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      11-29-2011, 12:37 AM   #2
Richbot
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Drives: Jerez Black E90
Join Date: Sep 2007
Location: STL

Posts: 2,036
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Well, I'm going to do this since nobody else has and it'll be fun to break relatively new ground for once. A full-on autocross build for stock class even with an expensive car like this is still less money than a few club racing weekends and since life has been getting in the way of racing, I'm going back to autocross. The car with the M performance exhaust and 12.5 gallons of gas weighs 3577lbs. Drop 7.5 gallons of gas and 20lbs in wheels and I'm at ~3515 without driver, with more weight to lose in the exhaust if I want it to be borderline on sound limits (drop the muffler and run with just the connecting pipes) which would put me just a hair under 3500. Not too shabby for a 4-door porkmobile with the whole interior still in it. A no-options coupe with the carbon roof would be a better starting point but we work with what we've got and I'm thinking the 30-40lb advantage and 1/4" lower CG are noise for someone like me. Plus tire fitment is an issue and the E90 is supposed to be a little roomier in the rear.

If I ever get them pinned down, RDSport is going to supply me with a front swaybar to start with. The 30.2mm bar is adjustable and provides quite a bit more wheel rate than the stock bar at full stiff, and I think this car is going to need a lot of help in the stiffness department. Added benefit will be better accelerative traction and more slalom stability which is good in a heavy car as tankslappers are hard to recover from once they start when you have such a heavy pendulum swinging back and forth. I'll evaluate the need for more bar once the rest of the setup is sorted. It's a tuning tool more than anything, and might save me some weight as the RDSport bar is hollow. I'm going to source some adjustable endlinks from somewhere once I know where the swaybar mounts on the shocks will end up.

Wheels will be the TR Motorsport MT1 18x8.5 and 18x9.5 for starters, with spacers and a stud conversion. Reasoning is that the high offset (ET44 front, ET35 rear) gives me more options for playing with tire fitment, which is important because I'm going to be stuffing some big tires under this thing. 15mm front and might have to have a custom spacer made for the rear.

When it comes to the most important thing for any autocross car, which is stuffing as much tire under the car as is possible under the rules, that requires a little more thought. To evaluate fitment, I made a handy-dandy conditional formatting spreadsheet for tire fitment and measured the clearances on the car as best I could with my winter setup. According to my calculations based on fitment with the current tires, there's room for the 295 in front. Only issue on the front might be the fender liners, but that could be solved by going to the shorter 285 without much loss of performance since the 285 is so wide, and it'd help the rake a little.

The 315's are going to be tight. I'm going to have to rely on the 9.5" wheels pinching them quite a bit relative to tire rack's measured width on an 11" wheel, because if they're 12.5" wide on the rims I'm about 5mm shy of fitting. I put the car on stands and put as much weight as I could on the right rear without pulling the spring which ended up putting the suspension at about 1" of droop. Got the tire close to everything it can hit. Then I used a straightedge, tape, and lots of contortions and dead reckoning to get clearances. There's only about 3/4" of room to work with between the tire and the fender, but the shorter height coupled with decambering in bump should help. Main interference areas on the inside are the shock boots, (1.5" and probably not an issue once I get something blingy on there) the front chassis braces (1") and the rear fender liner (1.125"). The 315's are about 1" shorter than the Conti DWS's on the car right now so that should help, but I'm probably going to need to space the wheels in somewhat and will be relying on the pinched tire to help me. The front chassis stiffeners are profiled around the curved shoulder of a tire so hopefully the 315's round off a little when they're mounted on 9.5" wheels. If the 315's don't work the 295's definitely will. And if the 295's don't work in front I might switch to the Kumho 285/30-18 and 305/30-18 which are a little narrower and more rounded than the hoosier.

Once I get the tire setup sorted and the swaybar in my hands I'll need to find a strut that works with the stock springs, bumpstops, and upper mounts, is high pressure gas charged (need some extra effective spring rate) and preferably isn't any heavier than the stock non-EDC dampers.

Hopefully I can get the car to a couple National Tour or ProSolo events with a decent AS field to show people that this car should be in FS with the pony cars or maybe CS with the Z cars. The only way this thing is faster than a Cayman S or the 'Vette around an autocross course is if the driver of the Cayman is asleep or the course is two straightaways with a sweeper in between them, but it'll be fun to try! At least it looks like it won't be suffering from woeful tire inferiority and the front camber isn't totally hopeless either.

Going to break some eggs making this omelet. I'll take lots of pictures once parts start arriving.
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Last edited by Richbot; 11-29-2011 at 12:57 AM.
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