Re: [CR] Frame "stiffness"


Example: Framebuilders:Chris Pauley

Content-Class: urn:content-classes:message
Date: Wed, 7 Jul 2010 13:40:05 -0700
In-Reply-To: <241072.25443.qm@web84101.mail.mud.yahoo.com>
Thread-Topic: [CR] Frame "stiffness"
Thread-Index: Acsd5ah8lUrx1M6ERKi5EfPBwYQDaAAJXuXA
References: <241072.25443.qm@web84101.mail.mud.yahoo.com>
From: Mark Bulgier <Mark@bulgier.net>
To: Anthony Taylor <ajft1942@yahoo.com>, <Classicrendezvous@bikelist.org>
Subject: Re: [CR] Frame "stiffness"


Tony Taylor wrote:
> I have a '93 Reynolds 753 OS Paramount which I have had from new.
> I really feel a bit beaten up on the last half of a century. I'm
> not sure if it is the geometry or the tube set, but I don't find
> it to be a plush ride, as others seem to.

Oh yeah, "plush" and "OS" are never going to go together. Diameter so thoroughly dominates the stiffness equation that you can almost ignore the wall thickness and just assume any OS frame is going to be stiff. Usually too stiff for my taste -- but of course YMMV.

Standard 753 frames with a 1" toptube (or older metric ones, 26 mm TT) are not just a little more flexible than frames with 753 OS, 1-1/8" TT. It's hard to even define stiffness let alone measure it, for the complex shape and loading of a bike frame, so let's just look at the torsional stiffness of the top tube. With the OS tube 12.5% larger and stiffness proportional to the fourth power of the diameter (for constant wall thickness) that tells us the OS tube is 60% stiffer. (1.125^4 = 1.602)

That's compounded by the fact that the OS tubesets probably weren't as thin-wall as the thinnest std. size tubes, the ones that gave 753 its willowy reputation. Let's say the thin unbutted part of your OS frame is 0.4 mm wall, vs the lightest std. 753 which may have been 0.3 mm (I never cut one up to measure the middle). Your tube is then 33% thicker, and stiffness is directly proportional to wall thickness, making that one OS tube (at least in the unbutted section, which is most of the length) 93% stiffer (60 + 33) i.e. almost twice as stiff in torsion. Those numbers are not intended to be exact, nor can you extrapolate that out to the "ride" of the entire frame, but they're indicative.

The one thing those two tubesets have in common, being the same alloy and heat-treat state, doesn't affect the ride at all. They are completely different animals, ride-wise.

Before someone jumps all over for me for ignoring rider fit and frame geometry and framebuilder mojo and all that, let me say I'm intentionally trying to tease out the difference that just the tubing diamter can make, in otherwise-identical frames. Obviously other factors can make one frame magic and the other a dog, even with identical tube sets.

Mark Bulgier
Seattle, WA USA