L Cubed

So I like to analyze suspensions and understand what makes cars handle well and turn fast laps with lots of grip. There are a couple of numbers that help define how a car reacts to cornering. The ones I am interested in are Roll Stiffness [lb-ft/deg], Roll Couple Distribution [% front], Front & Rear Ride Frequency [Hz]. There are others that matter as well and I welcome all other folks to through out whatever else they like to analyze/look at when setting up cars.

I have a 944 S2 and from a little research I was able to find that the wheel rates are:
Front = 136 lbs/in
Rear = 137 lbs/in (24mm T-bar)

This creates ride frequencies, based on a 3200 lb car w/driver and 51% front weight distribution:
Front = 1.35 Hz
Rear = 1.38 Hz

I measured the anti-roll bars when they were out of the car and found the motion ratios of the roll bars to be: [degree body roll / degree roll bar twist]
Front = 0.240 (stock 26.8mm hollow bar)
Rear = 0.134, 0.119, 0.103 (positions on 3-way 19mm M030 bar)
Do these sound right? Has anybody measured this and willing to share?

By my calculations my car has roll stiffness of:
Front = 920 lb-ft/deg
Rear = 1019, 1199, 1494 lb-ft/deg (3-pos roll bar)
Total = 1940, 2119, 2414 lb-ft/deg
Roll Couple Distribution (RCD) = 47.4, 43.4, 38.1% Front

Going a little bit further, I made a guess of axle grip available to help determine balance by dividing total tread width by the axle weight. I have 205 front and 225 rear tires:
Front "Grip" = 0.251 mm/lb
Rear "Grip" = 0.287 mm/lb
"Grip" Distribution = 46.7 % Front

Now, the reason I am doing all of this is to help define and refine the balance of my S2 since I use it for autocross and DE's.

My understanding from a previous car (FSAE) is that I want the car's grip distribution to be within a few percent of the roll couple distribution. The end of the car with more roll stiffness per it's grip will be the end with less grip, i.e. if a car had 50% front "grip" level (meaning each axle has the same "grip") and a 40% front roll couple then the rear axle would have a larger portion of the roll resistance duty causing it to have less grip than the front; creating what is commonly called oversteer.

This is a little simplified as there are a lot of variables that can effect how much grip the actual axle has. There are also personal preferences in how much grip balance each driver likes for the various arenas in which they drive.

To wrap this somewhat long post up, I am starting this to share the information I found in hopes it can be useful to others as well get critiques and input from those experienced racers. I think it would be great to get a couple of the popular setups around and get their numbers for comparison and discussion.

marc abrams

Wow there guy, that's a whole lot to take in!! Let me say 1st I don't race my 944. But I will say I think it more trial and error than science. Look at NASCAR, they can spend a week at a track and don't get that car dialed in till the last 50 laps. You need to sit down and have a Beer with someone like Paul Wolfe for a answer to that question.

Dimi 944

This is a great analysis but you will need to start colecting data while the car is in motion to get a better idea about the dynamics of the vehicle.

StoogeMoe

You said you're going to use this for autrocross and DE. Is this going to be used on the street also? Creating a dual-purpose car will affect how far you want to go with regards to ride comfort. You will quickly find that those spring rates are not adequate for track duty. The dive and roll are quite pronounced. As long as you match the front springs to the rear torsion bars, you will retain the balance you are looking for. Remember, get the balance/dive/roll right with springs, adjust with roll bars, and then fine tune with tire pressures.

Doing the calculations is fun, but probably a better approach is to copy a setup from someone else that uses the car the same way you intend to. It will be cheaper than trying to experiment with a bunch of different combinations. Of course you'll get get a different opinion from everyone what they think is the best setup.

JustinL

"Any suspension, no matter how poorly designed, can be made to work reasonably well if you just stop it from moving."
-Colin Chapman

SpeedyC2

If Harvey Ferris stops by to read this thread, you might talk to him about the spreadsheet he has developed to "tune" the 944 suspension.

I spent some time going through the spreadsheet, analyzed a number of options, and comparing them to the "benchmark" I chose, which is the M030 suspension.

Based on these calculations, I came up with a combination of sway bars, springs, and torsion bars with which I have been very pleased. I was looking for more a Club Sport than a full-on track car, and think I hit the sweet spot well.

L Cubed

I believe that suspension tuning and development is based in science and it can be measured. I am going through this analysis and looking at how the springs, torsion bars, and sway bars contribute to the handling balance of the car.

In response to Dimi, I do have Race Capture Pro in the car for data logging and have collected some data though I will need to do some more driving because I recently changed the bushings out.

In response to StoogeMoe, at the moment (until I get a truck & trailer) this is a drive-to-track track car, so I am keeping that in mind as I look at how much spring and bar I add to the car. Also, I am doing the math/science analysis to help make the right choice in changes. I am fine with using a setup that has been well used and liked in the past but I want to understand how it compares to what I am driving now and what I have driven in the past.

Speedy do you have the 26.8mm front roll bar or the 30mm?

On a interesting side note, the stock setup has the roll bars contributing over 50% of the roll stiffness to the chassis. A popular setup for 944 Spec (that I read) is the 350 lbs/in front spring, 30 mm torsion bar, 28 mm front roll bar, and 22 mm rear roll bar; with this setup the roll bars also contribute around 50% of the roll stiffness for the car (this does depend on the position of the roll bar setting though). So the handling balance of the 944 seems to favor roll bars as much as springs/torsion bars for the handling balance, meaning roll bar adjustments are somewhat coarse balance adjustments versus being fine adjustments.

SpeedyC2

My S2 has the 26.8 mm front bar and 19 mm 3-way adjustable rear bar. It also has the Koni sport adjustable front struts with 200 lb. springs and 27 mm torsion bars.

Personally, I find the car to be very well balanced with sharp turn in. It also happens to ride quite nicely.

L Cubed

Speedy, what tire sizes are you running front and rear? I think this is probably one of the items that either makes a setup work or not.
Thanks

StoogeMoe

I haven't heard you mention anything about ride height, camber settings, or shocks. They will have profound effects on handling as well. If you're driving this car to the track, then you're going to have to compromise on the camber. It will also depend on what tires you're using. Some like more camber, some like less.

Lowering these cars too far can cause MORE roll unless you correct the geometry of the lower control arm. You will need longer pins to the ball joints.

Lots to think about.

FrenchToast

(Not making fun, just had to post).

You've done quite the analysis there. It goes to show that slapping the widest tires and stiffest spring you can find, don't always make for the best handling car.

L Cubed

StoogeMoe, you are absolutely right. I think camber is something you either set by testing, manufacturer's recommendations, or tire data if you have it. For the ride height, I had read that level is best and took that as a baseline for the 944 platform, if people use ride height to make large adjustments to the handling of the car we should add it in this analysis. Speaking of the shocks, I am very curious as to when the Koni Yellows become inadequate and whether or not the Ground Control Koni Double Adjustable shocks are as good as they sound.

*It would be great to get an accurate suspension model for kinematics analysis if anyone has one they are willing to share*

This is essentially why I started this post. To better understand some of the fundamentals in suspension setup that make the 944 work at or near it's best. For example backing up statements like running even tires widths front and rear for best balance, or even wheel rates for best balance. To me many of the setups can be more directly compared by assessing the amount of roll stiffness and it distribution front-to-rear along with ride frequency because ride frequency (or wheel rates) show how much roll stiffness is contributed between roll bars and springs.

Until I looked into I would have thought that the roll bars only contributed 20-30% of the roll stiffness. With the stock springs, torsion bars, and roll bars more than 1/3 of the roll stiffness comes from the front anti-roll bar alone! The other interesting thing is that the stock S2 had a roll couple distribution of 57.7% front distribution which is part of the reason I felt it understeered badly and so I decided to buy the M030 19mm rear anti-roll bar.

L Cubed

Based on these springs and bars, and my guess that your car is probably similar weight to mine at 3200 lbs and 51% front weight distribution, I calculate that for your car:
Wheel Rates
Front = 169 lbs/in
Rear = 220 lbs/in

Ride Frequency
Front = 1.50 Hz
Rear = 1.75 Hz

Roll Stiffness
Front = 1002 lb-ft/deg
Rear = 1216, 1396, 1691 lb-ft/deg
Total = 2218, 2398, 2693 lb-ft/deg
Roll Couple Distribution = 45.2, 41.8, 37.2 % front

I would make the assumption that you have 225 front and 255 rear tires mounted, which would give you axle grip of:
Front "Grip" = 0.276 mm/lb
Rear "Grip" = 0.325 mm/lb
"Grip" Distribution = 45.9 % front

So from this it would seem that running on the softest position of the rear sway bar would give you a near neutral car in mid-corner because the roll couple distribution is almost matched to the "Grip" distribution meaning each axle is "carrying it's own weight" through the corner and creating a neutral-steer feeling.

Now for the big question.... Speedy does this match your feeling of the car and roll bar setting?

Arominus

Ive got a less balanced setup right now ,200lb springs, 24mm tb, 30mm/19mm sways on konis and lowered a bit, 225/255 tires.

I've been running the rear sway on full hard to keep it happy, otherwise i understeer greatly. Like this i still get some push and with the LSD i get lift off oversteer that i can use to tighten up my line. I'm thinking i should soften up the rear sway a step though. I'm thinking the 27mm tb bars would good for me.

L Cubed

Arominus, from your setup I calculate

Roll Stiffness's of:
Front = 1507 lb-ft/deg
Rear = 1494 lb-ft/deg (stiffest bar position)
Total = 3001 lb-ft/deg
Roll Couple Distribution = 50.2 % front

Based on the 225/255 F/R tire width the "Grip" Distribution = 45.9 % front, I am hypothesizing that this 5% difference is a big contributor to your understeer, as you probably suspected by suggesting a move to 27 mm rear torsion bars.

Roll Stiffness if you increase rear Torsion bars to 27 mm:
Front = 1507 lb-ft/deg
Rear = 1691 lb-ft/deg (stiffest bar position)
Total = 3198 lb-ft/deg (+6.5 %)
Roll Couple Distribution = 47.1 % front

This would be a good step to reduce some of the understeer without likely creating and based on Speedy's car it sounds like 200 #/27 mm is a good combo.

I think another option for you might be to swap out the 30 mm front bar for one of the 26.8 mm bars.

Roll Stiffness if you swap to 26.8 mm front anti-roll bar:
Front = 1001 lb-ft/deg
Rear = 1199 lb-ft/deg (middle bar position)
Total = 2200 lb-ft/deg (-26.7 %)
Roll Couple Distribution = 45.5 % front

If my assumptions and measurements are right this option would provide a close to neutral car with the ability to move the rear bar either stiffer or softer to create more oversteer or understeer.

A lot of this depends on some of the variables in how the suspension works, which with more input from others on how their setups respond we could have a better of understanding of the suitable ranges of Roll Couple Distribution vs Approximate "Grip" Distribution to predict handling balance... at least that is what I am working towards.