[hcj986s]

Insite,

You're dead right on that. The 944 Turbo has a front spring rate of around 160 lb/in, but the wheel rate due to the MacPherson strut geometry is more like 125 lb/in.

JayKay,

Thanks for the kind words. Controlling roll "as much as possible" isn't necessarily the right way to go about it -- decreasing roll means increasing the roll stiffness which requires stiffer springs. You could have a car with 0 degree of roll per almost infinite g loading, but traction would suffer, since the suspension would be solid and any bump you'd encounter would make the car just skip along. Not that softer is necessarily better either -- that's the trick of tuning these cars. Indeed, you can get your car to skip around by dialling up infinite damping, too, as the rear suspension becomes solid during bumps.

Let me try to clarify again: the three components are springs, dampers, and anti roll bars. The trick to all this is managing where the load of the vehicle moves when the vehicle is no longer static (that is, at a constant 60 mph in a straight line, the load is static, but if you accelerate, brake, or turn, then the load moves around from front to rear, side to side, etc.).

Springs manage the weight distribution during changes in pitch (nose up or nose down under acceleration or braking) and roll (left side up or down depending on direction of turn) AND when there are no changes in pitch or roll (steady state cornering). That is, stiffening the front springs will not only change steady state weight distribution in a turn but also how quickly the weight moves from static to dynamic distribution.

(Stiffer means the load moves FAST. This is the "go kart handling" people talk about, since go karts don't have springs other than in the tires, so the load moves extremely quickly. It's not necessarily desirable, by the way--undulations in the road surface make the weight move around, too, and a stiff suspension will make it move around quickly, making the car very darty and more work to control.)

Anti roll bars do the same ONLY for changes in roll. They do not affect pitch behavior. (There are also things called Z-bars that act opposite to anti roll bars--when one wheel goes up, the bar transmits the upward force to the opposite wheel which also rises. Myself, I've never seen one).

So if your car works fine with springs only with respect to pitch behavior but not with roll, then maybe you need to adjust anti roll bars. This is in steady state as well as transient handling maneuvers.

Dampers only work during transitions, when weight is in the process of moving around (force is proportional to velocity, while springs: force is proportional to distance, and mass: force is proportional to gravity. The equation is F = mg + cv + kx, where m is mass, g is gravity, c is damping factor in N per m/s, v is velocity in m/s, k is the spring rate in N per m, and x is the spring compression or extension in m. The force is what's transmitted to the chassis through the suspension. This doesn't take into account the tire's contribution to all these variables as well as the differentiation between sprung and unsprung weight, blah blah blah).

At steady state, the weight isn't moving -- it's already where it's going (of course, nothing is ever perfectly steady state, but you get the idea). So the dampers are contributing no force to the system (v = 0). If the car understeers when turning in -- not after its turned in and at steady state, but DURING turn in, or v > or < 0 -- then the dampers are making the front roll rate too high and are thus too stiff. Stiffening the rears has the same effect of softening the fronts, which would allow you to dial in more rotation during turn in.

This is a gross simplification, but ideally you'd set the car up with springs and dampers alone and see if you can get it to handle "properly" (maybe you like understeer or maybe you want neutral steer, etc., and the difference front to rear in spring rate contributes to pitch control -- you don't want the car to porpoise along after hitting a bump). If the geometry of the suspension isn't cooperating (too much roll = suboptimal contact patch, based on tire temperatures inside, outside and middle of tread), then add the anti roll bars and adjust the springs to restore the balance. Of course, adjusting camber will be a necessary step in here, too.

To control the spring & mass system, you need dampers. Install them, tune them so they're not too stiff (over damped meaning the car doesn't move when you hit a bump) and not too soft (car floats up and down repeated when you hit a bump). Then tune them, too, so that during transitions the car behaves the way you want (maybe you like lots of turn in oversteer, maybe you aren't so daring).

You're right about tire pressures: There is only one optimal tire pressure. It's the one that ensures the tire isn't over inflated (middle of tread tire temperature higher than average of inside and outside edge) or under inflated. It's really that simple. Yes, a stiffer tire increases spring rate and changes the damping, too, and you can use this as a stop-gap measure to adjust handling (in the rain, take out rear tire pressure to add understeer and soften the rear suspension for better traction). But deliberately running too high tire pressures is obviously wrong given the negative effect on tire wear.

If you can't get your tires to the manufacturer's suggested operating temperatures (where grip is maximized), then you have too much tire for the car (the contact patch is underworked).

You bring up ride height, which I didn't address much. Lowering the car is sexy, but mostly it reduces the vehicle's center of gravity. The force transmitted to the outside springs depends upon the mass of the vehicle, the height of the CG, the track, and the cornering Gs. Lower CG = lower weight transfer = more even weight distribution side to side despite cornering.

But lowering the car also has a pronounced effect on this weight transfer stuff, since lower ride height = stiffer springs. It also could affect geometry of the suspension -- excessively lowered 944s often suffer ball joint wear due to extreme angles of the tie rods. On my 944 Turbo, I lowered it 20 mm which is what the 968 Turbo RS's setup called for. I think that's the max drop with the rest of world M030 for the Boxster as well. Note that in this case, Porsche drops the front 20 mm and the rear 10 mm. All things being equal, this makes it harder for the front to roll (lowers the CG relative to the front roll center more than the rear roll center) -- increasing roll stiffness. Interesting, isn't it, that the ROW and USA M030 use the same anti roll bars? This illustrates the complexity of all this -- Porsche could have simply ordered up a larger front ARB instead of dropping the ride height to get the same effect, but for some reason they didn't.

I find it interesting that the Bilstein PSS9 setup doesn't suggest new anti roll bars (at least I haven't found anything suggesting it does). If we assume it gets to the same roll control as the ROW 030 setup, then the PSS9 must be much stiffer in pitch than the factory setup. Driving down the street, the PSS9 should bob up and down much more than the 030. Maybe you like that sort of thing, maybe you don't. Bear in mind that race tracks in Europe are smoother than ones in the US (generally), so the PSS9 might work really well in Europe but not as well in the US. I don't mean to rag on the PSS9, since I'm sure it's excellent in its own right, but one just needs to understand what it is they're handing over their Benjamins for.

My 944 Turbo had Koni coil overs front and just heavier rear torsion bars. If I wanted to adjust front/rear springing, I could swap out the front spring. It had Koni shocks all around, so if I wanted to adjust transient behavior, no problem. And I stuck to M030 anti roll bars. So with camber adjustments, I limited the variables to 5: single adjustable (bounce) dampers front and rear, front springs, camber, and rear anti roll bar soft, medium, firm. For the Boxster, it's ROW M030, period. No muss, no fuss.

Most importantly, while this is all complicated, if you give it careful thought and planning, noodling around with setting up your suspension can be a lot of fun. Change one variable at a time, see what happens, and if you likey, good. If you don't, then do the opposite. I'm not suggesting you not do it, just that your mileage will definitely vary and if you're not careful you can make a dog's dinner out of the whole thing.

Here's an idea: remove your car's anti roll bars and take it for a drive. Then put the rear bar back on and see how it handles (massive oversteer). This will give you a taste for what's in store for you if you want to go down this path. Maybe you'll love experimenting, maybe you'll say it's not your cup of tea.

If you want to learn more, try Fundamentals of Vehicle Dynamics (Gillespie) (not as much detail), How to Make Your Car Handle (Puhn) (a little hokey and definitely grass roots stuff), and the granddaddy of them all Race Car Vehicle Dynamics (Milliken & Milliken). The latter is THE authoritative tome on how to set up your car. It is 900 pages long, extremely complex, and gives all the answers but does not concern itself too much with ride comfort.

Hope this helps and good luck.