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      07-14-2010, 02:58 PM   #1
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Autocross nerd thread: The Ultimate E9X A-Stock SCCA Autocross Build?

No, I'm not doing it. Wish I was, wheel to wheel is too much fun. But since I enjoy bench racing, I will post my thoughts and somebody else can decide whether they want to do it.

The goals:

As much front camber as possible, as much tire as possible, and as little weight as possible, while being able to get on the throttle as early as possible.

The car:

No options 9/10 or later build 2011 coupe (for the manual seats) with 18" wheels. Carbon roof obviously. That should put the car somewhere around 3600lbs with a full tank of fuel, stock. DCT seems like a wash, either way you're going to be in 2nd most of the time. Going to 1st will just blow the tires off 99% of the time. It adds weight but you'll have 1st gear more readily available...pick your poison.

I would not choose ZCP and there's a very good reason why - tire sizes. Hoosiers don't come in good wide 19" sizes for the M3. The wide tires are also taller, 295/30-19 is 26.1" vs. 25.3" for the 295/30-18, which makes fitting wider tires a potential problem and also hurts gearing, which needs to be as short as possible. EDC is also going to get tossed because you can substitute shocks, so no point in having that either.

I guess paint color and interior trim could be optioned up, so spend all that tech package and ZCP package money on an individual color like LSB or Hellrot or Technoviolet.

The parts -

CCW classics in 18x8.5 and 18x9.5. This gives you lots of flexibility to build a wheel to suit the tire you can fit under the car just by changing rim halves, since tweaking the offset within the rules (+- 6mm) may be necessary to get the tires in there with no rubbing.

285/30-18 front and 295/30-18 rear A6's for starters. I would start with this fitment because it would have a good shot at clearing everything and then go to a larger rear tire if possible, perhaps a 315/30. The shorter diameter on both will help with clearance, and you'll also drop 2mph off the top speed in gear (70mph at 8300rpm with a 26.3" stock rear tire, 68 mph at 8300rpm with a 25.3" 295/30-18).

Straight pipes from the x-pipe back. Probably as easy as buying somebody's stock exhaust and using the stock connecting pipes with some 2.5" pipe welded on as extensions. If you want to be fancy put a 3" turn-down on the ends. It'll be loud but no louder than a CP car and you'll lose something like 40lbs. Now the car weighs about 3530 lbs with a full tank between the weight loss from the wheel/tire package (30lbs) and the exhaust (40lbs). Or maybe that Bastuck "sports" exhaust that just has the 3" bullet mufflers, but that's pricey for something you're going to take on and off for events. Plus the straight pipes could be stuck in the trunk and swapped out in about half an hour.

For shocks I'd go to a Penske dealer and have something custom made, but Motons are also available for the chassis already. Remote resevoir with nitrogen pressure adjustability would be a good thing since this car is still pretty softly sprung. The bodies are already aluminum and you're adding remote resevoirs so you're not going lose any weight here.

I'd have to look at swaybar options, but I believe RD Sport offers a 32mm hollow bar that would do the trick for starters.

I'd leave the brakes stock until they gave me a reason to change them. I have a feeling even autocross might be too much for the stock pads but who knows.

The setup -

I'd pull the pins for more camber up front and try to find documentation and/or convince BMW to give me documentation that it's an approved repair method, and zero the toe. Then I'd start with about 1/8" toe-in in the rear with maybe -0.5 degrees of camber. This car is not going to want to put power down and it will need all the help it can get, so keeping the tires square to the pavement will help get on the throttle earlier, along with a good amount of rear rebound. The nitrogen pressure can be used to tune balance at individual ends of the car without upsetting the whole works. This is going to be a slow-in, fast-out car because of the weight and lack of tire but it's going to put some distance between itself and the competition wherever the throttle is applied because it's higher up the power/weight totem pole than anything else in AS.

Anybody have real-life experience in a serious effort yet with one of these?
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      11-29-2011, 12:37 AM   #2
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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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      11-29-2011, 12:53 AM   #3
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Oh this is going to be a fun thread!!!!!!
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      11-29-2011, 12:03 PM   #4
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I'm just curious... Are there no tracks nearby? Not shooting Autocross down but I find it really boring compared to going on a real track.
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      11-29-2011, 12:15 PM   #5
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I race a Spec Racer Ford wheel-to-wheel with the SCCA. Racing is a high risk, high reward activity. There are only so many local race weekends and the time commitment associated with loading up a trailer and towing to Road America, Mid-Ohio, etc, doing a test day, qualifying, racing, qualifying again, racing again, packing up, towing all the way home, is substantial and I need a little break from the pressure of squeezing that into my schedule, so I'm going to get about 12-15 lower-intensity hits of the crack pipe while still doing a few races to keep my competition license current. Track days are fun, I used to be an instructor, but they're kindof like masturbation compared to wheel-to-wheel racing.

You can also use autocross prepped cars in SCCA Time Trials/PDX with a couple changes. The changes for Stock are relatively minor and in the case of the struts/shocks, front swaybar, alignment, exhaust, etc. are all easy to reverse and easily adapted to other classes if you want them to be.

Either you get autocross or you don't, it's not for everybody.
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      11-29-2011, 01:03 PM   #6
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Obviously you are more in tuned to SCCA classing rules...

But I didn't think you could modify the exhaust or increase the tire size by more than 2 sizes in the stock class. Also don't forget that the Lotus Elise is in AS!!!!
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      11-29-2011, 01:51 PM   #7
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Quote:
Originally Posted by Richbot View Post
No, I'm not doing it.
Quote:
Originally Posted by Richbot View Post
Well, I'm going to do this since nobody else has and it'll be fun to break relatively new ground for once.
Wow. You must've had a helluva Cyber Monday at work!
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      11-29-2011, 02:04 PM   #8
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Does "as big as a tire you can fit" always good? Because you keep adding weight as the tire size (width) goes up as well. Any thoughts?
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      11-29-2011, 06:57 PM   #9
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Nice write-up and I will be interested in your progress and results.
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      11-29-2011, 07:01 PM   #10
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dnvrdrvr - I posted this thread in July of 2010 and obviously nobody took the bait.

Bigger has been proven time and time again to be better when it comes to autocross, assuming the wheels you're putting the big tires on actually do something and you're not making the car 6" wider. Some FWD cars, where the rear wheels are basically just there to keep the trunk from dragging on the pavement, go with a narrow rear tire to heat it up faster and whatnot, but even on super narrow wheels the wider tires wear more consistently and are more consistent run to run along with more lateral grip, partially as a consequence of the extra mass. There are downsides but they're almost always outweighed by the upsides. The difference between the 295 Hoosier and the 315 hoosier is 3lbs per tire, which is noise and on the rear of the car probably only helps traction at these low speeds, remember we're in 2nd gear here and we all know this car will go sideways with the twitch of a toe in 2nd when you're already cornering at the limit. It comes down to weight of the car per mm of tire width as a rough indicator as how successful the car can be, and if you can give up 12 lbs of weight for 60mm of tire it's generally a good trade. Gearing becomes an issue too, you don't want to put 345/35-18's on the car just because they're the biggest because you might force yourself to go down to 1st in too many elements, though this car has pretty good gearing and power delivery for autocross.

Some people in RWD's stick with the narrower front tire for better steering response, scrub radius, clearance, width (narrower overall width of the car is better in autocross) but those are usually on stiff cars that can get good camber. Even though the front suspension on this car is worlds better than the E46/E36 design it's far from ideal. Another plus of starting with the biggest tire possible is you can always go smaller and you know where the limits are.
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      11-30-2011, 12:23 AM   #11
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Quote:
Originally Posted by Merked M3 View Post
Obviously you are more in tuned to SCCA classing rules...

But I didn't think you could modify the exhaust or increase the tire size by more than 2 sizes in the stock class. Also don't forget that the Lotus Elise is in AS!!!!
The Elise is in Super Stock competing against the GT3 and Z06's. The competition in AS is the Cayman S, several base Corvettes, and lots of other proven cars that aren't showing up because people seem to think the Cayman S is an overdog even though the 'vette won it all this year, and because the costs aren't really any better than Super Stock, you just go a lot slower. I'm going into this knowing it's an uphill battle in AS. The thing with SCCA is, once the car is classed unless it's just an obviously wrong classing, you have the burden of proving otherwise. Proving otherwise means building a car pretty close to the limit of the rules and driving it well and showing that it either is or is not competitive. I think it gives up about half a second to the Cayman on the tighter courses with a balls-out build. IMO, it fits best in F Stock with the other V8 RWD muscle cars, but if not there it belongs with the Z cars and RX8's (and miatas, and solstices) in C-Stock from a speed potential perspective. We'll see how badly outclassed it is next year. I suspect it's going to be really bad against a Cayman on a tight course and maybe within a few tenths on more open courses. Even as big as it is, the M3 is narrower than the Cayman and has a much better power/weight ratio, but the Cayman is lower, can fit almost as much tire, brakes harder and probably has better accel traction thanks to the layout. The closest analogue to an M3 in AS is probably the C5 Corvette, ironically.

In SCCA Solo2 Stock you can use any DOT-approved tire that will fit on the wheels without modifying the body or fenders in any way, you can use any exhaust setup you want from the cats back, so technically I could probably delete the resonators if I wanted to be really dumb. You can also change the air filter and use whatever oil you want!
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      12-06-2011, 01:03 PM   #12
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Richbot, I'll be running my E92 in Solo, also, out here on the Left Coast. Last season my wife and I competed in her Cayman S, so it will be interesting to see how we fare against the local hot-shoes, compared to last year. The Cayman is a natural, being true sportscar, but I was surprised how good the M3 was on an autocross course at the couple of events we ran it last Fall. It definitely drives smaller than the big beast it is.
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      12-06-2011, 03:40 PM   #13
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I have parts coming in now. Picking up a pair of lightly used 295 A6's for test fitting today.

Maypo - Are you going to run it on street tires?
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      12-06-2011, 05:57 PM   #14
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I AX mine in BMW club, but don't think it'd be competitive in SCCA AS. You need way more negative camber than you can get out of the stock suspension. You can change stocks in AS, but not springs or ride height, so there's very little to gain on the suspension side. The tire sizes that you're proposing will be at the outer limit of what will fit the stock rims.

Regarding sway bars, you don't need more stiffness in the front. Go with the E93 rear bar to stiffen the back a bit and improve rotation on tight AX turnarounds. The brakes are fine and all you're allowed to do is swap pads. I'm going to Stoptech's street/performance pad next year to see if I can get a little quicker initial bite, but I'm not expecting to gain much time there. I'll also go stainless steel, but I think that's not allowed in AS.

Unfortunately, AS rules don't allow changing the final drive ratio. My 4.10 ratio really works well for AX.

Regarding exhaust, you're limited to cat-back mods and you must still comply with sound limits at the venues. Check your club rules. Opening the back of the exhaust isn't going to gain you much anyway.

Go to a high-flow air filter, which'll gain you around 6-hp.

As you say, get as much negative camber as you can in front, but keep the back at one-degree or less. Run the back pressures at around 28-psi cold to improve acceleration traction and improve rotation.

Still, if you value winning, then this is not a good plan for AS. The car's too heavy vs. the competition in this particular class. Go futher, with coil-overs, final drive, power adders, etc. and join me and many others in BMW or other marque club AX events, where the rules are much friendlier.

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      12-06-2011, 05:59 PM   #15
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Quote:
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I have parts coming in now. Picking up a pair of lightly used 295 A6's for test fitting today.

Maypo - Are you going to run it on street tires?
No -- 295 A6's.
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      12-06-2011, 06:34 PM   #16
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Regarding sway bars, you don't need more stiffness in the front. Go with the E93 rear bar to stiffen the back a bit and improve rotation on tight AX turnarounds.
Having run on R1Rs and "well-used" 255/35x18 A6s, the biggest single problem with the car was putting down power. I expect a noticeable improvement with a larger front bar. I didn't realize there was bigger rear OEM bar available. A modest roll-rate increase in the rear could come in handy as development ensues.

Quote:
Unfortunately, AS rules don't allow changing the final drive ratio. My 4.10 ratio really works well for AX.
My calculated speed in 2d looks to be 70 on the 295s, which is just about perfect for our events (Seattle) and Tours, etc. Do I have that number right?

Quote:
Still, if you value winning, then this is not a good plan for AS. The car's too heavy vs. the competition in this particular class. Go futher...
I am actually more interested in taking on the ponycars in ESP... What's our opinion about that prospect?

Last edited by Maypo; 12-06-2011 at 06:35 PM. Reason: grammar
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      12-06-2011, 06:57 PM   #17
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Quote:
Originally Posted by Maypo View Post
Having run on R1Rs and "well-used" 255/35x18 A6s, the biggest single problem with the car was putting down power. I expect a noticeable improvement with a larger front bar. I didn't realize there was bigger rear OEM bar available. A modest roll-rate increase in the rear could come in handy as development ensues.



My calculated speed in 2d looks to be 70 on the 295s, which is just about perfect for our events (Seattle) and Tours, etc. Do I have that number right?



I am actually more interested in taking on the ponycars in ESP... What's our opinion about that prospect?
I don't see how a larger bar in front will help acceleration. The most likely thing you'll get, I predict, is more push. I've got camber plates and Dinan springs, running just under -2 degrees up front, toe is 0. Turn-in and cornering are great with that setup with EDC set at its hardest. I don't know where you'll end up with shocks, but I think that you really need to be careful not to get the front too stiff. That's not a real problem with the stock car.

I run 275-35/18s square and get around 72-mph with my redline at 8600 rpm (thank you Dinan). I find that I usually only spend a second or so against the limiter per lap, so it's close to perfect. The big advantage we have is the broad, flat torque curve, making it easy to adjust and control power through the tight sections and slaloms.

In ESP I think I'd go with the Mustang GT with the 3.73 FD and Brembo brakes. In FS, probably the same car, but I'd need to research a little. I used to run ESP in Texas Region and mod parts are widely available for the 'stangs, making it easy to build up a potent car. Of course, in ESP you'll have to contend with my old friend Mr. Madarash, but it's a hell of a class, as is FS. I wouldn't entertain any visions of trouncing a Mustang or Camaro in FS. Their basic suspensions and high torque work VERY well on flat AX circuits. Add tire and go...

IF an E92 were classed in ESP, then a very serious lightening would possibly bring it into competitiveness, but it's starting behind in almost every way these days. More weight and less torque means... slower. HP counts for very little when you spend more than half the lap in the middle of the torque curve.

You'll need more bar in back if you add to the front, for sure. The problem is that the subframe will need to be dropped for the install. That turns it into a several hour job. Still, I'm doing it before next season. Trailing throttle rotation is what you want for a fast AX time.

Dave
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      12-06-2011, 07:13 PM   #18
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dcstep- A front bar helps put power down by keeping the inside rear on the ground. If you need that explained to you then i think you might want to just sit back and watch this thread, and if you want to talk about developing the car for other classes or sanctioning bodies there is a "new thread" button up there at the top . I appreciate what you're trying to do but I can't be saved...

Maypo- we should compare notes on the a6's. I will have the fitment questions answered in a few weeks.
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      12-06-2011, 07:23 PM   #19
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dcstep- A front bar helps put power down by keeping the inside rear on the ground. If you need that explained to you then i think you might want to just sit back and watch this thread..
I'd rather race you smart ass. What region do you race in?
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      12-06-2011, 07:36 PM   #20
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I'd rather race you smart ass. What region do you race in?
I race all over the country, are you a club racer too?

I will be autocrossing with St Louis region and i plan to hit the lincoln tour

Sorry if i was douchey but that was a little bit of an elementary-level comment.

Back before you thought i was a smartass, before the e92 was a twinkle in bangle's eye, the e36 m3 needed a stiffer front bar in stock class to keep the suspension in its happy place. Too much bump travel and the suspension starts to gain camber. The consequences were better tire wear, better transitions, and no loss in steady state grip. This scenario plays out over and over in Stock. When roll stiffness is difficult to come by, the front bar can be a big benefit on a variety of cars including bmw's. The e9x is a better design than the e36 but it still stands the tire up during big bump travel especially with the wheel turned.

By the same token, more front roll stiffness means less droop on the inside rear. This means less weight off the inside rear on accel.

Come on down to stl or come out to Lincoln and run ESP and see if you can raw time me with all the stuff on your car. Loser buys the beer.
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      12-06-2011, 07:43 PM   #21
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I've been running the M3 in ST-U class since purchasing it. My previous car was a shoe-in for the class and was competitive (Mitsubishi Evo). The M3 is not so much in stock form... but I knew that from the start anyway. So far compared to my usual benchmark of regulars, I am down about 3 seconds on a 40 second course - that is a pretty big margin to gain back.

The car's power is not an issue, for me it was putting the power down without going power sliding around corners. Also, the soft suspension and lack of feedback aren't very reassuring. But I am getting used to the car now and a lot smoother than before. The car does have great turn in response though! Incredible turn in response.

Here's a video of one of my laps where I was throttle happy and go spinning.


The front stock camber is woefully inadequate even with a performance alignment. The tires are rolling onto the sidewalls in front. I'll probably start there and pick up some stickier street tires as well.

Great thing about a Street Touring class is that you run street tires which keeps the budget down and it is pretty open to a lot of mods as long as you don't change internal engine parts.
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      12-06-2011, 08:18 PM   #22
dcstep
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Drives: '09 Cpe Silverstone FR 6MT
Join Date: Nov 2008
Location: Colorado

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2009 M3  [4.00]
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Originally Posted by Richbot View Post
...

Come on down to stl or come out to Lincoln and run ESP and see if you can raw time me with all the stuff on your car. Loser buys the beer.
Lincoln could be a good possibility for me. What month is it? I'd probably class in STU or ASP, but running street tires. I always enjoy a good throw-down.

STL is a longer drive, but we do have friends and inlaws there.

Dave
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