911 SC: Suspension tuning for Street/Track car

[tvanbroekhoven]

Hi guys!

Last weekend I was giving it my best at the Zolder track in Belgium. During that time I noticed my a** was being kicked thoroughly by all kinds of hardware. Especially those little Lotus thingies.

On the straights I had more than sufficient power to keep up.

The problem was mostly situated in the corners, where my car seemed to understeer quite a bit (also in the fast corners). This could be caused by either my incompetence as a driver or as an engineer. Or a painful combination of both.

Now for the facts:

Setup (911 SC Coupe):

- 21mm front torsion bars
- 26mm rear Turbo torsion bars
- Bilstein B6 shocks
- Carrera sway bars. 22mm front, 21mm rear
- Corner balanced: with driver 10 kg difference rear and 2 kg difference front

Since the car is already pretty harsh on the road (Belgium = poor road quality) and I do not want to make that much worse. I want to be able to use it for a nice sunny sunday drive. But I've heard that overdamping could compromise ride quality and swapping to bigger bars that match the bilstein shocks better could even improve things. Is this true ?

Possible solutions:

- install 20mm torsion bars in the front
- install 27 or 28 mm torsion bars in the rear
- buy a Lotus


What do you guys think ?

Thanks!

[Grady Clay]

tvanbroekhoven, Tim;

Two big factors you are fighting are weight and center-of-mass; the Lotus has a 911SC on both accounts.
You can address these to a point but you lose the nice street-ability of your SC.

You are correct; the ‘harsh’ ride is most from the stiff shocks.
Keep the same (or slightly ‘softer’) shocks and enlarge the torsion bars to 22 mm front and 30 mm rear.
You may want an adjustable ‘through-the-body’ front sway bar and reinforce the rear sway bar attachments for larger sway bars.

You also may want to just waive the Lotus by and enjoy your nice (street) 911SC.

If you want to flash your light and go on past, build yourself a lightweight, low-CG ‘sleeper’ that appears the same as your SC but is all race-prepared chassis, appropriate track tires and 300+ hp underneath.

Best,
Grady

[tvanbroekhoven]

Grady, you've got a point!

But still I think I can make my car better without having to compromise to hard on the street-ability. Leaning towards 28mm rear bars for the moment, but still waiting for others to chime in.

Also, those Lotus guys may have the weight/center of gravity/ age advantage,
I'll counter that easily with a couple of ounces of shear madness that every 911 driver has stashed somewhere in his mind.

[porterdog]

You didn't mention your geometry; what are you running for camber front and rear?

Tires? Pressures?

[stlrj]

Since most understeer complaints are made of lowered 911s, returning to stock height/ alignment specs might be beneficial.

Even today, it would be very difficult to improve upon a stock 991.

Driven: The new Porsche 911 - YouTube

[onewhippedpuppy]

Here are the specs for my recently purchased '88 Targa:

Rear control arm monoballs
Weltmeister swaybars (front & rear)
Weltmeister swaybar poly bushings (front & rear)
Sway-away front torsion bars (23mm)
Sway-away rear torsion bars (29mm)
Turbo tie rod end kit
Banana arm monoball & bushing kit
Shock brace
Bilstein sports with custom valving

The list is from the previous owner and I don't have every detail, but it's a really great setup. Wheels are 16" so the fatter sidewall helps provide a little damping. It's a really good compromise, well controlled but still livable on the street. I haven't pushed it very hard due to the 13 year old tires that came on the car, but it feels very well balanced.

[Flieger]

I think 27mm rear bars would be good. Then send both front and rear dampers to Bilstein to re-valve them (change the damping rate to match the torsion bars and weight distribution of your car).

[don911]

My two cents....
If you want to corner like a lotus I think you'll need to buy one. My 87 has 22/30 T-Bars, #27 sway bars F&R, RSR struts/shock valved to match, monoballs everywhere, track tires, etc..... My carrera handles pretty well in corners but still no match for a lotus. I may suck as a driver but I'm not sure you can do enough to make it corner like a lotus.

[arrowSC]

Originally Posted by tvanbroekhoven

Hi guys!
The problem was mostly situated in the corners, where my car seemed to understeer quite a bit (also in the fast corners). This could be caused by either my incompetence as a driver or as an engineer. Or a painful combination of both.

Now for the facts:

Setup (911 SC Coupe):

- 21mm front torsion bars
- 26mm rear Turbo torsion bars

Tuthills (builders of 911 rally and track 911s here in UK) installed 27mm rear TBs in my SC while retaining std 19mm fronts, which should give quite a different balance to yours. Plenty of forum advice suggests this will promote oversteer, but their advice is based on many years experience and I'm very happy with the result on road and track. I don't think I'd want it stiffer for rough Euro roads.

Funny you should mention running with Lotus on track. Here's a video from a trackday at Goodwood earlier this year

[Grady Clay]

Matt J,

Get those 13-YO tires off your 911 NOW. I’ll come and help you take them to the shredder.
A few years ago, we had a ‘play day’ at PPIR ((with these cars) and someone with ‘like new’ similar age tires made 1½ laps before two exploded (almost simultaneously).


Tim, Matt J.’s configuration would be well suited for your SC coupe.
Every car/driver/track is somewhat different.
There is no ‘one right’ solution.
Everything is an engineering compromise.
Pick a well-balanced (and widely used - peer reviewed) configuration.
Test and then re-modify to get ‘best possible’ for you (this is a forever ongoing process).

… just don’t try and out-corner a Lotus with 500 Kg less mass and a 10 cm lower CG.
For grins, rent a ride in a very lightweight (<500 Kg) and low-CG formula car or Sports Racer.
Try some 125 cc shifter karts with fresh tires.
You will see.

Best,
Grady

[cajun]

Good thread. I was just about to start researching the suspension setup for my SC...

[onewhippedpuppy]

Originally Posted by Grady Clay

Matt J,

Get those 13-YO tires off your 911 NOW. I’ll come and help you take them to the shredder.
A few years ago, we had a ‘play day’ at PPIR ((with these cars) and someone with ‘like new’ similar age tires made 1½ laps before two exploded (almost simultaneously).


Tim, Matt J.’s configuration would be well suited for your SC coupe.
Every car/driver/track is somewhat different.
There is no ‘one right’ solution.
Everything is an engineering compromise.
Pick a well-balanced (and widely used - peer reviewed) configuration.
Test and then re-modify to get ‘best possible’ for you (this is a forever ongoing process).

… just don’t try and out-corner a Lotus with 500 Kg less mass and a 10 cm lower CG.
For grins, rent a ride in a very lightweight (<500 Kg) and low-CG formula car or Sports Racer.
Try some 125 cc shifter karts with fresh tires.
You will see.

Best,
Grady

Grady, the new Potenza RE11s are supposed to be in on Thursday. Putting around town is all that this car will do until it gets new tires.

[Bill Verburg]

I'd just change the sways to adjustable and have the shocks re-valved w/ a digressive setup up tuned to the suspension and car weight if that's not what it already has.

For better cornering wider stickier tires and lowering while compensating or bump steer issues and a performance alignment

8 & 9.5 x15 wheels w/ 225/45 & 275/35 Hoosier R6s will fit nicely on a lowered car.

[Craig T]

Considering equal driver skill, an SC coupe or Carrera can be made to run a faster lap time than a stock Lotus Exige or Elise fairly easily. It just takes money for a well sorted track suspension set-up (about $7K) with 2.5-3 degrees of front camber, some 225's up front and 245's rear (minimum), and requires you pull a lot of weight from the car.

An excellent driver in a "stock" Exige or Elise can be expected to run a 1:35-1:37 at Willow Springs International Raceway on R compound street tires (not slicks), and a 1:45-1:47 at Laguna Seca. A "stock" SC in GOOD mechanical condition is about 10-12 seconds behind this.

In my 78 SC set-up for Porsche Owners Club "IP" class (3.0 SC "Prepared") I ran a 1:34's at WSIR and 1:43's at Laguna Seca. This was a car that was still street legal with torsion bars, had all it's steel fenders, hood, decklid, doors, and sunroof. I even drove it to the PCA breakfast and on the Malibu Cyn roads occasionally...but there was HELL to pay on the spine if you didn't see a pot hole. Oh yeah, and there was only one gas station within 20 miles that I was high enough to get up the driveway without high-centering (Flieger knows the drill)

Maybe the previous poster was right...Enjoy your laps on the track and point the Lotus by

[Flieger]

Actually I don't have too much trouble with driveways. I guess we just have shallow ones around here. But there are a few places where you can't avoid scraping the front. Just have to remember that it is not a concours-mobile and let it scrape. You can always put some fiberglass filler on in the future and touch up the paint.

Latigo bottoms out the A-arms on the chassis, though.

[tvanbroekhoven]

Thanks for all your replies!

I know that trying to match a Lotus in handling with a 30 year old 911 is kind of a BHAG. . Altough I must say that arrowSC is doing a splendid job with that Lotus in his video. I think an Elise or even an Exige would have a hard time keeping him off.

I think my questions can be brought down to:

Should I go softer or harder torsion bar wise to make my car handle better AND if I go stiffer, will I still be able to use it for a relaxing cruise?

[Craig T]

Originally Posted by tvanbroekhoven

Thanks for all your replies!

Should I go softer or harder torsion bar wise to make my car handle better AND if I go stiffer, will I still be able to use it for a relaxing cruise?

Putting on a 28mm, ever better a 30mm, rear torsion bar will be a quick and cheap improvement. A bigger rear T-bar will noticeably lesson, if not eliminate, the understeer and you'll hardly notice the difference in ride quality. Changes to T-bar size in the front has a more dramatic affect on ride harshness, not so much in the rear.

Get rid of the push and you'll be faster. Loose is fast if you can handle it. Remember what Stirling Moss said; "The steering wheel is only used to present the car to the corner, the rest is done with the feet"

[Flieger]

It would probably be worth while to re-valve the dampers front and rear no matter what you do with torsion bars.

[tvanbroekhoven]

After much contemplating I decided to go with the 20mm front torsion bars and see what happens.

If I want further improvement after that I can still opt for 27 or 28 mm rear bars.

The roads here are really really, well, not good and we also have the infamous Belgian cobblestones to take into account.




If anybody has any use for hollow 21mm Sway-A-Way torsion bars (10k miles), please PM me!

[Roslin]

Tim ,

What result did you get from your suspension upgrades ?

Roslin