Re: [CR]frame flex

(Example: Framebuilders:Alberto Masi)

Date: Thu, 29 Sep 2005 15:48:48 -0400
From: "Steve Maas" <stevem@mail.nonlintec.com>
To: <classicrendezvous@bikelist.org>, <BobHoveyGa@aol.com>
Subject: Re: [CR]frame flex


---------- Original Message ---------------------------------- From: BobHoveyGa@aol.com Date: Thu, 29 Sep 2005 11:31:19 EDT
>Steve Maas writes;
>
>I'm still waiting to see some hard data showing that (1) frames flex to
>a degree that is genuinely perceptible to a human, and (2) that there
>is a significant difference between any two frames of the same general
>type. This is especially unlikely in our case, since we are dealing
>with frames made of identical materials to virtually identical dimensions,
>for a given size.
>
>
>Steve;
>
>I can understand an engineer's need for hard data. But I would ask why
>anyone would need hard data for an effect that is so easily observable? If
>someone were to insist that the light output of the sun varies with the number of
>sunspots, I'd say sure, show me some data, since the effect is probably too
>subtle for me to see. But I think I can trust my own simple observations to
>agree with someone who asserts that it gets darker during an eclipse.
>
>My own simple observations over the last 35 years are thus: When I stomp
>hard on the pedals, the chain rubs the front derailleur cage, when I ease up it
>stops. SOMETHING has to be flexing. The fact that it happens more on one
>bike than on another when both bikes have the same model crank, BB, pedals, and
>chain makes it quite plain that at least some portion of the flex is coming
>from the frame. I'll concede that it may be difficult or impossible to tell
>any difference between frames of identical materials and geometry... but the
>effect is obvious on frames built with different gauge tubing or frames of
>different sizes (I don't think any capable engineer or physicist could dispute the
>fact that a larger frame is going to flex more than a small frame built of the
>same tubing).
>
>I have never built a rig to test it, but it surely must be obvious to anyone
>who has ever climbed a hill at anything above a leisurely pace that a
>lightweight steel bike frame does flex.
>
>Bob Hovey
>Columbus, GA
>
>"You don't need a weatherman to know which way the wind blows" - Bob
>somebody or other
>

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