[CR] Differences in light weight tubing...can it really make much

(Example: Framebuilders:Bernard Carré)

From: "Mark Battley" <mark.battley@xtra.co.nz>
To: <classicrendezvous@bikelist.org>
Subject: [CR] Differences in light weight tubing...can it really make much
Date: Fri, 1 Jun 2007 12:56:47 +1200
Thread-Index: Acej1VAQTW+S+bSAR8WwEKA89a8EjQACiYsg
In-Reply-To: <MONKEYFOODu7BDPy4xZ00000eb3@monkeyfood.nt.phred.org>
cc: gillies@cs.ubc.ca
cc: gillies@cs.ubc.ca

_________________________ It is not about the weight but about the stiffness of the tubes. Their are 3 general tubing variables that effect ride quality. The wall thickness which can be as thick as 1mm on the ends and .7mm in the middle down (like Columbus SP) to the very lightest (except unusual tubes) which is .6/.3/.6. Tange Prestige used to make tubing in each .1mm wall thickness throughout that entire range. In addition they can be of several diameters and the 3d variable is tubing hardness. _________________________

The 531p tubing was a very light gauge of 531 tubing, a step above 531c. The 531p tubing has the same dimensions as 753, minus the tensile strength, so it probably builds up into a whippy frame, and may have been used mainly for the smaller sized-frames, or perhaps regular 531 (531c) tubes were substituted in some places (e.g. downtube) for larger frames....

This is oversimplifying things but I think it gets the point across. _________________________

This has been an interesting discussion, but there appears to be some confusion about the difference between stiffness and strength.

The various grades of steels have different strengths - the better the grade, the higher the strength, hence the tubes can be made thinner without the bike breaking. This in itself should make no difference to the "feel" of a bike - until you crash, or ride it long enough to generate a fatigue failure. Actually fatigue is a whole other matter, and different grades have different fatigue behaviour.

Different grades of steels (and most metals) DO NOT have significantly different stiffness. Only the strength changes. The material itself has virtually the same stiffness (in engineering terms known as modulus). But using a higher strength material, with thinner walls, will result (for the same tube diameter) in a frame that is MORE flexible.

As has been pointed out above the tube diameter has an effect (actually a big one) - if the higher grade steel lets you make a frame with larger diameter tubes with thinner walls, then you will gain stiffness that way, despite the material itself not being stiffer. This is of course why aluminium frames have to have bigger diameter tubes, because the material has only about 1/3 the stiffness of steel.

Presumably, this is part of the difference between "lively" (read flexible?) and "dead" (actually stiff?) frames?

Of course the overall frame geometry will also have an effect, as will wheels etc, which makes perceived differences difficult to interpret. A complicating factor is that the overall weight of the frame/bike also has an effect on how "lively" it feels. I notice it most when off the seat climbing up a hill - the lighter bike moves around under me much more easily. Whether that makes it any faster is open to debate!

In the 531 vs. 753 example above the 531 frame should be stiffer (not more whippy) than the 753 since the 531 is thicker walled. In principle, a department store "clunker" with really thick tube walls should be stiffer again - if the tubes are the same diameter, and the geometry similar (which they probably aren't). There is measured data around on various stiffnesses of high end frames, but I haven't seen any that include really cheap frames.

I'm not quite sure what the first post above means by "Hardness". In metallurgical terms this is usually related to indentation testing, and can be used as a test to estimate the material strength, but in layman's terms could presumably be used as a term for stiffness.

Sorry, didn't mean to turn it into such an epic... Most of you probably know all this already.

Best regards,

Mark Battley
Auckland
New Zealand