PedroNole

Larry, I've seldom found a reason to disagree with you and, based on your opening premise, that trend continues....

Rassel

That's another train wreck.

va122

Here we go again.

Larry Herman

I think that it is as much a personal preference as it is an engineering determination. After driving a plethera of cars, I prefer as stiff as possible (1400 lb+ spring rates for a 2700 lb car). So in answer to your question, for me, your car probably needs stiffer springs. Whether or not it is the ultimate setup, I don't know, but Porsche does believe in high spring rates in their race cars. I do know that I like it more and feel that I can get the most out of that kind of setup. With a softer car I feel that I am forever waiting for it to take a set. And I don't mind the hops and twitches of a stiffly sprung car. Others may like a more compliant setup and would be unnerved by a car that has such radically changing grip.

MUSSBERGER

Stiffer springs can sometimes offer more grip, and sometimes less grip, or sometimes both. But it sometimes comes down to intangibles.

stownsen914

Tom,

Yes, it's a six, but only a 2.7L. I used to run 300 in front and 175 rear in my 914 (at Doug's suggestion, by the way), and was always happy with that setup. I went heavier recently because I'm doing some experiments with underbody aero. From all the reading I've done it's important to keep the ride height as constant as reasonably possible when running a diffuser. Instead of going rock solid, I compromised and went somewhat stiffer.

Scott

Steve Weiner-Rennsport Systems

I think thats the most accurate statement of all,...

Track surfaces & conditions aside, personal preferences and skill levels play a big role in spring selection and chassis optimization.

Van

Instead of talking in "spring rates" we should be talking in terms of suspension frequency.

Spring rates are, of course, misleading - because it's really your wheel rate that's important (wheel rate is motion ratio squared divided by spring rate). On two cars with 400 lb springs, one might have a wheel rate of 200 lbs and one might have a wheel rate of 350 lbs, depending on where and how the spring attaches to the suspension arms - so comparing spring rates isn't "apples to apples".

Now, even wheel rates have their problems. Having a 300 lb wheel rate on a 1600 lb 914 will feel much stiffer than a 300 lb wheel rate on a 3400 lb 928. This is because there is a different amount of weight at each corner. I'm actually using incorrect nomenclature - springs are really measured in lbs/inch. For a 400 lb spring, it takes 400 lbs to compress it 1 inch; another 400 lbs to compress it another inch. So, in my example, a 928, being heavier, will compress the same "weight" spring more than a light-weight 914.

So how do we compare the two? Enter suspension frequencies! A formula that takes into account wheel rate and unsprung weight (how much weight is acting on the spring). This suspension frequency is usually measured in cycles per minute.

So, going back to the original question at hand, it should be: High suspension frequency = more or less grip?

Or, put in a more useful way: What is the suspension frequency sweet spot for different cars / tracks?

ervtx

Stiffer springs also allow you to run a lower ride height. Lowering the center of gravity leads to less body roll, putting even less demand on the springs in the first place (i.e. the effect is not linear).

A downside to stiffer springs is that the transition from "near the limit of adhesion" to "past the limit of adhesion" trends toward instantaneous. So stiffer springs require a lot more feel, and quicker reaction time in order to drive near the limit of adhesion.

A final thought... reaction time goes down as you age. Therefore, the question of "how stiff?" comes down to "how old?"

jgrant

+1

When setting up a car for multiple drivers, we tend to set it up with softer springs to accommodate the "Gentleman Driver". While the stiffer springs would suit the "Pro" better, the GD wouldn't be able to deal with it.

Premier Motorsp

Wow. No stupidity so far.

The way is it normally done is to figure out the roll gradient you want. You figure out how much roll you can tolerate before the suspension geometry really starts to compromise the grip. Roll gradient is simply degrees of roll per G of cornering force. A normal roll gradient for a fast GT car without downforce would be about .75 degrees/G. This is a stiff car, a bit stiffer than a 996 or 997 Cup.

Once you have that target, the math will tell you the wheel rates you need, and more math will tell you the spring rate you need.

On a 'normal' Porsche GT car you will wind up with about 1500 lb/in for a 2500 lb car that will pull 1.5 G.

Here is something interesting. In 2004 Porsche redesigned the suspension for the 996 RSR. They fixed all the suspension geometry which allowed the car to run above-the-ground roll centers for the first time. (Previous to this the GT cars had severely lowered street suspension, which put the roll centers well below the ground, which caused a pretty big increase in roll). These mods should have allowed Porsche to run much softer springs to achieve the same roll gradient. They ran the same old stiff (as high as 2150 lbs/in) springs anyway. Evidently they felt that the higher roll centers and stiff springs were both beneficial.

None of the Porsche GT cars run big anti-roll bars. Bars decrease the independence of the suspension and that is never a good thing. They also have other negative effects on grip.

It is almost always necessary to run a dual or progressive spring set up once the wheel rate exceeds about 80% of the corner weight(lbs/in vs. lbs mass). There does not seem to be any way to get around this and most of the opinions about stiff springs being bad come from attempts to ignore this rule.

Stiff springs require shocks with a lot of damping. Stiff springs contain a great deal of energy when compressed, and that energy is going to be released again in a short distance of spring travel. It takes purpose built shocks to deal with that correctly.

My untested opinion is that on a 'normal' Porsche GT car, stiff (>1500lb/in with dual or progressive rate) springs are faster in all conditions (provided that modern GT tires are being used) and faster yet on bumpy tracks. They however do provide less driver comfort and not everyone likes that. Ultimately driver comfort is a larger factor than suspension performance.

I like John Hajny's pyramid analogy. Somewhere along the range of springs, there is one setup that is the fastest, above or below that the car is slower.

BTW, a stiff and light T-bar car is using stiff springs is in violation of the 80% or corner weight rule. This is pretty much the reason why stockish 964s, 993s, and 996s corner better than stockish early cars.

Chris Cervelli
Spline Technologies

jrgordonsenior

Now where have I heard that before?....

RedlineMan

Hmmmm...

Of all the smart things you said (and they were legion), this is perhaps the smartest!

I only dwell in the realm of the Acute, Larry.

... The older I get, the stiffer I get.

Larry Herman

Chris, that was a great explanation. Your practical automotive knowledge never ceases to impress me.


Now as for you E Russell, reaction time may increase as you get older, but experience and anticipation increase. That allows you to recognize situations before most can ever feel them and can start corrections just as the car starts to go. You don't have to wait till you feel it, or worse yet, see it. So even though I am somewhat old, I still like it when it's stiff.

ervtx

Last night on a TV commercial, no doubt.
The stiffer I get, the older I feel.
Doesn't everyone?

Agreed. I actually run almost as stiff as my specific suspension allows, and I may never know how many spins I may have avoided as experience accumulates. I do know that on the few occasions when I have lost control (T4 exit at Laguna Seca in a SB Formula comes to mind), it was as though I never knew what happened.