SCCA STR Autocross Carrera Build

[winders]

Originally Posted by Trackrash

What torsion bar sizes will the calculations recommend to be used?

I'm guessing 22 front and 29 rear.

There are no new calculations to be done. The 22/29 combination leaves you too roll stiff in the front.....

[Flieger]

I use 22/29 with 22mm sway bars f+r. Considering removing the rear bar as it it a bit too willing to rotate on entry to the tight turns of my local canyons. It feels pretty balanced steady-state. Much better than the 22/27 combo. The biggest improvement is the ride. It no longer obviously oscillates in pitch after a bump.

You have to include sway bars and geometric lateral load transfer when discussing steady-state over/under steer balance, and damping for transients. For me it is cheaper and easier to remove my squeaky old weltmeister than get new dampers.

[winders]

22mm/29mm torsion bars are fine for a street car......not so much for a serious autocross car or track car....

[Driven97]

Originally Posted by Flieger

That chart is labelled incorrectly.

D'oh, I'm an idiot. Fixed it to say "Sprung Weight"

To make the chart, I blatantly stole the data in columns ABC from instant-g.com. To get the sprung weights, I took my actual corner weights minus what I estimated my tire + wheel + hub + brake + lower half of the shock + ~1/2 the suspension arm might weigh. Totally guessed on those though.

I am currently running 30mm bars in the rear, plus I stuffed my shock shafts with progressive bumpstops that also start ramping up my rates at any compression.

I ran 23mm front t-bars last season. As stated, I think that was way too stiff, at least balance wise in relation to the front.

For swaybars, I have a large (22.2mm) front underbody bar. Rules prevent me from running a through-body style bar. That made the front even stiffer. I have the stock Carrera rear bar. Even with all that, the car still rolls quite a bit:



The roll is ok as long as your contact patches don't get effed up. There is good camber gain in the rear, not so good in the front. I've countered that with as much static front camber as I can get, which honestly isn't much. The tradeoff is slightly reduced straight-line braking performance.

My two changes this offseason are to try and improve steady state balance. I dropped to 21mm front torsion bars. I also grabbed a larger rear bar to try off eBay. I'm concerned that the larger rear bar may make it harder to put down power on corner exit, but I'll give it a shot.

My goal is 100% front weight transfer - aka I want the inside front just off the ground. Like so:



In this photo the car looks setup pretty good - outside tires aren't perfect, but relatively square to the ground. Lots of roll countered by static camber + camber gain. Check out the inside rear though and you can see where it might be tough to put power down - especially with an open diff. Unfortunately, this was not the normal attitude, I couldn't get the front to bite like this often. Hence the attempted balance change.

Winders - do you have any idea what the practical limits for torsion bars are for a non-caged chassis? I decided to soften the front instead of stiffen the rear because of concern that I would be using the very undamped chassis as a spring too much if I went stiffer.

[BRAIDusa]

Oh my, this autocross setup stuff is complicated. Glad I do rallycross with mine.

BTW I wonder what the best setup for that is. Running stock right now and it's never been beaten on dirt.

[Driven97]

Originally Posted by BRAIDusa

Oh my, this autocross setup stuff is complicated. Glad I do rallycross with mine.

BTW I wonder what the best setup for that is. Running stock right now and it's never been beaten on dirt.

Get out of here rally boy.

[winders]

Originally Posted by Driven97

Winders - do you have any idea what the practical limits for torsion bars are for a non-caged chassis? I decided to soften the front instead of stiffen the rear because of concern that I would be using the very undamped chassis as a spring too much if I went stiffer.

Is there a limit? With my '78 Targa I had 21mm/30mm torsion bars. I would have gone stiffer if it had been a dedicated autocross car. With a coupe I would have no issue with going stiffer.

You are never going to have a situation where you have reasonable chassis balance and the front roll stiffness so soft you won't lift the inside front tire a bit in a turn near the limit. You have a better chance of keeping the front tire on the ground with an autocross car than you do a car on the big track. When I autocrossed, I wanted the car setup with almost too much oversteer. I wanted to fight the rotation, not have to try to induce it. On the big track, I still like a bit of oversteer...but not like I did autocrossing.

[Driven97]

Tweaks, as part of operation stop mid corner push:

Dropped front torsions from 23mm to 21mm.
Trying a 22mm rear swaybar from the stock 18mm. Grabbed this off eBay for peanuts, anyone know what brand it is?



Expected an oversteering monster yesterday, but the car felt much more stable than last year. Granted the course was on the slower side. Going to keep the rear big bar on a little more and try at a higher speed event. Also going to try reducing rear camber a bit. I think I'm slightly above 3.0 degrees, supposedly the Dunlop Z2 don't need as much camber as some of the other tires. I'm hoping this will also reduce my chances of inside wheelspin.



Sitting in paddock yesterday. Representing our host, of course!

[berettafan]

Originally Posted by winders

22mm/29mm torsion bars are fine for a street car......not so much for a serious autocross car or track car....

Depends on the weight of the car and the rubber.

[Flieger]

Reducing the rear camber will generaly reduce rear grip, but what you want to do is add grip to the front. When you add the rear sway bar you take from the rear and give to the front but when you make changes to the tires you are only affecting the one end.

[Driven97]

What I figure is reducing total roll via swaybar requires less static camber. I think I should pick up a pyrometer to back up the theory.

[msterling]

It seems like suspension setup is almost an art. It is hard to keep all of the determining factors in your head. I found the following chart on here somewhere. I need to print a big poster and hang it in my garage for when I get confused.

[Driven97]

Took the car down to the SCCA National Wilmington Match Tour this weekend. Had a great time.

Here is video of my Sunday runs.

The car felt a lot better than last year, especially on concrete. There was a long 180° sweeper taken at ~50mph that would have just killed me on the old setup, the car felt much closer to neutral.

Car felt better in slaloms, too. A friend pointed out that I was still pedaling the gas, which I think I started doing last year to plant the rear at each turn in point. Otherwise I would spin. Started trying to break the habit and drive slaloms more like a normal car. Felt way faster, but it was super hard to retrain my technique. Old habits die hard?

I ended up getting clobbered pretty good by the S2000s and Miatas, but on day 2 I cleaned up a few lines and got a little more confidence in the car. Found enough time to make it at least a respectable loss rather than an all-out beating.



I stick out like a sore thumb in a sea of Japanese roadsters in STR.

I had several spots where putting down power was an issue. The big rear bar plus the high grip concrete don't jive well with the open diff. I'm going to need to start an LSD fund if I want to try and keep pushing ahead with the car. I'd also really like to have my Bilsteins done by someone who knows autocross. I think the car can really improve in transition if they were done correctly.

So, overall: I'm not terribly pleased, but I'm not all that discouraged, either. The car is locally competitive. It's super challenging yet still a ball to drive. I'm still very far from full prep, so there's lots of improvement room in the car. And driver mod is progressing.

One final note - the car is a magnet. 20 people I didn't know stopped by to introduce themselves and ask questions. That didn't happen very often when I had 1 of 20 Minis or one of 15 RX-8s. Kind of neat, but also meant there were a lot of eyes that noticed how not fast I was.

[DG624]

Have you changed tires...I was going to Hoosier. It seems that a smaller rear bar (smaller than front) would work best for rear wheel drive.

[Driven97]

Originally Posted by DG624

Have you changed tires...I was going to Hoosier. It seems that a smaller rear bar (smaller than front) would work best for rear wheel drive.

Street Touring R requires 180tw minimum, 255mm wide max, 9" wheel width max. I'm on Dunlop Z2 225/255 f&r. All of the cars were on either those or the BFG Rivals.

I had a smaller rear bar in. I put the bigger one on to see. Ideally I should go with larger torsions in back, but trying the rear bar first was cheaper and less labor intensive. I should have enough front bar to keep a fair amount of load on the inside rear. I meant to bring the smaller rear bar with me and swap between days, but forgot it at home. Oops.

I think the increased speed of load transfer and reduced total body roll of the large bar are outweighing the inside rear tire lift in the benefit department.

[DG624]

I was going to to go with 21F 28R but decided to go to 23F 30R. What was your choice? Sorry I missed the earlier post 21/30 that seems too far apart...tell me how it works.

[Driven97]

Started at 23/30, way, way too front stiff. Dropped the fronts to 21mm. Car is better. Would really like to go to 21/32 and run the stock rear swaybar, or 21/33 and remove the rear sway altogether.

STR Carrera: Going Soft

EDIT:
Pics starting to trickle in from last weekend:

[DG624]

Others Ihave talked to RR and ER said that a 7mm difference F/R is preferrable. I was afraid I would get too much oversteer.

[Driven97]

You can't use a 7mm rule of thumb - stiffness isn't linear in relation to torsion bar diameter. It's diameter cubed or something.

I initially listened to the accepted recommendations. It didn't work for me, car drove and handled poorly. I've since gone to a traditional (non-911) approach to setting up a car and it's better so far.

This is what is working for me:

Fill out the chart I have linked earlier for your corner weights minus your unsprung weight. Aim for ~1.5 Hz for street, 1.75 Hz for dual purpose, 2.0 Hz for track on r-comps.

If you want to follow the factory balance, go a little front stiff, but not a lot. Modern performance cars (including later model 911s) are about equal or slightly higher frequency in the rear. That may be a little aggressive for our less than ideal rear suspension geometry. Still trying to figure that out.

If I keep getting faster, I'll keep on updating this thread.

[Trackrash]

A couple of suggestions. Remember that a 911 is a unique car. Lots of weight in the rear. This can be an advantage or disadvantage, depending.

Basically what you want to do is get the car balanced between over and under steer. Too much understeer and you are pushing your front tires around and scrubbing off speed. Too much over steer and you can't get the power down getting out of the turns.

Depending on the course, ie tightness of the turns, you may need to make some adjustments to your cars balance or your driving style.

From your pictures, it looks like you could go stiffer. I like your idea of going to a bigger torsion in the rear. Then tune it with the rear sway.

After basic balance, you can start working on transitions. Meaning, how does the car behave entering and leaving the corners. That comes down to shocks and alignment. You can do some research on alignment, some swear by some toe out, but I don't recommend it.

Shocks help on transitions. I know a guy, who gets to an AX and adjusts his rear shocks to maximum stiffness. Says it helps the car rotate. Probably does. Most successful autocross I know with SCs and Carreras use custom valved Bilstiens. Personally I like the adjustability of Konis.

I was having a problem with push in turns. I fixed it by lowering my front suspension a little, reducing my bump steer, and tightening up my front shocks. What was happening on my car was under power the front end would lift and the unweighted and miss-aligned tires pushed.

I was also having trouble getting the power down coming out of corners. I softened the rear shock settings which helped a lot.

Once you get to the point where the car is balanced you need to start fine tuning your settings. Have you measured tire temps or used shoe polish? At my last AX I was 6th top time of the day overall. The four cars ahead of me were all within .5 of a second of my time. That's like two percent difference.

Also I think that your rubber in the shocks may be causing you problems. Once the rubber stops start to compress beyond a certain point the spring rate goes up rapidly which will upset your cars balance. Koni makes racing rubber bump stops which are supposed to be progressive. Porsche put their rubber bump-stops in their shocks so on the STREET soft torsions would not bottom out.

JMO, Your results may vary.