911 SC race car handling problem

[1meansc]

Hi guys,
I have a problem that I am battling to sort out, and need all the help I can get.

The problem is as follows:
At the end of last year my SC based race car was like a train. Just went where I wanted it to go. Almost no effort to go fast, and was always getting faster. Amazing to drive.

Now the car over-steers for nothing. Tends to over-steer midway through the corner, with mild throttle.

Car has same suspension settings and tires.

The following are a list of changes through the off season.
Full strip down, with the removal of bolt in cage, to the fitment of a full roll cage. New cage has made chassis unbelievably stiffer.

The other main change was a weight loss of 50 kg.

The suspension setup is as follows.
23mm torsions up front with custom bilsteins & elephant everything
TRG rear coil-overs with custom 935 swing blades

Weight distribution of 62 % rear and 38 % front.

I expected the car to handle better with the stiffer chassis and the weight reduction.

We have found that the inside of the rear tires are running about 15 deg Celsius hotter than the outside. I have reduced the rear camber by almost 1 degree, and the handling is still not great.

I have softened the rear sway bar, to almost full soft, and the front is running stiffer than last year.

Any comments would be welcome.

[Tyson Schmidt]

Soften the rear spring rate.

When you stiffen the chassis that much, you can soften the spring rates.

What springs are in the rear?

Also, what sizes are the swaybars?


You'll likely find that you need to drop the rear spring rate by 50-100# and then put some of that negative camber back in that you removed.

[Jess]

When you say the "same tires" do you mean the set you've always used or the same brand.

Track tires lose their grip quickly. I find that ten sessions with Hoosiers is about all I can count on.

If you're running old tires, I'd bet that's your problem.

Jess

[1meansc]

Hi guys,

Thanks Tyson, this is the sort of info I was looking for. I will reduce the rear spring rate one level lower, at the rear.

The tires are the same brand.

The one problem with this car is I compete in a racing series and a time trial series. For the racing series I run on full slicks, and this is what the car set up is for. With this setup, the car was also great on the R type tires used in the time trials. A win win situation.

The car has just been handling like a pig since the upgrades, and has not been fun. You tend to get lazy when the car handles so great, and then doesn't. I have found that I was a lot more tired after the races this season.

I have also been about 1 second off my best times at all circuits.

Thanks for the info.

Out of interest, how much stiffer are the front 24 mm torsions from Chuck, compared to his 23 mm bars.

[Dave at Pelican Parts]

Torsion bar stiffness goes up with the fourth power of the diameter, so: 24^4 / 23^4 ~= 18% stiffer.

--DD

[Walt Fricke]

Tell us the spring rate at the rear. You may think that with your max sized front torsion bars your rear springs aren't all that stiff. But even with the biggest bars it turns out that the rates aren't all that high by coilover standards. My guess is you need a rather softer rear spring (until you convert the front to coilovers too).

I don't have the conversion handy (there are several out there), but on my 2000 lb track car I was running 400 lb springs up front and 500s in the rear. I bumped the rear up to 600 last year and had to disconnect the sway bar but otherwise the car seemed its usual self.

Slicks, at least bias slicks, don't seem to need much camber at all (especially with stiff suspension limiting travel and thus change).

Walt Fricke

[1meansc]

Thanks guys
The main problem I was trying to get my mind around, was what caused such a change, and why.

The extra chassis stiffness appears to be the cause, and softening the rear the cure.

A local friend of mine phoned me yesterday, and said that he could have told me to soften the rear. He missed the point that I wanted to know why.

Points that had concerned me was a lowering of the centre of gravity of the car, as a result of the removal of the sun roof, and all rear glass was replaced with polycarbonate.

I expect that the roll centre could also have changed and may be influencing the whole problem.

I have taken Thysons advice, and will be fitting softer springs in the next few days. I will report back on the results.

Walt: I am running 23 mm torsions up front, with 550 lb rear springs. I will be trying 500 lb springs for my next practice session.

Cheers guys.

[Walt Fricke]

My guess: 500 is still too much, though maybe with the sways disconnected they will work.

Steve Timmins' website has a torsion bar to spring comparison chart.

First figure is the T bar, second is the wheel rate, 3d is the coilover needed to give the same wheel rate (the difference is due to the spring geometry effects)

Front
23mm = 327 lbs/inch wheel, 355 spring
Rear
31mm = 351 wheel, 340 spring.

Quite a change when you went to 550 lb springs in the rear.

Walt

[gsmith4965]

I may be off base here but, its curious what would cause such a dramatic change in handling....... Im wondering if the car is equipped with LSD and its has a malfunction. I have driven many locked rear end race cars (and LSD) race cars and the fully locked rear ends can be evil. Just a thought.

Other things id check is a bad shock, broken swaybay or other obvious suspension failure
greg

[Tyson Schmidt]

Greg, he removed a bolt-in cage, and installed a weld-in cage, thereby greatly stiffening the chassis.


I see it all the time. You'd be surprised how much this affects the handling.


I'd just like to add that you should also re-corner balance the car, as welding in a cage will almost always introduce a little twisting into the chassis.

[cstreit]

What type/size tires are you running?

[gsmith4965]

Excellent point Tyson......

[1meansc]

Hi guys,
I re-corner balanced the car after all the work was done. Not all work was listed, as it does not relate to handling, but minor changes in weight distribution.

Car was almost correct when it went on the scales. Just took a little weight off one front corner, not even 1 turn on the TB adjustment.

The feeling I got when driving the car was that the rear was just too stiff, and needed to be softened, but I wanted to know if I missed anything.

The tyres are 235 x 16 up front and 265 or 270 x16 at the rear. The car runs on R rated or slick tyres.

The roll cage is so stiff, that we can have the car on three stands, two on the rear TB area and one on the RH side of the front, and we can still open both front doors, easily.

Here is a pic at a time trial earlier this year, at a facility that is used by many motor manufacturers to set up suspension systems. Track is just a little wider than the car. Pic is a result of too much right pedal, and too much speed.
Enjoy

[cstreit]

I'm actually asking about type/mfg of tire as well. Some tires have more sidewall flex and will not use the suspension as effectively as others... Stiffening the car will force other bits to work harder including the tires...

[1meansc]

Hi Chris,
The tyres we use are Dunlop SP Sport.
We do not have the range of tyres that you guys have in the USA.

The car was great last year. I will be trying the softer srpings in the next week. I have has a small engine problem at the moment, and hope to have the motor back in the car this week. I will report back asap.

Cheers

[jester911]

Beautiful race car. Post some more pics.

[1meansc]

Thanks Jerry,
Just a few more pics from our last race at the Kyalami Race track in Johannesburg, South Africa.


The second pic was taken on the out lap.
The cars that I currently compete against, in the same class, are an original 3.8 964 RS Club Sport, 993 RSR bodied 3.6 l 964, 3.6 l 73 RSR replica, as well as race preped 964s.

[1meansc]

Just an update.

I found out that the rear springs were in fact 500 lb springs. I tried 450 lb springs, which improved the situation, but had the rear sway bar disconnected.

I re-fitted the 500 lb springs, and upgraded the 23 mm front bars to 24 mm Elephant Racing bars. Chuck sent them the same day I ordered them. They arrived within 5 days, including the time zone differences. Thanks Chuck.

The corner balance was done, and the car now feels the same as it did last year. The sway bars are in the middle of their adjustment, and the car feels quite neutral.

I have unfortunately taken the car off the road, in preparation for the new motor. The car will be great in about 4 weeks with its 3.5l motor.

Thanks for all the advice.