Jeffrey Behr

AMEN!!!!! Me too.

A1Aflorida

As for 245's and 295's, best to get an answer from someone here that knows or a good P-car independent shop, especially one that mods cars on a regular basis.

Soul Performance

There shouldn't be. GT4's come with 245/35R20 front tire and a 295/30R20 rear tire. Can you find a 245 front? Other wise the actual contact patch difference between a 235 and 245 isn't much, depending on the run width, etc.

Dave @ SOUL

haripin

Thanks, Dave! Super helpful. I can get a 245.

What do you think about going with a 295/35/19 instead of a 30 sidewall?

Also, TireRack only seems to have Michelin PS4s in the Mercedes MO1 designation, which I know is used on their AMG cars. I can't seem to find a standard PS4s tire in the sizes I need. Anyone have issues running MO1s?

haripin

Hey Dave, thanks for your help.

I just order Michelin Pilot Sport 4S's for my 987.1s in the following:

Front: 245/35/19 and R: 295/35/19

Do you think the 35 aspect ratio on the rears will be an issue? My car has stock suspension and alignment. Instead, should I have ordered 295/30 instead?

hueyhoolihan

as the difference between the width of the front and rear tires WIDEN, the aspect ratio, in order to prevent excessive diameter and resulting rake, will need to be reduced. the stock tires have a difference of 30mm tread width between front and rear. it's 50mm on the tires you mention. i would drop the aspect ratio from 35 to 30 on the rears.
doing that will also prevent your speedometer from becoming inaccurate to the point of annoyance.

haripin

Thanks — this makes sense. The 295/35 would make the car taller in the rear by 1.3” where the stock set-up is 1.1” inch.

To add a wrinkle to this: I could only get Michelin PS 4S’s with the MO1 (Mercedes AMG) designation in the front. They make the MO1 compound in 295/35 but they do no for 295/30. So I would have to mix MO1s in the front with standard — non mfg specific — rears.

any thoughts on this?

Zach L

@haripin regarding your question about whether those sizes will be detrimental to handling performance, the answer is yes. Any increase in rear tire width while not increasing front tire width by the same amount or more is going to be detrimental to handling performance. Specifically it's going to increase understeer on a car that already has a problem with too much understeer.

The GT4 weighs more than a 987 and has a slightly more rear-biased weight distribution. This results in a similar weight over the front axle but more weight over the rear axle on the GT4. This is one of the main reasons the GT4 has more tire in the rear for more mechanical grip yet still has similarly neutral handling at the limit while cornering. Another major factor is the adjustable front camber on the GT4... this also aids front grip while cornering. Also, the GT4 has more power so you'd want a bit more tire during on-throttle corner exit.

So while the GT4 may perform well and have fairly neutral handling with those tire widths, running them on a 987 is going to result in significantly more understeer, on a car already engineered by the factory for understeer at the limit.

haripin

Yup. Agree with all this. But this is the predicament I'm in. The PO left me with these wheels, my current 235/35/19 (front) and 285/30/19 (rear) needs to get changed because the tires are shot and the rear tire is way too thin for the 11 inch wide rear wheel. I also don't want to drop $3000 on new wheels, so I'm trying to make the most of what I have. I'm happy with the current performance of the car, even with 235 in the front and 285 in the rear. I'll be keeping that same stagger with 245 up front and 295 in the back.

What I'm actually most concerned/focused on is the delta in diameter and circumference with the set-up. I've been doing some excel modeling, and it seems like since I'm constrained to a 295 wide rear tire (assuming an aspect ratio of 30) on a 19" wheel, I might have to go with a 235/35 to keep me within the 5% circumference threshold from factory set-up. This is a bit unfortunate because that is going to promote even more understeer.

Also, I should say, plenty of people run a wider stagger on track and on street. Yes, the car is inherently understeer biased but there is a reason people run the wider stagger.

I've included the chart for anyone who wants to nerd out.

Zach L

Yep, I'm one of them. 19x9 front, 19x11 rear with 245/35 and 305/30 for street setup. The fitment looks great, but it does induce more understeer than was already present. That's even with running max negative camber on stock LCAs.

If I'm looking to drive hard or go to the track I'll swap on one of my other wheel/tire setups... I run 255/35, 275/35 on 18x8.5 front, 18x10 rear for my track setup. I have GT3 LCAs in the garage about to go on and will probably swap to an 18x9 setup on the front soon.

None of these setups are right or wrong, but it's ridiculous for people to suggest these increased staggered setups don't harm handling performance. Just have to decide whether function or form is more important... or just get multiple wheel/tire setups for different purposes.

​​​​​​ Street:


Track:

haripin

Nice car! I guess my situation is im already running 235/35 and 285/35 (thanks to PO). That’s a 50mm stagger and I dont find the car has tremendous understeer. I also drive fairly spirited although not on track (yet).

if I’m keeping the 50mm stagger with 245 and 295, it doesnt sound like I should notice a lot of relative difference. I am wondering about height tho, and introducing more positive rake. Stock Cayman R has 0.8” lift in the rear. My 245 and 295/35 set-up will have 1.3” of lift in rear, with a total circumference difference from OEM of just about 5%.

Do you think there’s a whole lot of performance difference between 295/35 versus 295/30? Like, would you go with one over the other if performance was the goal?

not sure how extra sidewall affect handling. I’ve heard it may actually give me some more roll which will help on cornering.

Zach L

All other things being the same, I wouldn't get caught up on how a one-step change in aspect ratio will affect handling. There are other things like tire widths and alignment that are far more influential. You could throw on an adjustable rear swaybar and have far more influence on car control than a subtle change in sidewall height.

haripin

True. What can I do alignment-wise to help reduce any understeer?

Zach L

Biggest help will come from adding more negative camber in the front. Achieved through the use of GT3 adjustable lower control arms (LCAs), front camber plates, or both. Max negative camber with factory 987 LCAs is about -1.5* ...GT3 LCAs are capable of around -3.0*. It should be the first suspension mod on these cars. If you lower the car you'll need adjustable rear toe links as well to get the alignment back where it needs to be. Those two are the bare minimum parts needed to have proper, functional alignment on a lowered 987. I started running max negative camber on my factory 987 LCAs pretty much as soon as I got the car, until able to throw on the GT3 units.

Once you have them installed take it to a local race shop that normally preps Porsches for the track. They'll know best baseline alignment settings for how you use your car... then you can make gradual alignment adjustments if/as needed from there.

PushingTin007

Apex wheels website has a really good page dedicated to the different size wheels and what will and will not fit the 987. I just used it and found it very helpful
using their advice I just ordered 255-35-18 and 295-30-18 along with their vs5rs wheels