SCCA STR Autocross Carrera Build

[rennch]

Originally Posted by Mehoff

Ha! Hard headed is right....255 is WAY too much tire for an AutoX car. Hell, it's almost too much tire for a track car with a stock drivetrain (factory weight).

I was running on 17 X 10 & 13 wheels (245/315) and it was a killer autox setup.

That said...I'm going with 245/275 x 15 when my Group 4 Wheels arrive.

[rennch]

Originally Posted by neilschelly

I'd expect an autocross car to have more use of wider tires than a track car. I don't recall seeing too many Miatas on 255s or 275s on a track, but they sure do love them on an autocross course. I ran my Impreza autocross car (same weight as a Carrera 3.2) on 255s, and I was the odd one out when everyone else was running them on 275s.
-N

I think the 275 Hoosiers bump him out of class. He's right...not a lot of street tires in 275. Trust me, I've looked.

[rennch]

Here's my car on 15 X 7 & 8 (Soon to be 9 & 10) on Hoosier R6 225/275.

If only they made street tires in 275-35/15!





This was originally built to be a BSP autocross car.

I realize it's kinda hard to see the contrast because of the murdered out wheels. Sorry 'bout it.

[aschen]

OP, Cool project. Ive always wished mid year 911s were a little more favorably classed for autocross. Your car is the same color as mine and I think the grassroots motorsports car. I think it will be tough to keep up with the top STR cars but I wish you the best of luck, Ill be following the progress. I think you are right about tire width. More always seems to be better for autocross even if it means other compromises. THis is suppoted empirically and quantitatively.

Michael, my vocabulary is not adaquate to describe how fantastic your car looks.

[Mehoff]

Originally Posted by Driven97

Come beat me and I'll listen to you.

Lookout! Wasn't trying to be a dick, apologies if it came off that way.

I guess I'll keep this in the vain of MY experience: without the need for the extra tire (ie: too much rear slip angle, wheel spin, etc which can be induced by speed and/or power) the wider tire is actually slower than a narrower tire. Whether this be a function of lower roll resitance, aero, etc I have no idea, I just have the empirical data to back it up. If a racer can't get the car to rotate proberly at low speeds with a 245 or 225 on the back the issue starts and ends in the drivers seat (same with wheel spin on a short course). I have to imagine getting temp into a wider tire is more difficult on an autocross course as well, but not sure.

If there's one thing I've learned about racing these cars it's this: look at what the fastest guys are running and replicate.

[rennch]

Originally Posted by aschen

Michael, my vocabulary is not adaquate to describe how fantastic your car looks.

Thanks man. That makes me feel happy inside.

[neilschelly]

Originally Posted by emptyo

I think the 275 Hoosiers bump him out of class. He's right...not a lot of street tires in 275. Trust me, I've looked.

Yes, for ST classes. I was more or less responding to the idea that 255mm is too wide for autocross tires in general. If it's not too wide for Hoosiers, then it's definitely not too wide for street tires. And as for getting a wider tire up to temp, there's gonna be a whole lot of other variables involved there... but yeah, if you can't get them warm, that's too wide.
-N

[Driven97]

Originally Posted by Mehoff

Lookout! Wasn't trying to be a dick, apologies if it came off that way.

Oh no, not at all!

Check out this NC Miata thread on STR car builds. 38 pages that ends up delving into roll couple, changes in setup based on tire manufacturer, even how to optimize for specific sites. These guys are at a whole other level.

The S2000 guys got to 194 pages before they had to reorganize the info into different threads. WAT.

I personally don't have either the knowledge or driving talent to compete with the big dogs, but my true purpose in documenting my build is to try and inspire others who do have the knowledge and talent to give one of these wonderful cars a shot. I've been autocrossing for 16 years and can probably count the 911s I've seen on one hand, and maybe one of those was a serious effort. STR is only a few years old, and it's already exploded into often being the most populous class at events. That's exciting.

Maybe my car will be quick enough to at least make someone wonder, maybe it'll end up hopeless. Best case scenario I'll meet you and a dozen other 911 owners at Nationals to use our 30+ year old cars to lay the smack down on the young roadsters. Worst case scenario, at least I'll have the best looking car in grid all by myself.

[Driven97]

Michael, that car hurts my feelings. Just gorgeous. If this whole STR thing doesn't work out, BSP is the next step.

[Driven97]

Did a corner balance today.

Before:



After:



This is with 1/2 tank, no spare, but without me in it. With me in there, cross weights weren't affected much, but the total weight sure was.

[Driven97]

FYI, I would avoid the eBay monoball camber plates. Looks like they use some sort of fabric type insulator between the ball and socket. It worked out on one side causing a very irritating rattle, the other side was on it's way apart. In under 1000 miles.



I dropped the rubber donut stockers back in for now. I'll remeasure camber to see where I'm at with those, but judging by the shaft position it isn't much less.

[Elombard]

Scheez, what did they cost?

[berettafan]

Originally Posted by winders

For an autocross car, I think a 23/30 torsion bar setup is too stiff at the front. Personally, I would go 21/31 or 22/33. Otherwise, you will be fighting understeer on all but the fastest courses. I would get the same size anti-roll bars front and rear as well...

Scott

Front is going to be loose, I agree. You might try those bars without the front sway and see how it goes.

[Driven97]

Originally Posted by Elombard

Scheez, what did they cost?

They were $275 but I had a fair amount of eBay bucks. They have a "warranty" listed on their site, so I mailed them, but I don't have high hopes.

Originally Posted by berettafan

Front is going to be loose, I agree. You might try those bars without the front sway and see how it goes.

Indeed. The car pushes a lot in sweepers.

I planned to balance with a big rear bar, but haven't grown a pair to weld up the sway mounts yet. In retrospect, I should have gone big rear spring + small rear bar, soft front spring + big front bar. I think a big rear bar might make it hard to put power down on corner exit.

[berettafan]

remember to lift a touch when it's pushing. any hotshoe 911 drivers in your area that could give it a run and offer some feedback?

[Driven97]

Originally Posted by berettafan

remember to lift a touch when it's pushing. any hotshoe 911 drivers in your area that could give it a run and offer some feedback?

Right now a lift, even slight, blows right past Pleasant Rotation Town and goes straight to Oversteer City. Here's me trying to balance the car on a decreasing radius sweeper:



I've got a few tweaks I'm going to try. I guess the suspension toes out in jounce. Since I'm at zero static toe, I am getting toe out when I lift, which is rear steer. I'm going to crank in some toe. I also have a super secret trick up my sleeve.

[Driven97]

Long time no update. I've been having a ball with the car. Just attended my first ever SCCA National Tour event. 22 entries were in STR:
18 S2000s
3 Miatas
...and me.
The car sure stuck out like a sore thumb in grid:



The car is slowly getting better to drive. I'm now 99% certain I am way rear soft, and heavily underdamped all around. I have a temporary fix in place which has helped, but I'll be raising the rear torsion rate properly this offseason. On this surface in particular (brushed concrete) the car has been rather tough to drive, it's more behaved on asphalt.



I ended up 17th of 22. Not great, but every other car was specifically set up for STR, and well driven. No class filler. Not bad for a car with very little applicable knowledge base, only a few months development, a low budget build, and a hack driver. Especially when most people have written the car off as noncompetitive.

Regardless, I had a blast and the car got a TON of attention. I'm not convinced I can get it to the level of the S2000s (which are still being figured out) but I've at least proven the car can run with them and not be embarrassing.

Link to video

[NYNick]

This is a great, well done, fun thread. The video is a gas.
Thanks for sharing!
Nick

[Trackrash]

Michael, say hi to the crew at Qualcomm for me. I sure miss that place.

Matt, stiffer is definately better for the suspension. The quicker 911s that don't have a ton of power, will use 15" wheels with sticky rubber. If you are serious, strip as much weight as possible off the car, then work on bottom end power and getting the car to rotate.

Speaking of San Diego. With PCASDR at Qualcomm you will definitely use third gear, have twelve or so laps, and can have an instructor.

By the way, good luck beating the S2000s

[aschen]

I always love to see unusal and cool cars in the "spec" autocross categories. I suspect you have an uphill battle against well preped modern cars, but your fight is a noble one.

Do you have an LSD? It sounded like some inside wheel schenanegens in your vid, but it may have been the front tires I was hearing.