Falcondrivr

I noticed the Sunset special on RS bars in excellence this month. My car has what I believe is excessive body roll when the suspension is fully loaded on the track. I have some questions:
1. how much does this effect lap times?
2. is it possible to dial it out and still have a comfortable ride on the street?
3. Do the RS adjustable sway bars do this?
4. If not, what does?
5. What questions should I really be asking?

I drive my car as a DD on urban streets (some brick paved from the 1920s) and 6 or so DEs a year...

My car now has:
Bilstein HD lowered to RS+10 (ROW-10)
M030 springs
stock M033 sway bars
R compound tires (nitto NT01, pics below were on worn Hoosier R6)
Pics of what I'm talking about:

levtron

Nice pics...

pirahna

nine9six

Maybe a PM to Bill Verburg is in order? Pirahna, are you playing with multiple windows again? ^^^

Bill Verburg

your spring rates are way low for a track car, not surprising as it's a compromise where you are on the street side
M030 springs are 166/314 lb/in
RS are 246/457 lb/in
Cup are 1142/1256 lb/in

A common track biased dual purpose setup w/ good digressive shocks will be ~600/800 lb/in

I'd start there, the RS sways are nice but are more of a tuning aid than a main spring is

Suspension is a system of components adjusted just right. You need all of them for max effect, RS sways are a part of the package

you want to lower the car as much as possible, this means shorter springs and shocks, RS wheel carriers, solid rear sides, stiffer than stock bushes, RS sways, 8.5/10 wheels(x18 gives the best tire selection)

As the car is better on the track it gets worse on the street, especially so if roads are bad

jscott82

Soft Springs + R Comps = Queen Mary

As Bill said, you need to get the springs in the ball park and use bars to fine tune... With a stiff spring a good shock is even more important, especially for the street.

Or

Go back to good summer tires and slide it around like you are on Top Gear...

Paul902

Good input, Bill. What springs are ~600/800, though? What is body roll with RS springs and sways like compared to M030, and will RS springs work with HDs? I've wondered this in the past, as, like the OP, I long for less body roll when at the track. I also have HD/M030s, and find the HDs could take more spring. Here is a fast sweeper, where you can see the compression on outside. At the time my car was also about RS+10mm.

Kika

Certainly Bill is much more knowledgable than myself on this topic, but...

I would think that maintaining all tires on the road with equal weight on them would maximize traction and result in best track times. Reducing body roll would help to maintain equal weight all around.

That is up to the point where the tires break loose.

So, to me it seems logical that faster lap times would result from reduced roll.

I don't track my 993, but I did put row turbo bars on my car, and it certainly does reduce body roll and is still quite livable on a street car.

jscott82

You are not really talking about a pre-packaged setup here... To get into the 600-800 range you are going to buy an off the shelf spring and get shocks custom valved to match (add in adjustable shocks to fine tune).

To make this work you will need some trial and error (or someone that has done it before) to get the spring length, helper rate and length right.

jscott82

Actually softer is better.

... but ....

Like all things its a trade off, the body roll causes so much movement of the suspension you cant control the alignment and that hurts you more than the softness help... so its all a trade off...

Bill Verburg

I have Eibachs but i'm sure that all the usual suspects can supply them

You need perches to match, these are 9" x 2.25"ID x 600# fronts on shorter 300mm Cup shocks, stock Monroe are 347mm and RS Bilstein are 320mm


Spring rate needs to be correlated to grip, you would use lighter springs where there is less grip, either due to tire compounds or conditions and use commensurately stiffer springs where there is more grip

To eliminate roll stiffer springs are necessary, correlated to the grip levels expected, you also want the car low w/ little suspension travel, then setup the camber to take advantage of what body motion remains.

jstyer

You are right about the first and last bit, but you're a little off base on the stuff the middle. It's a common misunderstanding, but for a given height, increasing roll stiffness will not reduce weight transfer. The amount of weight transfer is set by the RATIO of front to rear stiffness, not the overall values. Increasing overall roll stiffness at the same f/r ratio just allows the car to react more quickly to driver input, and can actually decrease overall grip levels. (all else being equal, soft is sticky)

The reason experienced drivers almost always prefer higher roll stiffness is due to the confidence it gives during quick direction changing at speed and negative effects due to roll on suspension geometry. They no longer have to wait on the car to react to their steering inputs, and the wheels stay properly aligned to the road surface. On the flip side, super stiff springs can get an inexperienced driver in trouble. Since vehicle dynamic load changes are now happening in a much smaller space of time, the driver needs to be "on their toes" with inputting small steering corrections.

(Deleted my last paragraph, Bill's last paragraph above is gold for a quick and dirty break down of what needs to happen on a track friendly suspension. Also, jscott said pretty much the exact thing I did... But way shorter/better. Don't know how I missed his post too. I'm out of it today. )

Paul902

Hmmm, unfortunate... Is there any data/anecdotal evidence about how RS springs work with Bilstein HDs? Are RS springs the same nominal length and diameter as M030s?

race911

The only contribution I'll make is that there's no mention of data acquisition--are you within about the last sec/min of lap time?

The one suggestion I'll make is a single phone call to Steve Weiner, and he'll fix you up with what will work for: 1. your experience level, 2. your goals, and 3. the tracks you run. _______ around on internet forums once you get to that multi-adjustable damper discussion is both futile and expensive. And if you're going to seriously change parts, you're going there.

Kika

Thanks for the explanation, I was pretty sure it was far more complicated than my simplistic view.