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Re: Chassis stiffness

From: Keith Clements
Category: Technical
Remote Name:
Date: 04 Sep 2002
Time: 23:28:50


Page 29 of Gerald's Autobiography mentions the torsional stiffness but not a value. I would not have thought it too difficult to measure. How about jacking one corner up at the suspension location point and seeing how far it moves before either the opposite or adjacent corner lifts. The other corners would need to be on some stands on the suspension mounting points. The jack could be in the form of a lever and weight, unless you have some corner weight equipment.

I noticed the torsional stiffness phenomenum recently when building the soapbox which had no added suspension apart from tyres and things that bent. Really the test needs to be done laden with the driver/navi and fluids as the stress and strain will obviously alter.

Anti-roll bars, panhard rod and other suspension locations will also affect the eventual stiffness but without a good chassis roadholding is doomed.

It would be difficult to find a chassis in original condition. Either rust or additional weld and plates would have altered the stiffness.

We do know lots of testing was done on cobbles and pave to test dynamic effects. How quantiative that was and what instruments were used, I do not know. They probably relied on the drivers bum!

Gerald did say that the straight chassis box sections to which the suspension was located was the main reason for such good roadholding and since these were welded to the front and rear bulkheads this compensated for lack of an all steel body. My only critism, having just looked at the Evo 7 that my friend is preparing for rallying, is that there is a lack of stiffness to opposite corners as this goes down the thin posts either side of the windscreen.

On my Jup I had designed a roll bar that located on the chassis each side of the radiator, went through the centre just behind the windsreen and went between the heads of driver and navi to locate on the roll-over hoop that also located to each rear shock absorber mounting. I never fitted this. But as a result, the door catches wear with all the yumps of rallying. The bending stiffness of the Jup is not good.

Last changed: November 19, 2008