Larry Herman

Nope, less roll resistance = more grip at that end. Less roll resistance allows the inside tire to have more "pressure" against the road, and makes for a greater amount of grip between the two front tires, not considering any other variables. It is the balance of roll stiffnesses between front and rear that determines the understeer/oversteer characteristics of the car.
I prefer stiffer fronts springs than what the GT3s come with too. I'd balance it with softer front /stiffer rear sway bar settings.

mitch236

I really don't have a firm understanding of how springs work towards the balance of the car but I can say that after changing the rates to 750/950 from the stock PSS9 springs, I had alot of oversteer where prior to the change, I had mostly understeer. Then I changed the rear rates to 850 and had less oversteer. Maybe I am wrong but to me the best way to tune a suspension would be to start with spring/damper combinations to get the broad stroke of car balance the way you like it and then fine tune with damper settings and sway bars.

The reason you won't (or shouldn't) find known best setups is that it is so driver dependent. Although, you may find alot of opinion about broad stroke setups which you can fine tune for your style.

AudiOn19s

Thanks for clarifying for me. Still gonna take more research on my part to determine spring rates for my car...luckily I'm not doing coilovers until this winter so I've got pleanty of time to change my mind over and over again trying to come up with a good starting point!!!

Andy

AudiOn19s

Also...going back to Mitch's issue...here's an oversteer / understeer tuning chart that I have used as reference in the past and found useful...of course with the M3 I'm always leveraging the understeer chart and had no use for the oversteer chart...though I did finally get the car neutral to my liking. Not 100% sure if all of this will apply to the rear engine 996 or not?...maybe we could put together one that's 996 specific?

gbaker

I avoid this subject like the plague. With any car being represented by at least two masses and two spring systems (the real springs plus the tires), there is no simple equation to describe the optimal setup for a given environment. I was once chatting with the chief engineer of a nationally-known pro team and casually asked how one goes about designing a suspension. Man, was that a mistake.

The best thing you can do is take the advise of the experts here.

Geo

That is not my understanding of this subject. Some elements are correct, but the cause and effect is wrong.

First of all, the determining factor is the rate of the load transfer F/R that determines understeer/oversteer, not roll stiffness (all other things being equal of course, e.g. same suspension geometry). Roll stiffness is an influencing factor, but is not the true cause. Stiffer springs and swaybars speed up the load transfer (but do not affect the total load transfer).

While this may seem a subtle distinction, I personally think it's helpful in understanding chassis tuning. Carrol Smith's Tune to Win is an excellent reference for discussion of this material.

Larry Herman

George, I am certainly not an expert in this subject, but I do know that what both you and I are talking about affects the balance of the car, and it does so in different ways. That is why I mentioned " not considering any other variables". Depending on the overall roll stiffness, the f/r spring rates, the shock valving and a host of other things, you can affect the balance of the car in different places in the turn. I feel that roll resistance is a gross adjustment that affects the car throughout the entire turn, whereas some other things may affect the car more while you are on the brakes, at initial turn-in, during transition to power or at trackout.

RedlineMan

Hmmm...

It is helpful to keep in mind that all of these things are on a bell-shaped curve. Until a certain point, too soft can cause understeer, and as you transition over the hump of the curve, you get the same result for the opposite reason. Let's talk just springs.

For instance, front understeer can be result of too low a spring rate. There is not enough resistance to weight transfer, or perhaps not a quick enough resistance of it. As I mentioned previously, there is not enough spring pushing back against the chassis, nor pushing the tire into the road, and/or not quickly enough. This often also manifests itself in front brake lock as well.

If you continue to raise the front spring rate, you will reach a point of diminishing return where the car then starts to understeer because there is NOT enough dynamic weight transfer, or TOO much force pressing down on the tire and overloading the grip potential. In between is a range where you find varying degrees of success, depending on what you are doing elsewhere with dampers and sways.

You can also have too little and too much roll stiffness, with a similar plotted curve and results. As Larry mentions, you can gain too little grip from the inside front tire if the car squats too much to the outside and lifts load off that inside tire. So too can you have too much roll stiffness where the inside tire is lifted off the ground. This usually results from too stiff a sway bar, used for the wrong reasons. That reason would be using huge sways to control roll. When you get down to it, it is SPRINGS & DAMPING that control roll in high performance track environments. Bars are for front/rear balance tuning. This is particular problem in the older 911s with their wishy washy chassis.

Now, in the first instance with soft springs, you may have some reason for wanting soft rates, like perhaps SEBRING! In that case, you may be able to increase compression damping to compensate for dive while still allowing the suspension to react with some suppleness to the more general humps, bumps... & MOUNTAINS you find there. But that is getting complicated.

My suggestion would be to start with a well-matched set of sway bars, meaning neither was outside a "normal" range for that weight and inherent balance of car. Then, I would say that a 150lb spread from front to rear spring rate would get you in the ball park. It would be very helpful to first tune the suspension with springs alone, and find a good balance. You would then be relying less on more "artificial" means of tuning the balance, like sways and damping.

I might guess that the stock 996 sway setup would represent that sort of balanced ratio, where the GT3 setup would not. I'm not saying the GT3 does not work, as Larry clearly proved it did... quite! However, they seem to have played one end against the other a bit, and so to me that is not a truly linear basis on which to begin your quest.

I would be very inclined to start with a stock 996 setup and work from there. As I said earlier, my experience with the 930 shows great success with that ratio. Not that there is a direct correlation between that old dinosaur and a 996, but they are not strangers either.

Premier Motorsp

The key to understanding sway bar adjustment is to understand that they affect the total lateral load transfer distribution (TLLTD), with distribution being the key word.

When you soften the front bar, you make the front take a little less of the load transfer, and you make the rear take a little more. Normally this results in a decrease in understeer. You traded rear grip for front grip.

You have to realize that this is always a trade front to rear or rear to front. Many people get confused and think "Well I adjusted my front bar softer, and now the front grips better, so I will go softer yet and get even more grip. Then I will soften the rear bar too and really have some stick." It does not work like that.

On the other hand it is possible that your front tires hit 100% of the limit and because the front bar is too stiff your rears are only at 80%. Softening the front bar in this car WILL increase the total grip.

Chris Cervelli
Premier Motorsports

RedlineMan

Very good point;

It is always hard for me to believe that people don't get this, but I know there are lots that don't. I give them an analogy. Think of your car as a cake pan, half full of water, held up by full suspension at each corner. If you make an adjustment to the damping, spring, sway, you will be shifting the amount of water around. You want to shift enough, but not too much. To get more grip up front, you want to get some more water to the front, but you don't want to spill it either.

If you spill, you are too soft, and that is probably is a good indication of an oversteer situation. If the level does not change, you probably have understeer. Sometimes giving people a solid tangible object to focus on - in this case the water - helps them visualize dynamic load shift. Like this load, water will seek the path of least resistance.

My rule of thumb is start out soft, and only stiffen things up until your objective is reached. Anything balanced is workable to some degree, but too soft is more manageable than too stiff in most cases.

AudiOn19s

I know I didn't start this thread but I really appreciate good reads like this esp since I'm currently trying to choose a suspension for my 996.

The wealth of knowledge shared is the reason this is such a great community!

Andy

mitch236

And there's the rub. I am always being asked what I am doing since I started this project. The truth is, my setup will eventually work, FOR ME that is. I tend to like a well balanced car that tends towards understeer. That suits my style. There are a fair amount of drivers that like a little oversteer. I also prefer a setup that is a little soft because of the tracks I drive on. That probably won't work for many of you who don't regularly drive at Sebring.

But overall, this thread has alot of good info and I want to keep it alive until I feel confident that I understand exactly what spring changes do to my car. Next will be damper settings. I figure at the end of my exercize, I will have a firm grasp of the setup for my C-2.

Geo

Buy and read "Tune to Win." Very educational. Easy enough for someone with a semi-technical mind to read while at the same time having solid information and plenty of formulas where appropriate. I read it with a highlighter.

mitch236

Thanks and I have been reading it.

This is what I think so far:

(I want to keep the principles simple so I will think of the suspension system consiting of three parts, the springs, the bars, and the dampers.)

The springs control the ultimate movement of the sprung mass. So if the front compresses 2" under braking then with higher spring rates, the front now compresses 1" in the same time frame so the mass transfer happens faster.

The dampers control the time the mass transfer takes but not the ultimate movement. So if the front compresses 2" in one second, increasing the damper would increase the time to say, 2 seconds.

The bars tie the two other components together, while acting somewhat as a spring themselves.


Does this sound (very simplistically) correct?


Now for the big question. When tuning a suspension, which component do you start with and do you start soft and go stiffer or the other way around?