Ak3249

Hello All,

Just ordered a set of 993RS speedline wheels for my Carrera Cab. Also ordered a set of Michelin Pilot Sport 4S's, would anyone know what tire pressure I should be putting in front and rear?

Tlaloc75

I would do 31/33

jadatis

If specifications of tires are the same as original tires, the pressure advice of car still counts.
Those are:
1 maximum load or loadindex.
2 kind of tire ( standard load or XL/REINFORCED/ Extraload).
3 . Speedcode ( less important to be exactly the same)
If not the same , give old and new tires 1,2,and 3, together with weights on axles ( most GAWR's are used to calculate the advices for, but real weight in your use are better), and I will calculate advice pressure for you.
Then also give max speed you use, AND WONT GO OVER FOR EVEN A MINUTE.

IainM

Don't 18" wheels need more pressure than 16"?
ive heard so much contradictory info on the correct pressure for 18"s
im running 36/39lbs just because it feels kinda ok steering & cornering with my 18s but have 0 scientific support

Tlaloc75

I’ve experimented with a lot of tire pressures, doing back to back runs on my favorite roads. My go to is now 31/33 on Michelins and 29/30 on re71r. Both feel just about perfect at those pressures and don’t get too greasy as they heat up.

IainM

I'm also running RE71-Rs. Are you on 18"s?

Endoman

Manual states 36/36 psi (2.5 bar) for 17" 36/44 psi (2.5/3.0 bar) for 16" and 18" set cold. 16 " wheels may not fit on car fitted with "Big Reds" (RS and turbo)
These may be regarded as safe figures.
A pedantic traffic officer could give you a ticket for running 33 psi rear on 18s in UK and the tread profile will not be ideal. You may invalidate your insurance if it is considered a contributary factor in an accident.. For comfort 2-4 psi lower than manual is often used.

Tlaloc75

Yes, correct. On track people start much lower cold, but I’ve found these pressures work well for spirited street driving.

btw - I’m quoting cold pressures only, they are a bit higher once hot, of course.

Tlaloc75

I can’t speak to what would happen in UK, but where I drive 31/33 (or 29/30 on re71r) results in optimal handling and reasonable tread wear. At 36/44, handling is quite a bit worse, especially once hot, and ride quality suffers.

I know that’s what’s on the sticker, but you owe it yourself, and your 993, to try lower psi and see how it feels. I think you may be pleasantly surprised.

IainM

jadatis:
Can you calculate tire pressures for me please?
1: max load FR = 1587 lbs, RR = 2348 lbs, actual FR = 1218 lbs, RR = 1968 lbs
2: Bridgestone RE71-R, 200AA, 86V,
3: Car: 168 mph, tire 149 mph

Tlaloc75

This is the formula:
[Original Tires Max Load Rating] * [Sticker's
Recommended P.S.I.] / [Original Tires Max P.S.I.] *
[New Tires Max P.S.I.] / [New Tires Max Load Rating]

Only works if you agree with original sticker recommendation though .

Ak3249

Appreciate the help and replies. I'm currently running the 16 inch standard wheels (car has low miles so believe it or not still on orig tires from 95). Instead of replacing the now unsafe to use tires, I decided better off keeping them on the orig wheels and getting another set to be able to use the car a little.

I don't believe my car being a standard Carrera shows the 18inch option in the door, only a 16 or 17 inch. Appreciate it, gonna run 34 upfront, 42 at the rear helps both comfort and prob better for the seals in the 2 piece wheels.

IainM

Seeing as OP seems to be on a path to success, allow me to hijack this interesting dialog for a post or two.

As AK notes, my '95 Owners Manual only provides data for 16" and 17" wheels:

FR: 205/55/ZR16 = 36 lbs
RR: 245/45/ZR16 = 44 lbs

FR: 205/50/ZR17 = 36 lbs
RR: 255/40/ZR17 = 36 lbs

Anomaly #1, Why are the 17" pressures square while the 16"s are asymmetric?

Then Endoman presents compelling evidence (which I have previously seen and embraced) showing:

FR: 225/40/ZR18 = 36 lbs (BTW, these are the size RE71-R's I'm running
RR: 265/35/ZR18 = 44 lbs

Anomaly #2. Why do 18" pressures go back to asymmetric?

Thankyou Tlaloc for the equation. I read that as:

You increase new tire pressure from the recommended (sticker) tire pressure when either or both the following are true:
a) the new tire maximum load rating is less than the recommended tire max load rating
b) the new tire maximum PSI rating is more than the recommended tire maximum PSI.

Anomaly #3. How do the manufacturers (liability driven) maximum ratings have anything to do with the required mechanical design parameters for suspension design such as compliance, volume etc. (Im an EE using electrical analogs so please correct my errors)?

The two methods for judging ideal tire pressure I've read are:
chalking the sidewall-tread corner at several points around circumference and seeing where chalk is left after driving
using tire pyrometer to measure temperature of tire tread across width to ensure even friction/pressure/wear

Anomaly #4. How do these measures provide insight into the tread grip footprint vs lateral load?

Has anyone seen some physics for this? I run out at Boyles Law and haven't thought long enough about how different size/aspect ratio air volumes might perform differently with sidewall construction vs tread. I suspect this is a complex science/art (and ultimately, is the benefit worth it)


I'm finding the same as Tlaloc, I'm feeling and performing better with lower tire pressures on the 18" wheels vs published info. You've encouraged me to try lower.

I also measure cold. I see 4 lb increases with hot tires and this was what drove me to try lower cold pressures so that at temperature, my pressures would be 36/44 (rears were heated most for me).

jadatis

For Gawr's up to 149mph,Fr 38 psi , rear 60 psi.
For GAWR's up to 99mph, Fr 29 psi, rear 47 psi.

For your given axle weights.
Up to 149mph,Fr 29 psi , rear 50 psi.
Up to 99mph, Fr 23 psi, rear 39 psi.

Assumed tires to be rear and front the same and XL/ Extraload/reinforced wich carry their maxload upto 99mph AT 42 psi, so give that info if mayby standard load AT 36 psi, then advice lower.

The weightdivision tells me motor in back, oh Porche forum I think.

Advice must be recalculated if standard load, or camber angle ( most rear done) above 2 degr( alignment, wheels like this on axle /-\).

Here max technical carspeed higher then maxspeed of tires, was not allowed in the Netherlands , where I live, only for winter-tires . Law has chanched, now you may but need to stick a sticker on dashboard with that lower maxspeed of tires.

So let me know if I used the right data.
And how did you determine the axle- weights you gave?

IainM

Thanks jadatis for the detailed reply. My axle weights come from my most recent alignment when the car was corner balanced with 1/4 tank of gas and 180lbs in drivers seat
so should be pretty accurate.
I have to ask: are these the minimum tire pressures for safe operation? Is the higher tire pressure at higher speeds required to prevent deformation due to
centripetal force reducing the road contact patch?

I can't see how you can predict the optimal performance of the car in various cornering conditions based on the tire pressure, maximum axle weight and
maximum wheel speed. I would have expected the volume of air enclosed by the tire, the aspect ratio of the cross-section, materials & construction as
well as weight, pressure & temp to be major factors. But if this pressure advice is purely a safety consideration then it makes a bit more sense.

So is that how it works? You have to have a minimum tire pressure for the maximum speed you will achieve, beyond that its whatever feels good and makes
you fast?

It would then appear that for typical "street" speeds, we can operate at much less that the advised 36/44 lbs. If only because I'm in the USA and the
highway patrol won't check my pressures or look for a maximum speed sticker on the dash (good grief I'm a naughty boy)