[hcj986s]

A little late to help the posters above, but I think this issue needs some clarification.

I've have seen too many people at the track all but bragging about how stiff their springs are (ie "which springs are the stiffest?"). I've also heard people say adjustable shocks are the bee's knees because you can stiffen them up for the track and soften them for the street.

Thinking about your suspension in these terms is completely ridiculous.

Let's start with the springs. As a suspension moves, its geometry changes. Modifications to the suspension for track use should therefore focus on "correcting" these movements. For example, in bounce, MacPherson struts generally add positive camber -- not what the tire contact patch needs for maximum grip. Likewise, the inside wheel adds negative camber. If your car is set up with, say 2 degrees of negative camber, then the inside front tire might go to 3 or 4 degrees, based on the rate of change in camber (in degrees/g) as the suspension compresses.

The issue here is the ROLL RATE (deg/g), not how stiff the springs are (which contributes to roll rate, to be sure). As a 4-wheel vehicle turns, weight is distributed from 50/50 right to left to some amount where the outside is greater than the inside. In addition, differences in front and rear roll rates cause the weight to shift from front to back (or vice versa). Hence, a vehicle with a static 50/50 weight distribution might go to 60/40 in a turn.

This happens because the front roll rate is generally in excess of the rear roll rate (well, this is vastly simplified, since the height of the roll center and other variables need to be taken into effect, but let's think in these terms for now). The following is counter-intuitive, but imagine a board placed across two springs, one 50 lb/in and one 100 lb/in. In total, the spring rate is 75 lb/in, so if a 150 lb person gets on the board, the springs sag 2 inches. But in fact the stiffer spring must take more load because the softer spring compresses less in order to keep everything level. You might say, no, the board is now tilted, but Shazam! The weight must move towards the 100 lb/in spring-- hence the stiffer spring takes more of the load relative to the static weight distribution!

As you stiffen the rear suspension roll rate, say by installing a stiffer anti roll bar, then we all know the car will tend away from understeer ("US") and towards oversteer ("OS"). In our example, the person on the board moves back, towards the 75 lb spring. Shazam! again.

So if the car has 50/50 front to back and 50/50 right to left weight distribution, then in a turn, perhaps 60% of the weight is on the outside, and because the front is stiffer than the rear, of the 60%, maybe 35% is on the front outside and 25% on the rear outside.

As the front outside tire gets more load, it grips better (the friction force is proportional to the weight on the contact patch times the coefficient of friction, mu), but the nature of tires is that the additional grip is not linearly proportional to the weight on the tire (mu changes). As a result, the front tires together as a combined system do grip more due to the weight transfer to the front, but not proportionally more than the rear -- in fact, it is proportionally LESS than the rear. Hence, the REARS grip more relative to the fronts -- and the car starts to US.

It should be intuitively obvious, then, that simply bolting up the stiffest springs and anti roll bars will modify the roll rate with no clear idea of what happens to the weight transfer. This could make a car handle worse than stock.

If you want maximum performance from your car, all this must be balanced. You cannot solve roll stiffness by anti roll bars alone, since at some point the combination of springs and anti roll bars starts to act "funny". Likewise, bolting the stiffest springs you can find and keeping stock ARBs means the car rides like a truck -- and more importantly, when the car isn't turning, the ability of the car to maintain traction is compromised, especially in the wet or at bumpy apexes.

So in fact we want the softest springs possible while maintaining minimal geometry changes in the suspension from roll. When I tuned my 944 Turbo, I used photos of the car on the track, measured the roll and the lateral acceleration as well as the change in camber, and then determined what I wanted out of the suspension in terms of roll rate, etc.

Moving on to dampers, they, the springs, and the unsprung mass of the vehicle comprises a total mechanical system. Underdamping the vehicle means that when it hits a bump, it oscillates as the chassis moves up and down relative to the vehicle. Ideally, it should move down (relative to the wheel, which is really the only thing that's moving as the road shocks move it up and down -- the chassis stands still), then up past equilibrium, and then back to equilibrium. If too firmly damped, the body goes "down" then back to equilibrium. This contributes to a harsh ride because HIGHER frequency vibrations are transmitted into the chassis. In other words, firming the damper increases the resonant frequency of the suspension. Softening the damper reduces the natural frequency of the system, which means as you tool down the road and hit minor undulations (ie low frequency inputs), where the stiff suspension wouldn't really notice them, the soft suspension wallows around -- which is also a degradation of "ride quality". Softening your dampers for street use will make your car handle like a Lincoln Continental -- if you want to do that to your Porsche, so be it.

So what's the deal about dampers? Well, the key is they act like springs when the suspension is MOVING. Springs act like springs when the suspension is STATIC (including steady state turns). If the car AT TURN IN is understeering, then from above we know the front roll rate is too stiff. As the damper compresses, it is adding to the roll rate, so we need to soften the front damper. You get the idea.

Softening the damper for street use and firming it for track use results in a car that has bad ride on the street. And mindlessly dialling up the dampers to "9 or 10" on your PSS9s is not the way to tune your car's handling.

One final word: Porsche race suspensions are softer than you think. The 968 Turbo RS front springs were 225 lb/in vs the stock 160 lb/in (fronts). NOT 400 LBS. NOT 500 LBS. NOT 1,000 LBS. At those levels, the suspension begins to be nearly solid, which means the tire starts acting like the primary spring in the suspension. While the suspension geometry changes are vastly reduced, traction is severely compromised. Stiffer springs also means the vehicle's weight transfer reacts faster to steering inputs, which makes it harder to control, especially in the wet, as the weight distribution changes rapidly.

Hope this helps.