[CR]Stiffness of metals


Example: Framebuilding:Norris Lockley

Date: Tue, 12 Feb 2008 09:52:49 +1100 (EST)
From: Mark Battley <mark.battley@xtra.co.nz>
To: classicrendezvous@bikelist.org
Subject: [CR]Stiffness of metals


Just to be a pedant: Most engineering metals have very similar "specific" moduli of elasticity: modulus divided by density. E.g. while steels have a modulus that is roughly three times that of typical aluminium alloys, the d ensity is also approximately three times. Similar ratio holds for Titanium. Different alloys and heat treatment processes can very significantly chang e the strength of the material, but they have virtually no effect on the mo dulus.

So in the case of rims if you use a higher strength alloy you n eed less of it for strength, so actually your stiffness will go down for th e same shape - but of course in practice it all depends on the rim geometry , spoke count and pattern etc.

In the case of frames, the high strengt h alloys let the wall thickness and hence weight be reduced, but for more-o r-less fixed tube outside diameters (as in most steel frames before "oversi ze" tubing appeared) this means that the stiffness of a high end frame WILL be less than a clunker. Same stiffness material, more of it in the clunker because it needs it to be strong enough.

As an aside, we tested a few frames for bottom bracket to headtube torsional stiffness as part of a pro ject I had some uni students on last year. The 1970s "gas-pipe" clunker was stiffer than a current carbon-fibre Scott CR1 (although more than 3 x heav ier..), almost 3 time stiffer than a 531C tubed frame, and only a few % les s stiff than a Cannondale CAD3. How stiff is stiff enough? Who knows...

Off soap-box.

Mark Battley Auckland, NZ

----------------- ------------
>> The old MA2 is softer old technology

material.
>> The new aero type rims and material s are far stiffer and stronger so you can
>> u se less spokes and be plenty rigid.
>> Look at

all the 3 & 4 spoke wheels of today.
>I'd

just add that the added stiffness of aero sectio n Al alloy rims is
>not due to materials changes

but the increased sectional area, which
>is why t hose aero rims are heavier as well. All post WW II Al alloys
>(and really all engineering metals, Be ryllium notably excepted) have
>similar moduli of elas ticity.