Hey guys -

I'm freshly returned from putting Ohlins R/T coilovers on my '05 RS and attending the Evolution dial-in school in Fontana this weekend. I'm mulling over my skidpad and tire temp results, and my conclusions on the suspension direction seem a bit off the beaten track - thought I'd trot them out here and see what people think.

I started at basically stock ride height, Falken 215s on 8.5" rims, -3.0 front with a bit of toe-out, -1.5 rear with zero toe, corner weights set. Springs are 9kg/mm front 11kg/mm rear. Rear bar was Hotchkiss set at center (+30%) setting. Shocks were full stiff both ends for skidpad testing.

Initial tire temps showed a 10 degree difference across the front, 5 across the rear, and 30 difference front/rear, with the car obviously understeering. Baseline time was 12.8 sec.

First tweak was to lower front 5 threads (about 3/4"). Retest showed lower temps in front by about 5 degrees, higher temps in rear by about 5 degrees, so overall f/r spread was down to 20 degrees, similar temps across tires, car felt much better but still understeering. .4 second time improvement!

Second tweak was to lower front another 5 threads, which took the springs out of preload at full droop. Retest showed minor improvements in temps, car felt marginally better but same understeer, less than .1 time difference.

Third tweak was to return front to previous (first tweak) height, and tighten bar to hardest (+50%) setting. Car feel noticeably improved, understeer was still present but noticeably lessened, rear temps went up another 10 degrees, fronts stayed about the same (track was heating up by now). .2 second time improvement.

That was it for skidpad, I was out of tweaks to change. Went to a slalom course for transitional work. I ended up at full stiff on the shocks in front, and 9-11 (of 20) in the rear depending on how I wanted the rear to feel. Both myself and Andy Hollis agreed that was the best the car felt that weekend (he had driven it several times throughout the weekend). I was surprised at the shock settings, had thought I'd end up running stiffer in the rear to help with the understeer, but that made the car too twitchy on transition. So my description of the setup would be that I still have steady-state understeer, and need more rear spring/bar rate to fix that, but can adjust the transition settings with the shocks to keep the rear drivable. Car is great in transition, but still has noticeable steady-state (mid-corner) push - I don't think I can fix that with rear toe without losing transition control.

It seems to me that I need significantly more front/rear spring rate variance. Fronts can maybe even come down a bit to 8kg/mm, and bump up the rears to 12kg/mm? That should help with both front compliance, and shift more weight transfer to the rear, and I still have enough shock rate range left to control more spring rate in the rear. My concern is that both the overall rear rates, and the front/rear spread, seem to be well outside of most of the recommendations/setups I see out there. I don't want to be chasing a wild goose, but the times/temps are what they are. What am I missing here?

KeS

 

The spring split and rear rates aren't that radical. I will be surprised if you can dial out the understeer with just springs.... If you are still at ~stock ride height in the rear there is plenty to gain by continuing to adjust ride heights. By juggling ride heights, tire pressures, spring rates, and alignment eventually you can get the car to turn midcorner and still be driveable (will take quick hands....) in transition.

Eric

 

When you dropped the front 3/4", did you reset your toe and camber? I dropped mine 12mm (1/2") and lost almost an 1/8" of my total toe out. Also had to readjust the neg camber slightly to maintain -2.75.

This past weekend, I increased my front and rear spring rate by 50lbs, dropped the front 1/2", increased the rear 25mm solid bar to the medium setting, and added another 1/16" front toe out while keeping the rear at zero. I ended up increasing my skidpad time by .265 and did not affect my transition control. My steady state cornering is good, but my acceleration on corner exit needs help (a little understeer). I plan to increase my rear sway bar to the stiffest setting and add 1/16" total toe out in the rear. I will also play with the rear tire pressures. Need a little more rotation on exit. I'm running Kumho MX's on 18x9.5. I'll be running the RT-615's shortly.

Eric, I'm running a bit lower in the rear than you are, maybe to low. Without rolling, the 245 tire fits deep. I'll be returning my rear shocks this week because Lex finally has the new versions.

Dave

 

Subscribing

 

Dave, I doubt that you are really running lower than me in the rear. If so then you are way too low. My short 285 tire is tucked up into the rear fender.....

Eric

 

Good feedback, thanks guys. No, I didn't readjust camber/toe in front, no equipment there. I'll be taking the car in tomorrow to find out where it's at now.

I stopped lowering in front because the benefit gain went down noticeably - possibly due to the associated alignment changes. Thoughts that I should actually go UP in the rear to transfer the overall weight back a bit as well as raise the CG? I hate to give up CG, though.

Other thought - is there anything like a "spring bank" for swapping used coilover springs? This is going to get mega-expensive as I work through minor changes - I'd already like to run a longer spring in front just to keep some preload on it and avoid street clunking, as well as the rate changes. Seems to me there's probably a zillion 2.5" springs sitting around in people's garages!

Re fast hands, I used to autox an ESP Camaro, so that's not a problem, though I like to keep the rear tame enough that I don't inadvertantly throw a run away. For the setup I had this weekend, I was able to get the transitional response pretty nailed down to a setting where one click softer or harder affected the driveability noticeably - of course that will change as the other settings do, but it's good to be able to make predictable changes in any case.

KeS

 

If you look at your roll centers and the associated axis inclination it will be clear approximately where the front and rear need to be....

As far as springs go, if you were running 2.25" units you could have access to my extensive inventory. I run a tender/main combination to have a high main rate with shocks that are a little longer than optimum.

Eric

 

Kevin,

I autocross STU in Milwaukee region pretty seriously, and I can feel your understeer pain. I've been dealing with it for some time now even with the rear bar full stiff and all kinds of shock settings. What I can tell you that helped quite a bit is taking the rear camber down to -.8 degrees with about 1/8" toe out (that was the lowest we could get it at with that toe). I run -2.8 front with 1/8" toe out in the front too. I really like this setup with my Buddy Clubs right now. Less push and good transitional. You may want to take out some negative camber in the rear.

Tim White

 

You would be surprised. My rear wheel lip (not tire) is only 1.5" from the fender. I have pictures of the wheel compressing deep into the rear fender on corner exit. The distance between the wheel center and the rear fender lip is 11.5". BTW, this is not by choice. Waiting for the 2nd versions to arrive.

Eric, do you have a set of 8", 650 lb, 2.25" I.D. springs you want to get rid of? how about 700 lb? PM me.

Dave