Corner weight adjustment

[TMAPV]

If we have a sprung system, so springs on all four corners, why does the system not have 'design' weight distribution at any ride height? Adjusting my car I can change the load on the corner by adjusting the ride height through the coilover spring seat height. But the spring preload on each corner should always be the same as it's displaced the same amount by the mass, I should only be changing the total length of the strut by moving the spring seat. Is the subsequent change in cross-weight because of actual CG being tilted or is there some other effect?

 

[GregLocock]

A car sat on 4 wheels is indeterminate, jacking one contact patch up, or lengthening one spring, will result in a different force at each cp. The ultimate of course is to have the car on 3 wheels with the 4th hanging in mid air. This is known as the 4 legged stool problem.

Cheers

Greg Locock


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[SwinnyGG]

The change in cross weight is due to lateral movement of the CG. When the CG moves laterally, it also moves closer to some corners and further away from others, because the contact patches and the CG do not reside in the same vertical plane. This is why when you adjust one corner, all the other corners also change (though the change may be small).

 

[gruntguru]

The change in cross weight is due to lateral movement of the CG.

Disagree. Increasing preload at one corner will raise that corner marginally and displace the CG in plan view only very slightly. Almost all of the cross weight change is due purely to the increase in vertical load at that corner and its diagonally opposite partner and the decrease at the remaining two wheels.

je suis charlie

 

[GregLocock]

It is just high school maths to test the two hypotheses.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[gruntguru]

Even simpler. Take the case of very stiff springs (and tyres). A very small preload adjustment will move all weight to three wheels with almost zero displacement of anything - including CG.

je suis charlie

 

[SwinnyGG]

I should only be changing the total length of the strut by moving the spring seat. Is the subsequent change in cross-weight because of actual CG being tilted or is there some other effect?

When you lift one corner of the car, you're increasing the amount of load carried by that corner and the opposite corner (ie increasing right height on LF increases load on LF and RR). Think about a 4-legged table - if you pick up one corner, all of the table's weight is now being supported by the corner you're holding and the opposite diagonal corner, with none on the other two legs. This would be the case with a car too if the suspension and chassis were all infinitely stiff.