jstyer

This is the situation, the situation is this:

- MCS Single adjustable shocks
- 514 lbs/in rear mains 290 lbs/in front mains
- Rear Alignment: .09 degrees toe-in per side, 2 deg camber, 2.0 kinematic toe on the motorsport gauge
- Front Alignment: 2.5 deg camber, 4.8 deg caster, zero toe
- RS sway bars full soft rear, full stiff front
- 245/40 front tire, 265/35 rear

I'm running yellow group in PCA, but have had many other advanced instructors drive the car and consensus is the same. Oversteer everywhere. The rear takes a set and is fairly predictable, but is seriously short on stick. Steady state cornering, slow technical corners and trying to use throttle to push the line out, fast long sweepers, it just doesn't matter... the car is just always running out of rear grip. And while the smokey sideways action keeps me awake, it's also keeping me from advancing with this car.

To me the front/rear spring ratio is not crazy, and I'm already maxed out on sways. Would you folks chock it up solely to the wide fronts and lack of width on the rear tires? Or should I be looking elsewhere?

Thoughts always appreciated!

rlm328

There is a chart at this website that you may find informative

http://store.uucmotorwerks.com/artic...ybar_setup.htm

jscott82

Something does not sounds right... 993 naturally want to understeer.

Sounds like you have confidence in your alignment... But I would have another look at the rear k-toe. So many shops seem to mess that up.

Short of that, I would guess something in the rear is binding. I assume you have spherical top mounts? Id start there, I had one seize up after getting water in there.

Make sure the sway bar can cycle though its range of motion freely.

Worse come to worse, Id pull the springs and start cycling things by hand....

38D

Has the car been in an incident ever? Could be a bent link in the rear.

coryf

IMO:
Too soft front spring, too small rear tire, not enough rear toe in, more kinematic toe if you can get there at your current ride height.

You did not mention: Ride height. (rake?) Tire choice, Limited slip?

Thundermoose

How reliable is the LSD on the 993? Can you fit wider rears?

What tires were your running? What rear pressures? It was pretty warm at TWS yesterday and they could've been getting greasy.

RickBetterley

We ran 285/35s Hoosiers on the rears. That was a dedicated track car though so no guaranty on that fitment

jstyer

Good point! Sorry I left that out... At the factory measuring points, I am at 120 mm front, 110 mm rear. So basically RS.

No limited slip, but it's not oveesteering due to power. It's definitely a chassis issue. Tires are direzza star specs.

First time I took the car out, the rear sway bar was binding. I have never had that happen before, and it was scary to say the least. I would have the same low but predictable grip until apex, than as I unwound the rear would just snap out very unexpectedly and never in the same place. Regreased all the sways, and checked the top mounts. Things are now much more predictable, but still oversteer.

I actually just bring my own gauge and tools and rent a rack at the shop I align at. It's a brand new hunter optical unit so I have confidence in the rack.

Heard a few murmurs that my KT might be a bit aggressive. You guys suggest maybe bumping up to 3-4 KT when I go with the bigger rubber?

car_slave

Try a 225 tire in the front. Not much split between 245 and 265.

jdistefa

A few possibilities:

-something is seriously wonky, bent, binding. I would string the car to doublecheck the alignment numbers and look carefully underneath at every control arm, sways, etc. I think this is unlikely.

-the more likely possibility is your spring rate - 993s really like a front/rear split that is close to 75%. You are at 56%. Per Cory I agree you simply don't have enough front spring. Change 'em.

-I would put a 275/285 on the rear

-if you have factory KT control arms you can increase your KT setting (another option is just to run heim jointed KT and toe control arms - this helps to reduce dynamic toe changes and 'tie down' the rear end)

jstyer

So, definitely going to more rubber in the rear, and going to change the KT.

When changing to a spherical bearing style do you still set the KT to similar numbers on the MS gauge or do you just go off of thrust angle/caster?

Let's continue to talk spring rates, as this path really interests me!

Here's the ratios that I've been able to find based on Bill V's amazing chart: http://forums.pelicanparts.com/uploa...1343088015.gif

ROW: .58
993TT: .53
RS: .54
RSR: .79-.87
GT2: .82-.9
CUP: .91

KW V3: .33 (Seriously nuts)
KW CS: .54

PSS10: .58

Bill V: .8

Me currently: .56 50N/mm & 90N/mm

So, it's pretty apparent that the street cars were all around .5-.6 and the ratio bumps up quickly as rate jumps up. And I mean waaaaaay up. Cup car is the lightest sprung of the track group and it was running 220 N/mm. That would be nearly un-drivable at Texas World Speedway.

This car still sees 3-5K street miles a year, so I want to err on the side of softness. Would you recommend going to 60 or 70 on the front (putting me at .67 or .78)? Or... I could go to 70 on the rear, which lands me at .71

Thing that stands out to me is the RS ratio of .54 I'm very close to RS rates and ratio, and I can't seem to believe that Porsche would've gotten it so wrong.

jstyer

P.S. Matt, finally put your old pauter rods to use in my rebuild this winter. 25 hours on them so far!

Bill Verburg

f/r ratios aren't the # to look at, the difference is
spring rate difference ~150 is the base set up for all the race cars and sport street except for RS which has 211

before doing anything check all the bushes and arms for worn, rotten or bent parts

RS bars will work fine at neutral w/ the rest of the suspension right


bump up the front spring rate

decrease the tire width spread, 225 or 235 front w/ 265 is fine, 235 or 245 w/ 275 or 285 is fine, the lower the spread the less under steer

RSR steering arms help

Bump toe and KT is idiosyncratic, you want the smallest # on the Motorsports gauge that is comfortable for you, the higher the # the slower the rear reacts start ~4 go down to ~2 till you find your comfort level,


an effective lsd helps a lot

jstyer

Yes, I'm running 8.5 front and 10 rear. I was going to try to go to 285 on the rear.

RS/GT2 uprights up front. I have solid/sport rubber rear control arm, solid front arms, solid rear toe, sport rubber rear camber and KT. Solid subframe.
Every bushing in the car is brand new.

Would you go to a 342 lb or 400 lb front? Conversely, I could drop to a 457 lb rear.

Currently on Direzzas, was going to try a year on RE-71R's before I jumped to NT01. Want to tune the car on something a little slower tire before I start really leaning on it.

craina

I got to drive Justin's car over the weekend. It should also be noted that he has removed the stock rear wing assembly... That is my best guess. He really has created a pretty wild 993, my mostly stock (pss10, sways) feels pretty tame by comparison. His car needs fine tuning but boy is it fun to drive even though I was constantly fighting/worrying about the rear.

Having tried both, Re71r >= NT01 in terms of grip IMO. Biggest difference is the sidewall on the nt01 is stiffer.