Looking for ideas from the suspension geeks

[clausen]

Hi,

I've got another curly question that relates to a pretty different racecar to an FSAE. There are a lot of clever vehicle dynamics thinkers on here that I thought might be interested in coming up with some theories.

I currently work on racing a large sedan with a 650hp engine, wishbone front suspension, live axle rear with watts link, and a spool diff.

Some apparently typical behaviour with these is that when they have understeer issues, more rear spring can make it worse, and more front spring make it better.

Can anyone think of why? I was thinking that increasing front spring mihgt just be reducing bodyroll therefore reducing camber change at the front, but I don't think this is the case, especially as more rear spring doesnt help.
I thought that maybe the damping ratio was so bad that increasing the rate brought the damping ratio to a better place, which might increase grip. But I know that the dampers aren't that far out.
Something else I've read is that drivers can complain of understeer when the car is too tailly and after correcting a bit of oversteer on the way into the corner, the car starts understeering becasue they're turning in for the second time.

Has anyone got any other ideas? Anything that might relate to tyre sensitivily not working as per normal in certain situations?

[clausen]

Hi,

I've got another curly question that relates to a pretty different racecar to an FSAE. There are a lot of clever vehicle dynamics thinkers on here that I thought might be interested in coming up with some theories.

I currently work on racing a large sedan with a 650hp engine, wishbone front suspension, live axle rear with watts link, and a spool diff.

Some apparently typical behaviour with these is that when they have understeer issues, more rear spring can make it worse, and more front spring make it better.

Can anyone think of why? I was thinking that increasing front spring mihgt just be reducing bodyroll therefore reducing camber change at the front, but I don't think this is the case, especially as more rear spring doesnt help.
I thought that maybe the damping ratio was so bad that increasing the rate brought the damping ratio to a better place, which might increase grip. But I know that the dampers aren't that far out.
Something else I've read is that drivers can complain of understeer when the car is too tailly and after correcting a bit of oversteer on the way into the corner, the car starts understeering becasue they're turning in for the second time.

Has anyone got any other ideas? Anything that might relate to tyre sensitivily not working as per normal in certain situations?

[kwancho]

Aside from the obvious more front spring = less weight transfer in the front = better overall front grip?

[clausen]

If you add more front spring, more of the weight transfer is supported by the front, which means a greater difference between the contact patch normal forces at the front, which should usually mean a reduction in lateral force capacity at the front end.

Is this not always the case?

[Brian Evans]

I've worked on cars like that many times. What was happening for me was related to camber curves and excess roll. Increased roll resistance from spring or ARB would keep the tires happier. think about this if your large sedan has typical short a-arm lengths, there usually isn't enough room to have them long enough to minimise camber gain.

So it's all about the tires - weight transfer is only one part of the equation. Look at the roll center height of your rear suspension - if it's typically quite high, you probably have a lot of weight transfer from that alone, and can run very soft rear springs. I've seen cars like this, particularly with a spool, that basically would lift or close to zero load the inside rear tire. So looking for a change in weight transfer at the rear from increased rear spring is moot - you can't increase beyond 100%.

Brian

[Frank]

Are you ABSOLUTELY sure that after a spring change the tyre temperatures (or condition of compound) has not caused different behaivour? (with perhaps, a relatively small spring change doing nothing)

The ride height (rear roll centre height more importantly) has not been altered in the process? obviously these beasts have rediculously high rear roll centres...

Is this your observation from data?... or is it just a report you're hearing?

Have you got your car's numbers correct? I assume there's TWO motion ratios for the rear. one in roll, and one in bump...

My last guess, camber curves..

As far as I've always seen... its 2 golden rules
more rear roll resistance = more overster
more grip available from tyres/track surface = more oversteer

Best of Luck!!!