fbirch

Your front readings are similar to what I see on the rears of my car with Yokohama AD08R street tires on a hot track. My fronts run about 8 C cooler than the rear, but my home track is fairly fast.

My camber is only -2.45 F and -2.25 R, stock springs and shocks. I see 7-10C of temperature spread from the outer edge to the inner edge of each tire, with the inner edge being hotter, especially on the rear. What would you consider an acceptable spread?

pete95zhn

I have no idea, but fortunately Michelin does: 10F (12C). I've also heard that Toyo's intructions for R888 is 15C.

fbirch

The following link has some useful info:

http://949racing.com/using-a-tire-py...49-racing.aspx

993GT

great thread! very nice work/setup you have

rbahr

Hi Pete,

I didn't see tire pressures or sizes - might have just missed it, any way some possible explanations - lots of understeering, bars or springs are off - to much in the front, to little in the rear, tire pressures...

I hammered mine over the weekend put 200 miles on the car in 1 day at WGI - took some time to get tire pressurs correct, now comes a lot of fine tuning...

I still have questions on rake and ultimate ride heights, but that can wait

Ray

pete95zhn

Tires are MPSCs, 235 / 295 -18. Hot pressure F 2.1 / R2.3 bar. Understeering...not that I can notice on fast corners, maybe just a little on slow ones.

I'm not an expert on rake and ride heights questions. Have I promised to answer for some?

rbahr

Sorry about being confusing: What I meant to say was that for under-steering some possible causes are:

bars or springs are off - to much in the front, to little in the rear, tire pressures...

In addition - I was wondering what you had your rake / ride heights set to?

Thanks

Ray

pete95zhn

The last time I measured ride heights: F 105 / R 125 mm...rake's then 20 mm. OEM points, with my street tires (225/40 & 265/35 -18). Since then I've raised rear spring perches by 5mm. I checked the distance from garage floor to fender arches: F 635 / R 624 mm. Highly unscientific measurement, just after changing track tires to street ones, so the suspension is not settled.

Now some wheel size differences effecting the rake (size vs height):

225/40-18: 637 mm
235/40-18: 645 mm

265/35-18: 643 mm
295/30-18: 635 mm

24/64-18: 640 mm
27/68-18: 680 mm

So when I've set ride heights with street tires and change to track ones, my ride height changes from 105 / 125 mm to 109 / 121 mm. Changing to slicks would change the ride height like this: F 106 / R 143 mm. Or if put differently, tires' part of the rake is
street (= mkI Gt3): 3 mm
track (= mkII GT3): -5 mm
slick (Cup or RSR): 20 mm

pete95zhn

Going a little further with this rake vs tire size issue one can find out this:

Original ride height:
mkI GT3 102-112 mm / 125-130 mm (rake at lowest setting 23 mm, of which from suspension 20 mm)
mkII GT3 115-120 mm / 128-133 mm (rake at lowest setting 23 mm, of which from suspension 31 mm)
'04 Cup 86 / 107 mm (of which from suspension 1 mm!)
'02 RSR 82 / 112 mm (of which from suspension 10mm)*

What should I learn from this then?
1. when setting the suspension have the tires you're going to use at track. (I did'nt have them at hand then...)
2. variables are plenty
3. CUPs have their rear tires really deep in the wheel wells

* '02 RSR's Tech Info states that it's ride height is measured from the same points that street's and CUP cars's. Compared to mkI GT3 one could see some (interesting) things:
-it's 13mm lower than GT3 at it's lowest setting, that means the engine is 33 mm lower (because of subframe solution)
-RSR uses 37 mm taller tire, so effective rise is 18.5 mm
-taking into account two previous things the top of the rear shock floats 31.5 mm lower
-At front RSR sits 20mm lower than a GT3; 5 mm comes from suspension setting and 5 mm from subframe solution
-body rake is 30 mm, but if/when GC is taken into account, the rake is just 10 mm (30 mm -20 mm from rear subframe lowering = 10 mm)

All above if I've calculated/thought correctly....

powdrhound

I'd set the car up with whatever you will run at the track..

rbahr

Hi Pete,

The changes you made to the rear side members should have lowered the rear ~20mm (at least that is the difference in mine). With that in mind and using a 10mm actual rake value, then you want a height differential of (10 + 20) or 30mm.

This was part of an offline conversation I was having with outline.

WRT springs - I suspect that we have similar considerations but coming at it from different directions - I have 1000/1200 and iirc you have about half that. When I run the numbers, given my weights, I end up with ~700/900 as starting points. This implies that we are both off by roughly the same amount...

But then, (for me) it's time to drive the car.... :-)

With all that in mind, I was at WGI ~week ago and really liked the way the car felt - downright toss-able.

Ny next outing will be at LCMT in ~3 weeks, I will give you some feed back then. I am running the Nittos 235/315 while I get used to the car. Not certain, but may put on some sticky tires depending on how comfortable I feel.

Ray

AudiOn19s

I enjoy this rake conversation...it's one topic I studied quite a bit when setting up my car.

Do you guys have profile pictures of your cars as they sit with said rake? I ask because the only thing I don't really like about my car is the way it LOOKS (not drives) based on the rake in the Chassis.

I have 20mm of rake currently and the car looks like a '78 Chevy Camaro that should have ladder bars hanging off the rear suspension it has soo much rake.

I'm at 105 / 125 currently and my car ever so slightly tucks the front tires up in the fenders. I can't imagine you guys under 100mm how low the front must be. I made myself feeler gauges to put in the measurement points to make sure my base line heights were spot on before corner weighting.

I leave well enough alone though because I love the way it drives. Trying to find a profile picture myself and I don't really have many....cause I can't stand the way it looks from the side.

rbahr

From my conversations with outlline, the 996 folks seem to like 10-15mm of rake.

This would imply that you could drop the rear ~5mm and lose the 'chevy with ladder bars' look and have acceptable rake - not sure what it does for all the other values...

That said, Pete's car and my car have a very different rear geometry which can tolerate and actually is designed for the lowered ride height.

Currently my ride heights are pretty high roughly yours and Pete's. I will likely go to more the cup car heights which is ~90mm in the front and ~120mm rear to keep the 30 mm differential I want

Ray

pete95zhn

Andy,
see my post #19, there's few pics, but from quite close distance.



Ray,
the chassis is 20mm lower compared to the rear subframe, but the height of the car at rear is measured from the subframe. So the rake looks out slightly different with our side members. Still adjusting suspension heights from spring seats moves both subframe and body. What I have not yet measured is how body is positioned compared to the ground. Ideal would be level or rear slightly higher. With my flat bottom it's easy to measure if the surface is level. (Not the case in my garage bacause of drainage well in the middle.)
Anyway street GT3s and CUPs run almost equal rakes, but one has to keep in mind the tire size issue affecting it, which I did not yet do... Back to the drawing board.

I also may go lower, but need to still maintain 80 mm ground clearance at minimum.

Yes, i think our spring rate split is more or less the same. My springs are 456/685. Most likely I will go to stiffer ones next year. I'm waiting for your feedback of your setup. There's already few running very stiff ones with success.

How have you managed to stuff those rear tires there?! Pics pls! Regarding tires, what's the widest you can use at front?

Another question about tires: Does anyone know how much is max tire size difference F/R the ABS unit can understand?

AudiOn19s

My old red car only had 10mm or rake...105 / 115 and while the suspension was different and it was a C2 with C2 geometry it didn't turn near as well as my GT3 does...part of the reason I'm afraid to take rake out of my car. I think 20mm should be a minimum for us...as others have pointed out Cups run closer to 30mm

Sure was sexy though...