may be involved, too, especially in
> the area near the spindle, where the force is actually quite high. In any
> case, the movement of the chainring edge is only something like 10-20 mils.
> Steve Maas
> (No more posts today; it's getting late in)
> Dublin, Ireland


I would consider the components outside of consideration if, as I mentioned in my last message, they were the same make and model on each bike and bearing tolerances were similar, etc..

For many vintage race bikes, a Campy crank, spindle, front derailleur, pedals, etc. is pretty much a 'standard' (not trying to start another Campy vs Simplex vs Sun Tour vs Huret etc. discussion, this is just for sake of argument and based on the high-end bikes of the period that I've seen). I've heard way too many stories from friends who rode a frame that's a few cm larger who found that it flexed more (or rode one that's smaller than they were used to and finding it stiffer) to discount the stories as figments of their collective imaginations, especially since I've noticed the effect myself. And to repeat with emphasis, I've noticed different amounts of flex on bikes that bore identical components.

I'd be willing to concede that my old Stronglight 93 crank with the de-webbed chainrings was more flexy than a stock Campy NR crank, so why is it so hard to concede that a frame flexes to varying degrees as well, based on the length and thickness of its tubes? Both component and frame are part of the structure of the bike, both are subject to the collective forces applied, both may have varying dimensions. When dozens of folks weigh in with tales of different bikes (with identical components) exhibiting varying degrees of flex, I find it hard to consider it a case of mass hypnosis just because there might be an absence of documented tests to verify their experiences.

Most framebuilders on this list could set this matter to rest without any fancy lab equipment... all they'd have to do is bolt a bare frame's bottom bracket shell down on their (horizontal) surface plate, then rest a 20 lb weight on the seat lug. Would anyone be crazy enough to bet that there will be no measurable deflection? Nope, didn't think so. And since it doesn't take a very strong cyclist to put more than 20lb of torque on a bottom bracket, I feel pretty safe in believing that frame flex IS a reality in lugged steel bike frames.

Bob Hovey
Columbus, GA