NC - autocross ride height & suspension setup questions

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rpmchris
Posts: 245
Joined: Thu Jan 18, 2007 4:55 am
Location: Murphy, Texas (NE Dallas)

NC - autocross ride height & suspension setup questions

Post by rpmchris »

Some late season setup questions for Brian, and maybe Ron can chime in, too...

Any specs on ideal ride height for an autocrossed MX-5? This is my daily driver and weekend autocross toy, so I can't slam it even if it would be faster to do so...

Background - I've been on Koni Sport shocks with MS/Eibach ProKit springs and 17" RPF1 with 245/40 BFG R1s, and I have roughly 4-1/2" space from lower frame rail to ground (at the jacking points). I had it cornerweighted and balanced (50.3% crossweight), and it's currently aligned to -2.5 camber front and rear, max caster and 1/16" toe out in front with 0 toe in back. Jason Saini at Team MER did the suspension work for me at the beginning of the season, and except for a little less negative camber (was -2.7 front/ -3.0 rear), it's worked very well.

I'm switching to coilovers and 18" RPF1 with 285/30 V710s, which are slightly taller - should I try to keep it at 4-1/2" or should I go a little lower, as the much higher spring rates should keep it from compressing too badly on the street, right? Also, I know Hoosiers like a little more negative camber - how 'bout the Kumhos?

Thanks in advance!
'06 MX-5 Sport 6-speed
Autocross junkie!
Brian
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Post by Brian »

Hi Chris,

We are on the same road....and I am actually a little jealous of how low you have it. Because I was running a local class here that limited me to 8.5 wide wheels I was using what was available from Enkei (40mm offset) and tire clearance dictated I stay closer to 5 inches using your measurement method. Now that I plan to run SM2 with the Cosworth supercharger I too plan to finally get our 18x9.5 combo on my own NC and hope I can actually get a little lower. However, currently I have very similar alignment numbers to yours and I won't try to lower until I see how much more fender roll work I need to accomplish the fit of the 285/30/18 combo all around.

At the our recent SCCA divisional several customers had done really nice jobs of rolling the lips to fit the big 285s....I am just waiting for our next really hot sunny day so the metal moves easily without (hopefully) any paint issue.
Brian Goodwin
Good-Win Racing
www.good-win-racing.com
rpmchris
Posts: 245
Joined: Thu Jan 18, 2007 4:55 am
Location: Murphy, Texas (NE Dallas)

Post by rpmchris »

I won't have to wait for a "really hot sunny day" as it's been 100+ here for a week or more! My only fear is that my stripes will need to come off if I must resort to a heat gun in front.
'06 MX-5 Sport 6-speed
Autocross junkie!
vman
Posts: 11
Joined: Sun May 06, 2007 10:28 pm

Fender rolling

Post by vman »

Guys, I rolled my fender's many times and kept hitting. I also watched a bunch of rolled fender NC's at the national tour in SD hit their fenders in front under braking and have all sorts of problems. All these cars were running Kumho 285 30 18's. Note - if you switch to Hoosier A'6s get ready to have less clearance, as they are a 1/4 wider than the Kumho's. I finally gave in and had Tony at AWR racing build a set of flares for the car and cut back the wheel wells...no more rubbing problems, or worrying about cut tires when running the 18 x 9. 5 ET 45 RPF1. At $450 or so dollars for the flares and about $800 in labor, its about the cost of a set of tires to get rid of the worrying about hitting your fenders and slicing a $1,200 set of tires. As to lowering the car, don't go too low, it will cause some strange handling. As to camber and toe settings you seem pretty close to where you need to be ie. and ride height about 2 inches lower than stock is a good starting point. Note - the NC is very sensitive to ride height. Also, we found that corner balancing the car with the stock rubber bushings was a waste of time and reduced its stability, better to just set all perches the same height. Also, I suggest you measure ride height at the fender lip, as the frame side can bend...Note - stock cars sit about 27.5 inches at the fender lips. If you go to flares, you will need to measure using a bar across the back of the open trunk and hood as the fender lips will never be as exact as the factory.

VM
Brian
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Post by Brian »

Thanks VMAN...but I might as well roll first. I will ROLL ANd ROLL AND STRETCH it as far as hot paint on a hot August day will allow and see what I get. If it cracks then nothing lost but a little Sunday time and we do the flares. If I get it far enough....then perhaps I won't have to bother with the flares.
Brian Goodwin
Good-Win Racing
www.good-win-racing.com
vman
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Joined: Sun May 06, 2007 10:28 pm

To each their own

Post by vman »

Brian, good luck with the rolling of the stock fender, life with flares has its ups and downs...the good - lots of clearance and no more cut tires, the bad, they can crack a bit when you get a cone stuffed up there and you have to use a straight edge bar to on the hood and deck to measure ride heights (which is a pain).
rpmchris
Posts: 245
Joined: Thu Jan 18, 2007 4:55 am
Location: Murphy, Texas (NE Dallas)

Post by rpmchris »

Will be working on my fenders this weekend - these tires are huge!!! :shock: I'll post pics pre- and post-work.
'06 MX-5 Sport 6-speed
Autocross junkie!
ronbauer
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Post by ronbauer »

Hey guys,

First off, you can roll the fenders enough to avoid any rubbing on the metal of the fender. I haven't had any problems with mine at any corner. The fronts are what need the aggressive roll/flare, while the rears can get away with less.

The fender liners will rub and self clearance as they go. Other than that, I have had some rubbing away of the paint in the inner portion of the frame of the car on the rear, but it hasn't caused any issues with the tires.

As for alignment, we've changed ours recently to where we're running about 2.7 front and 2.0 rear camber, with about a 1/16" toe out in the front and 1/16" toe in in the rear.

I'll have to measure my ride heights with the 285s on the car when I get it back this weekend (Jim has it currently from the tow to Wendover.) It is sitting a little higher now than previously, mostly because my Moton/Hypercoil setup ended up not allowing me to go any lower in the rear. I think that having some rake (front lower than the rear) is the way to go with the car to help with the slow speed push.

My car has been corner weighted as well. Sorry Vasili, but I have to say that I haven't noticed any negatives to this or going pretty low on ride heights.

The biggest change though was going with the Motons. I highly recommend them!

Ron
rpmchris
Posts: 245
Joined: Thu Jan 18, 2007 4:55 am
Location: Murphy, Texas (NE Dallas)

Post by rpmchris »

Thanks, guys! I had it cornerweighted/balanced/aligned on Friday, and got the fenders rolled Saturday (wasn't nearly as bad as I expected, since I'd already started the process when fitting the 17x9s), before driving up to OKC for a local event. I won CSP, was third in PAX, and set the full-fendered FTD (only one CM car was faster) - I was even faster than Michelle Seelig in her Nat'l Champion STS2 Civic.

I'm now using my 17x9" RPF1s for my street setup, with 255/40-17 Kumho SPT tires all 'round. My used 17x7.5" RPF1s are now for sale (along with used 245/40-17 BFG R1 competition tires and 235/40-17 Goodyear F1 GS-D3 street tires).

I ended up with the following alignment settings:
-2.5 camber front & rear; 1/16" toe out front & 0 toe rear; max camber front; ride height (at jacking points) is 4-1/2" front, 4-5/8" rear; balanced front-to-rear with driver @ 50.0%; cross balance at 50.1%. I'm sending pics to Brian to post here...
'06 MX-5 Sport 6-speed
Autocross junkie!
mr2ace
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Joined: Thu Jul 31, 2008 6:56 pm
Location: Dickinson Tx

Post by mr2ace »

That's Great Chris Congrats !

A couple of questions your jack point measurements exactly where on the lower frame are you taking these from the seam, or inside if the seam, or ?
Your cross weight is that "without" or "with" driver? I assume "without" but if it is "with" can you share how this was accomplished?

Thanks
Richard
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