Recommended tire pressures 18 inch rims?

Other then what is recommended on the owners manual of 44 rear and 36 front(from memory) what is the norm here for tire pressures on 18 inch turbo rims. I read that maybe its best at lower tire pressures for better traction. And how low should it be from what is stated on the manual. Thank you in advance for ne comments. Take care. Mike '99 996 coupe red/black

Just about everything I've seen says lowering pressure LOWERS grip. IOW, to reduce understeer, you lower fronts and/or raise rears. As for pressure, you'll get as many different answers around here as people you ask. One thing is for sure though, whatever pressure you use, you'll want to decrease the difference between front and rears (ie: more like 4 to 6 lbs vs. the factory recommended 8).

Mike: There was an extensive thread on that question on this board in April or May - though I'm not sure whether it was lost from the archives in the system crash last month. In any event, I keep my tires (18" rims) inflated at 36 front and 40 rear (rather than the factory recommended 44). The 44 is actually the maximum recommended cold pressure rating for the tire (a little scarey since the pressure will build significantly as the tire gets hot when driven). In the prior thread, others seem to keep the fronts at lower pressures (around 32).

Ben, you said that backwards. If a car is understeering it's because it has lower grip at the front relative to the rear so that the tendancy of the rear tires to "push" the car in a straight line overcomes the ability of the front tires to turn in. Based on your correct assertion that lowering pressure lowers grip and raising it increases grip (to a point), to reduce understeer you would raise the front tire pressure and/or lower the rear pressure. But then that was probably just a typo. FWIW, I'm running 32 front and 34 rear on my new 225 and 285 S-03's on 8X18 and 10X18 Kinesis wheels. I ran my old Conti's at 32-36 but the rear tires wore more heavily in the center, so I'm trying the lower rear tire pressure. With 225 front and 285 rear tires and 1 degree negative front camber, the car feels well balanced and predictable. Mike

Hi Mike in CA. I have 225 front and 265 rear so would the same tire pressure's u are using would be the same with me. Alos with respect to camber what is that and how to u adjust for that camber? I have P-zero rosso's if that matteres. thanks

"Ben, you said that backwards. If a car is understeering it's because it has lower grip at the front relative to the rear so that the tendancy of the rear tires to "push" the car in a straight line overcomes the ability of the front tires to turn in." Yep, I said it backwards, but I meant it correctly :-) (see the "lowering pressure LOWERS grip" comment). Sorry for the slip up.

The is a really good article on automobile suspension design at the following location: http://www.rqriley.com/suspensn.html In that article, part of the description of camber reads as follows... ""Camber is the lateral inclination of the wheel. If the wheel leans out at the top, away from the vehicle, it has a positive camber angle. With a negative camber angle, the wheel leans inward at the top. Camber-changes occur when the body leans during a turn and when the wheels move vertically through jounce and rebound. The purpose of the negative camber angle is to maximize cornering forces by keeping the outside tire upright or at a slightly negative camber angle as the body leans to the outside of the turn. The second purpose of negative camber is to minimize lateral movement, or tire scrubbing, at the contact patch."" You need to have a competent alignment shop adjust your camber for you. Be aware that while the additional negative camber will improve grip, it will also make the inside edge of your tires wear faster. There is no free lunch! 32-36 psi front and rear worked fine with my 225 and 265 Continentals except for a little wear from overinflation at the rear. That's why I'm going to use 32-34 for my new tires. The main reason for the high tire pressures recommended by Porsche is for sustained driving at rated speeds like those on the Autobahn. We don't have that issue here in the US Mike

We also had an extensive thread on the effects of tire pressures on handling on the TrackQuest/Rennlist board (a great board if you track your car). The conclusion was that there is a pressure setting for each tire which will deliver maximum grip. This pressure setting depends upon many things that are too numerous to list (road temps, air temps, suspension set up, tire construction, tire wear, etc.) and can only be determined by repeated pyrometer checks. Any deviation from the setting will reduce grip. So raising or lowering pressures could either increase or reduce grip based upon where it currently is relative to that ideal setting. Since all 996s are factory designed to understeer for safety, you'll want to find a pressure that makes the rear tires have less than optimal grip, while finding the optimal grip setting for the fronts. With all of the above in mind and not knowing whether to increase or decrease pressures for better turn-ins and getting the car to rotate easier and push less, most 996ers running street rubber at the track use around 34-to-36 hot in front and 36-to-38 hot in the rear. Camber is a much better method of taking out some of the factory understeer in 996s. But it can not be done with the factory shocks and springs (I pretty sure Ben in LJ says it can be done with the M030 kit). I had to put H&R Springs (1.25" lower and stiffer) to allow negative camber to be dialed in. Other higher end coil-over kits also allow camber to be dialed in. My mechanic set my car up with -1 degree in front and -2 in rear and corner-balanced my car. Much firmer turn-ins and better rotation earlier in the turn. But it will still push a bit if I put in too much throttle too early in the turn. (nothing can overcome poor driving skills ) I might increase the front to -2 to help get even better rotation at the track, but it comes at the expense of even more uneven tire wear driving straight on freeways (It's always a trade off between grip and wear). Hope this helps more than it confuses. Bret

Good input to the discussion, Bert. I should have mentioned that my 32-34 psi street settings were cold, which all else being equal (and as you correctly point out they never are) correlates pretty well with your hot track recommendations. I have the ROW M030 suspension on my '99 C2 and it is possible to get 1 degree negative camber in front with that suspension. Mike

" But it will still push a bit if I put in too much throttle too early in the turn. (nothing can overcome poor driving skills ) I might increase the front to -2 to help get even better rotation at the track, but it comes at the expense of even more uneven tire wear driving straight on freeways (It's always a trade off between grip and wear)." If you're not already on the bubble point wise, adjustable sway bars will take care of this.

Just my 2 cents worth, I have SO3 235 fronts and 285 rears on 18" alloys. My tire pressures are 30psi fr & 32psi rears. Grip is phenomenal! If I adhered to Porsche recommendations, I would be running higher PSI than recommended on tire cold air PSI. The most important thing to keep in mind is that as the tire spins, it generates heat inside the tire that subsequently raises the PSI, roughly between 15 to 20% of the starting cold air PSI. That is a huge increase and certainly takes the tire close to the MAX manufacturer recommended PSI. I recently stopped and checked the PSI in my SO3 after cruising on the motorway constantly at 70mph for 30mins. This was early morning and so the outside temperature was about 3 degrees. The results were as follows; Front normal cold air PSI = 30psi After driving on motorway for 30mins at 70mph constant = 36psi Rears normal cold air PSI = 32psi After driving on motorway for 30mins at 70mph constant = 40psi (higher PSI may be attributed by extra heat generated from engine compartment or fact that car is rear wheel drive) MAXIMUM manufacture recommended PSI front or rear is 40psi. One thing I have noticed with my SO3 compared to previous tires are that the tire wear front and rear are even � especially rear where I normally get the middle portion wearing out quicker (perhaps signs of over inflation perhaps)

After puting in adjustable sway bars and setting the front to max grip (softest setting - hole closet to the end) and back to next to the last (one back of softest - still have two more settings to firm the rear up) together with 1 deg negative camber in the front, I was still getting a slight push in very slow corners (ie: decreasing radius) which would eventually turn into oversteer with the use of throttle all the way through the corner. Today I lowered the pressure from the 34/42 used in the above example to 34/38, and the push is so slight before rear rotation occurs that it's almost not mentionable. So, I don't think anyone's gonna eliminate push with tire pressure given it took me: new suspension (coilovers), GT3 adj sways (set to reduce understeer), and 1 deg negative camber. Even with this setup, the diff between 38 rear and 42 rear is somewhat neglible (although it was just enough to make it perfect). PS. The car beats me to death with the factory 36/44 and is a pretty nice ride with 34/38.

The general guidance about lowering pressure to raise grip, or vice versa, can not be used without understanding where you are starting. A tire has maximum grip, when hot, at a certain pressure. Any change, up or down, will reduce that grip. If you start the rears at 44 cold, then dropping pressure will increase grip. If you start from 28 cold, then the opposite is true. I prefer to find the right pressure for max grip and then RAISE it to reduce grip if needed. That way, you are also stiffining the spring rate (tire stiffness is part of the overall spring rate) and are working the two factors in the same direction. If you drop pressure in the rear from optimum, you are adding oversteer with reduced grip, but you are adding understeer with the spring change. That makes it very difficult to sort the car out. One caution is that traction falls off more quickly with overinflation vs. underinflation.

After reading this topic I went and tried different pressure settings. I had pressure at 36/44 per manual recommendations (sitting cold in 40 degree garage) Drove it since purchase that way for 2K miles. Handling has been phenomenal but ride is bone shattering on anything but glass road. This weekend I reduced to 32/36 and found the road handling felt slippery at speed (>100mph on slight bend in road) and I felt a decline in sharp cornering as well. I moved back up to 35/39 and that seems to be the ticket for my cab. Great balance between ride quality and handling. As usual good info from this page has improved my experience with my car. Thanks to all for sharing....

I agree that the factory tire pressure settings make MY2002 coupe ride choppy. Will lowering the pressures increase risk of bent rims? I have to ride on rougher roads here in the NE with lots of railroad crossings to contend with also. How low can I go without significantly increasing chances of bent rims while improving ride/handling. I have the standard suspension with 18" rims and the Continentals. Thanks

Hi Guys, I agree with just about everything posted so far. And I feel the pressure for optimal grip will vary with the tire itself and its construction. The sidewall construction alone can in many cases provide a little extra negative camber if the inside wall is harder than the outside (the old R1's come to mind.) Also the best way is to measure the temps (also noted above). However, I will add to avoid all of this cold vs. hot problems I recommend you get your tires filled with nitrogen. It has ALMOST zero pressure growth due to heat. I'm not a chemist (if we have one here maybe they will chime in an correct anything I'm about to say wrong :-) ) It has something to do with the fact that nitrogen is a simple small molecule vs. "air" which has all kinds of stuff in it. The good thing is that it is not so small that if will leak out through the rubber or hurt the rubber of the tire. Also not flammable. You would have also considered Hydrogen or Helium if not for the fact that it would add a whole new meaning to the word "blowout" and that helium might interact with the rubber. Also believe both would be prone to leak out due to being too small and would go right through the rubber. I have a good friend of mine who preps about 12 very competive races cars (had a couple of national champs last season). He tells me that since the went with nitrogen they are seeing no changes in preasures (even at Daytona in the summer!) I know not everyone here just happens to have a tank of this stuff at home :-) However, I know some people here are also running adj shocks like the JRZ's. It is recommended that you put nitrogen in those too. So I figure, the cost of having one of those tanks with a regulator is not too bad and it will last me a few years. So find yourself a good race shop near you, you'll find they have the stuff and ask them what they will change you to fill you up. Should not be too much..the stuff is real cheap!!! Then never worry about hot vs. cold again and just get the balance for your car. (also remember it will vary by track too!!!!!!) The perfect setup might require 4 unique tire preasures.

Gas behavior is defined by a particular equation (Boyle's law, as I recall) which tells us that the pressure vs. temperature behavior of air vs. nitrogen is nearly identical (largely because air is mostly nitrogen). You will see the same pressure increases with nitrogen as air. There are two benefits of nitrogen - 1) It is dry while often the air from a compressor has quite a bit of water. The water gives you inconsistent behavior with pressure vs. temp. 2) It is much easier to work from a cylinder of gas, at the track, than deal with a compressor for most people. I forget there is a #3 - You can impress the H#ll out of everyone by having air tools running in your paddock area. Hope this helps.

you guys are nuts. nitrogen and stuff. what are we the Porsche GT3 team. I cant even drive over 85mph without being afraid of the troopers in nj. But, hey what the heck we can all dream a little. Drive safely!

Nitrogen or not - What about tire pressure/bent rims?

Thanks Mark D. for the corrections above...and the added comments...that is why I always leave the door open to be corrected. (the extra stuff I mentioned was the water you noted!) All I was trying to say was than nitrogen was better and provided more predictable pressures.

Guys, sorry to dredge up the tire pressure topic, but I think its an important one, and one that'd I'd like further advice on. In looking at all these postings on this thread, it seems that many people recommend two things (1) going lower than the factory recommended settings of 36/44, and (2) reducing the 8 psi spread between the front and the back. My question is as follows: I started with the standard 36/44 setting which felt OK, but then switched to a 32/40 setting based on comments received. At this setting, I felt that car was too soft! The sound going over Chicago roads was a low thud-like noise, as opposed to a clean, tight sound (sorry, a little difficult to describe). Nonetheless, I prefer a hard ride to a soft, and so have changed the settings to a 37/45 psi level. The question then is: does inflating slightly higher than the recommended 36/44 pose any dangers with respect to overheating, grip, etc. My impressions of driving with this tire level have actually been very good. I also checked with my authorized Chicago Porsche dealer and they said that for highway/city driving this level wouldn't be problem at all. Caveat: keep in mind that I don't track the car at all, which is what some of the earlier tire pressure recommendations were for. Most of my driving is done on highways, at average speeds of 85 - 100mph. However, when I am on the highways, I keep on them for extended periods of time. Your comments are greatly appreciated. Regards.

You do want to avoid exceeding the maximum cold pressure inflation point of the tire. Note, the rear 18" contis that came with my '99 c4 had a maximum cold pressure of 44 (the S03s I replaced them with have a higher maximum of 51, I believe). I tend to inflate my tires to the factory recommended 36/44 (though I experimented with reducing the pressure on the rears to the 40 range - I didn't feel that big a difference so reverted to factory recommended settings)- most on this board inflate at lower pressures. I've not heard of anyone posting an intent to exceed the 36/44 recommended setting. Generally, the higher the pressure, the firmer, but harsher, the ride.

Thanks B -- I forgot to mention I've got Conti's. I am going to keep it 1 psi higher on front/back over the recommended 36/44, because I prefer the firmer ride, and will see what happens. Hope I'm not doing anything too crazy. Will report back in a bit. Regards.