SCCA STR Autocross Carrera Build

[westcoast911]

Originally Posted by 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.

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[Joe V]

Nice thread. I also AX my 84 and have put lots of thought and time into suspension, tires, exhaust....never ending!!! Tried a set of 17 with RE-11 tires and car felt like a dog! Maybe it was the added weight of the wheels but it felt dead. Even when stopping felt like I had an extra 500 lbs in the trunk. Didn't even bother AX'ing it with those wheels. Went back to 16's and it handles nicely.

[Driven97]

Thanks westcoast - very helpful.

Right now, steady state, the car is finally pretty balanced. At the event two weeks ago there was a fast ~50mph 180 degree sweeper. On last year's setup it would have been excruciating - I would have been angry waiting for the car to push it's way through. Instead, I had to play with the steering angle a bit as I was trying to find the right throttle position. Just how I want it.

Probelm is, I don't like how I've gotten there. My 30mm rear torsions are way too soft. So I have that large rear sway on, AND I have my shock shafts stuffed with engineered bump stops to bring up the rear rate some more - though I have no idea how much.

On top of that, I dropped my front torsions from my first guess (23mm) down to 21mm. I'm much happier with the action of the front - it feels like it's digging in and biting now, where before it felt like I really had to be gentle on my inputs to keep the front from "skittering."

My next move is going to be to remove the two band-aid solutions in the rear, and try more rear torsion. I'm still open diff in the rear (LSD is out of the budget for now) and have a little trouble putting power down on occasion. The big rear sway made it worse, so being able to remove that sway should help. Also I'm probably getting progressive rate in the rear with the bump stops. A nice linear rate should be more predictable when I can pull those off.

As far as shocks - that's something I'm pretty sour on. I understand what they are supposed to be doing, and I can feel how they are not doing it well on my car. It transitions poorly. Here's where I probably make 90% of this forum angry - I had Chuck at ER valve my Bilsteins. I told him what I wanted, what the intended use was, but didn't know enough to give him hard numbers. He said he knew what he was doing, not to worry. I've had several people take a look at my shock dyno charts, and say they're not even close to enough damping. In fairness, ER is not an autocross company, and probably not familiar with how to make a car work well between the cones. But I'm not happy about my hard earned cash spent there. It wasn't cheap.

[msterling]

Originally Posted by Joe V

Nice thread. I also AX my 84 and have put lots of thought and time into suspension, tires, exhaust....never ending!!! Tried a set of 17 with RE-11 tires and car felt like a dog! Maybe it was the added weight of the wheels but it felt dead. Even when stopping felt like I had an extra 500 lbs in the trunk. Didn't even bother AX'ing it with those wheels. Went back to 16's and it handles nicely.

17" wheels and tires will definitely be heavier, plus your gearing is effectively higher if tire outside diameter is increased. Most 17" tires end up being well over 25" dia. My dinky 15" 225/45-15 tires are 23". Acceleration and braking are both effected negatively by larger diameter. Much easier to spin-up and to spin-down a rotating mass of 23" diameter compared to one that is 25.5" diameter and heavier. 17's for show and 15's or 16's for go. The trade-off is that there are very few tires available in the sizes you want for 15" and 16".

[BRAIDusa]

Originally Posted by msterling

17" wheels and tires will definitely be heavier, plus your gearing is effectively higher if tire outside diameter is increased. Most 17" tires end up being well over 25" dia. My dinky 15" 225/45-15 tires are 23". Acceleration and braking are both effected negatively by larger diameter. Much easier to spin-up and to spin-down a rotating mass of 23" diameter compared to one that is 25.5" diameter and heavier. 17's for show and 15's or 16's for go. The trade-off is that there are very few tires available in the sizes you want for 15" and 16".

Didn't we have this same discussion about 18 months ago Matt?

[msterling]

Paul,

My name is actually Mark. Maybe I have discussed this before. Do you have 17's on the Braid car for autocross? How do you like 'em? I am considering getting a set of 17" x 7/9" replica Fuchs with 225f/255r ZIIs but it would be an expensive test. Certainly the taller tires would look much better on the car and I could use the setup for street-only if they were less effective for autocross. If I had money to burn I would be trying a lot of stuff.

I just went to a car control clinic as an instructor with test and tune autocross in the afternoon. I ran new BFG Rivals, 225/45-15 on all four corners and ran some pretty good times with two different drivers. They seem about as quick as the 205/50f-225/45r Nitto NT-01s I ran the last two years. More testing is needed and I can get an event or two out of the Nittos before they cord.

[Driven97]

I'm Matt. Paul and I both did pretty well yesterday, a pair of trophies and class wins for old P-cars:



Although he did so well because the condition of the lot made it more like a rallycross than an autocross. Check out that gravel spray:

[msterling]

Matt & Paul,

Great job! Old 911s can still do it. Where is that pavement? I need to stay away from there. Our pavement was pretty shabby as well. There was a section of the 360deg that was treacherous yesterday.https://www.facebook.com/photo.php?fbid=4289539774055&set=gm.10152396873227264&type=1

[Driven97]

Originally Posted by msterling

Matt & Paul,

Great job! Old 911s can still do it. Where is that pavement? I need to stay away from there.

Pontiac Silverdome, former home of some high school football team they let play in the NFL for some reason.

It's replacement Ford Field in downtown Detroit opened 12 years ago and the poor Silverdome has been slowly rotting since. After hosting a few more monster truck rallies, it was a roller rink for a while, then they turned the parking lot into a drive-in theater which failed. They finally deflated the dome last year.

After the Lions departure, the city of Pontiac began to experience several years of serious financial problems. Due to the continued high maintenance costs of the structure, it made several unsuccessful attempts to sell the stadium.[26][27] In early 2008, United Assurance Company Ltd. made the highest purchase offer to date, with a bid of $18 million to convert the Silverdome into a Hollywood-style entertainment complex, following an earlier bid of $12 million by an attorney.[28] However, the city announced in October 2009 that the property would go to auction with no minimum bid, and that zoning regulations would be relaxed for any buyer in order to spark development. The city engaged the firm of Williams & Williams to conduct the auction in November 2009.[29]

After reading about the auction in a newspaper, Greek-born Canadian real estate developer Andreas Apostolopoulos, CEO of Toronto-based Triple Properties Inc., submitted a winning bid of US$550,000. Real estate fees of 6% raised the price to US$583,000.[30] The sale of the Silverdome, completed in 1975 at a cost of $55.7 million (approx. $225 million in 2012 dollars), and sold in 2009 for $583,000 was viewed by many as a symbol of the collapse of real estate prices in the Detroit metropolitan area though many local leaders and residents claimed the sale was brought about due to the incompetence of city management and their not having a vision or future plans for the stadium and surrounding area.[31]

[DG624]

Matt, I am still reluctant to go with the large rear T-bars with 21 mm fronts (Currently have 21F/26R). I drove the car at Mid-Ohio and it seemed stable but I was not pushing it becaues I didn't want to bend it and I was very tired.

I really must do more testing where I can push the limit and see what the car is doing. I would like to stay with the front 21mm bars. I am going to replace the control arm bushings and add an adjustable front sway bar to allow more tuning. I have old Koni sport shocks that may be another problem. The bump stops seem to be a way of increasing rear spring rate without going to stiffer bars...a quick way of testing.

I did add the turbo tie rods and bump steer spacers. The car seems solid and is not sliding but I have alot of camber -1.8 R -1.5F. The tires are also getting hard and I flat spotted them tring to avoid a group of stopped cars.

I have too many variables that may be causing trouble.

[Driven97]

Oh, for track work I would definitely be more conservative. I would follow the tried and true formulas on this forum to keep the heavy end behind you.

Autox likes a loosey goosey rear, as we don't see above 65mph too often. And the worst thing we can hit is a cone.

While I have no problem going 11/10ths on an autocross course, 8/10ths was enough for me on the few track laps I did last year. Car was really pleasant and rewarding to have on track (I know why they are so popular out there) but my impending doom warning bells played in my head just a little louder than they have in other cars for me.

[DG624]

Matt, have you added an adjustable front sway bar to allow solfter front Tbars? I noticed you have different bars but have you considered this option for tuning?

[Driven97]

I have an aftermarket front bar, but it's not adjustable. It's an OE style bar made my Addco that's 22.2mm, up from the stock 20mm. That should make it about 50% stiffer than the stock bar. Through-body bars aren't legal in STR.

STR Carrera: Front Swaybar

I've looked at piecing together a custom adjustable bar that would be legal. I think it could be done with Speedway Engineering bits. A straight bar would fit in the OE underbody location. I'm concerned about clearance of the arms with the steering linkage, and not enthused about modifying / welding my a-arms to accept drop links. After all that, the kinematics wouldn't be ideal. Plus, the swaybar mounts to those weird loose brackets by the steering rack - I don't know how strong they are. So basically I bail on that idea every time I get that far.

Another option for me is that Addco has a custom department. I don't think it would be very difficult to call them and ask for an 827 made out of 1" wire. That would be a 25.4mm bar, which I think would equate to 160% stiffer than stock. I contacted them once and they were swamped, never revisited that idea to know how much it would cost.

Swapping underbody front bars is a PITA. I don't like how they attach at all.

[Driven97]

While I still have some improvement to do on the national level, I won STR in the Detroit region SCCA again yesterday making me 2 for 2 on the season.

Car works great on the crappy / broken asphalt. Just eats it up. Now if I could only make the car work that well on concrete...

Ride along video

[msterling]

Congrats. Seems like you are getting that thing sorted out! The hybrid tire combo has got to be unique.

I'll be at a SCCA event in Medina OH this weekend competing in CS. BRZ, Z-cars and Miatas will be the main competition.

[BRAIDusa]

Originally Posted by Driven97

While I still have some improvement to do on the national level, I won STR in the Detroit region SCCA again yesterday making me 2 for 2 on the season.

Car works great on the crappy / broken asphalt. Just eats it up. Now if I could only make the car work that well on concrete...

Ride along video

Good job mate. When are you going to get that thing dirty? We got spanked in the RallyCross due to a slipping clutch. Well, that was Scott's excuse, mine was slipping talent too.

[DG624]

Several companies make drop links...you can modify the control arm to accept the link mount. The stock part is mounted in a rubber bushing that is not ideal for quick response. A hollow bar would be better and can be used on the rear with a after market drop link. I think that adjustable pieces are very important for tuning. A rear adjustable bar with your front bar would be better.

[Driven97]

Had a national event last weekend. The larger torsion bars I ordered showed up on Thursday, so I figure that's as good of time as any to pull apart the whole rear suspension on the car.



Old 30mm solid bar on top, new 33mm equivalent hollow bar on bottom. Note the bar is now the same size as the spline on one side. Engineering wise, I'm pretty sure this means that these are the biggest rear bars that are wise to use.



Someone suggested the compliance in the spring plate bushings might be making the rear feel unsettled, so I bought a pair of Delrin ones to put in the outer locations only. I left the rubber on the inners just in case I hate the Delrin. Only an hour or so to change back to the rubber outers. Rules don't allow me to increase metal content in a bushing, so the ER ones are illegal. On the bright side, the Delrin ones were nice and cheap.



Quick re-corner balance and a half assed string alignment (this car is a pain in the dick to align) on Friday. That's without me in the car, btw.



...and headed straight to the Pro. With some young whippersnapper great grandson watercooler behind me.



Out of the 21 car class, I was the only one with a roof.



Ok, this is going to sound really bad, but I ended up 18th out of 21. Now keep in mind that this was against the best drivers in the midwest in the best prepared cars. I expect to see many of them in the trophies at Nationals, and would not be shocked to see any of the top 6 crowned National Champ. Tough crowd.

On the LH course the car felt really good - best it's ever felt on concrete. My 32.085 over there was midpack in the field. On the RH course I struggled to find grip. It was a little bumpier, and my Blisteins are valved for 23/30, not the 21/33 I am at now. I went nearly a second slower on what is supposed to be a perfectly symmetrical second course. Several people were faster on the right, so I know it wasn't a course issue.

So positive thinking, if the car were to work as well on the right as it did on the left, I would be about 1 second out of the trophies over sixty seconds of course. I've got room to improve the car - the open diff was noticeable. The shocks are all wrong and not exactly high tech. There's not enough front swaybar. I still have maybe 100lb of legal weight reduction left to do. I'm on an OTS Steve Wong tune, stock headers, on a motor that's likely a little tired. And I aligned the thing myself in the garage. Do most of that and I think that I would be capable of battling for a trophy. Put in one of those freaks of nature drivers too, and maybe, just maybe, a Carrera could be nationally competitive vs. the S2000 / NC Miata.

[Trackrash]

If you are allowed headers, I would not hesitate. It makes a big difference on those 3,2s.

Beating a well driven and prepared S2000 is a lofty goal. Keep up the effort and who knows.

Ever thought of entering a PCA event?

[Driven97]

Originally Posted by Trackrash

If you are allowed headers, I would not hesitate. It makes a big difference on those 3,2s.

Beating a well driven and prepared S2000 is a lofty goal. Keep up the effort and who knows.

Ever thought of entering a PCA event?

Headers get a little goofy - as there is a catalytic rule. I'd have to get creative and likely very custom. $$$. Just out of curiosity, where do something like SSIs shine? I'd be looking for a big bump in the midrange to really make a difference - I have a LOT of digs from 3000-4000rpm.

When the race budget gets big enough, next big purchase is going to be an LSD. It's really going to hurt the wallet, but I think it will pay off big time, judging by how much wheelspin I'm now getting.

I'd love to do a PCA autocross, but neither of the Detroit area clubs seem to put them on.