Geometry question?

[DCL]

My car was working well on Kumhos but this year I've been swapping tyres to meet 1b list. In particular, I've been trying the ZZS 215/55R13 and the R888 225/45R13. Now there's about 30mm difference in the diameter of the rear, so it changes the rake by 15 mm (or about 7mm using the usual measuring points).

I assumed that adjusting the rear ride height, to maintain the rake, was the correct thing to do, but that seems not to be working. My question is whether adjusting the rear ride height (to achieve a constant rake) with different size tyres is valid. Bigger tyres raise the rear roll center, but adjusting the rear ride height does not change it (just moves the C of G).

So, should I really be adjusting the front ride height to preserve the relationship between front and rear roll centers and the roll axis angle?

Just trying to avoid a lot of testing really.

Confused. . .

[Equus]

In terms of weight transfer, yes, it's maintaining the same roll axis inclination that is what counts... but then you'll find you are screwing up a lot of other stuff in the process.

For example, because you're raising the CoG, you'll need more roll resistance, but not necessarily any more ride rate (OK... maybe a little more to dial out the slightly increased pitch and squat), so you can kiss goodbye to your previous spring optimum, ARB and damper settings.

If you keep the same roll axis inclination, but change the rake, you're altering the underfloor aerodynamics (not much, being a Seven, but even so), and the effective trail on the steering, etc. etc.

Bottom line is I don't think you'll find a simple formula, and a certain amount of testing trial and error will be necessary.

[Amris]

I'm with Equus here, I recently had my car flat floored and i couldn't believe the difference it made to the handling (especially breaking). I stayed and watched the guys do it and they walked me through the fine art of suspension setup. It took a whole day and was setup with my specific body weight. In conclusion i would say get it professionally setup with the tyre diameter you are happy to stick with and don't touch it if at all possible :-)

[DCL]

Equus said:

In terms of weight transfer, yes, it's maintaining the same roll axis inclination that is what counts... but then you'll find you are screwing up a lot of other stuff in the process.

For example, because you're raising the CoG, you'll need more roll resistance, but not necessarily any more ride rate (OK... maybe a little more to dial out the slightly increased pitch and squat), so you can kiss goodbye to your previous spring optimum, ARB and damper settings.

If you keep the same roll axis inclination, but change the rake, you're altering the underfloor aerodynamics (not much, being a Seven, but even so), and the effective trail on the steering, etc. etc.

Bottom line is I don't think you'll find a simple formula, and a certain amount of testing trial and error will be necessary.

Thank Equus, that certainly bears out in what I was finding. I suppose I'll just have to tweak until I find the sweet spot of each set of tyres and carefully note the settings. But the next question is what effect do changes in the roll axis (specifically the angle) have on weight transfer/balance, and what's the mechanism that produces it. I understand how the basic roll rates of front and rear interact, but less so how the axis along which they act can change it.

Thank also Amris, the car is a sprint car and this year has been flat floored prior to each event. That's perhaps why I asked the question as I'm confident about what is changing and it didn't add up, until I realised that the De Dion tube fixes the roll center at the A frame bush. Adjusting ride height does not change it like the front, but tyre diameters do. Although we're talking about relatively small changes, they do become apparent at the limit of grip. The commonly held view that rake should be 10-15mm is a good starting point but the best actual figure can vary depending on tyre sizes amongst other things.

[SimonRogers]

I think you will find that there will be a prefered setting for each tyre - no doubt. But they will be marginal. The big difference is the tyres themselves as you have found and no amount of tweaking is going to recover that gain or tyre preference you have.

Luckily when I was swapping from slicks to Kumho the front was only a 5mm diameter change and it was the rear that was the big big change. 1" on the rear spring collar. I had a rule marked up to ensure the change as acurate and could change on the day if required. (I was using the Kumho as wets or at events where I wanted to save the slicks).

I needed to shift some grip rear to front by using the damper rebound adjustment and that was all. Still perfectly balanced.

Your issue is complicated it seems from the big change from Kumho to others at the front. My starting point is always the front ride height so I would agree that all 4 corners need changed to find the same balance.

I do have a solution.

Fit some slicks :-)

[DCL]

Thanks Simon, I'm sure I'll be able to easily dial out the issue, and recover the balance, with the Penske's. There's no doubt that the Penske's just keep getting better with grippy tyres, and slicks are a 'must' at some stage. Having just knocked 2 seconds off the lap times with Penske/Kuhmo, I was just wanting to understand the process that was causing the change with the 1B tyres. But having spent a few days reading up, it's clear that the interactions can be very subtle, and are too complex to assume anything. Best to treat the symptoms and not worry too much about which medicine I use.

Edited by DCL on Sunday 21st August 09:36