Puzzled about tire pressures & ride harshness

When I bought my 996 a year ago, I set the tires up per the manual for 18" wheels: 36 psi front; 44 psi rear. The car came with 235/40ZR18, 285/30ZR18 Toyo Proxes T1R, siped (not my 1st choice, but supposedly still a good tire). Right away, I noticed that the ride was way too harsh @ 36/44 and have since dialed the fronts down to ~30 and the rears down to ~36. I just got back from a very fun, but quite frankly bone-jarring & noisy 400 mile roundtrip in the car with the tires @ 30/36. The tires had a huge variation in ride comfort and noise level on the trip and, at one point when I pushed the car past the century mark, the front tires exhibited some concerning "hopping" (other runs to similar speeds were fine, so I think this one time was the tires reacting to the road surface). My questions: * Is this level of harshness and extreme road noise normal? * What do you guys run your tires at? * Do I need to go down even lower in pressure? * Does anyone have any thoughts about the Proxes T1R? I'm a Pilot Sport PS2 guy myself (have them on my 330i & swear by them), but I'm curious if I'd see similar extreme harshness, noise and surface reaction from from them as well? I'm used to a sport-biased ride (my 330i has the Sport Package), but this seems out of whack...

How old are the tires? They should have a manufacturer date on them a four digit number stamped on the side wall to show the date XXYY where XX is the week number and YY is the year number ie 2207 is the 22nd week of 2007. As tires are heat cycled they get harder, lose grip and make more noise; this is life. The TR1s which came on my car sucked and I replaced them with PS2s. The PS2 will be much more quite, smoother and have gobs more grip than the TR1s. It sounds like your wheel may be out of balance if it is, 'hopping'. I run 37 front 42 rear psi HOT. I would suggest you run the same or factory.

I can't find a date code on the tires (at least in XXYY format), but I know they were purchased in December of '06 from Les Schwab because I have the receipt from the previous owner. Assuming they weren't sitting on the shelf for a few years, they're not that old. I wonderered if the wheels were out of balance, too. I think I need to get them looked at. Interesting points comparing the T1R to the PS2. That's what I'm looking for. The grip on the T1Rs seems OK right now (although I haven't tracked the car to really see), but I am really disappointed with the ride, even with pressures lowered.

It is dangerous to underinflate so much below the recommended pressure: 1. Weakening of side walls 2. Excessive heat 3. Deformation If you hit a pot hole with underinflated tires, you could also bend your rims and rupture the tires. If you want a softer ride, consider different shocks and springs... or get a mini-van? PS: if your Highway 400 ride was at around 1:40 AM or so, I was following you between langstaff and 401 (you went East, and I went West) PS2; your plate last 3 numbers are 928?

How to tell the age of a tire http://www.tirerack.com/tires/tirete....jsp?techid=11

Get new tires! When I first got my car, it had worn Conti summer tires, and the ride was harsh, and the car lurched around at every bump in the road. I put on nice new Potenza Pole Positions, and the car rides beautifully now! It IS a 911, so you will hear road noise from the huge tire footprint, but the ride shouldn't be harsh like that.

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Underinflating like that is just inviting tire and wheel damage, and is probably hurting mileage as well. One thing you're probably not seeing is rollover; when you corner with your car, the lateral forces want to take the tire and roll it under your wheel. Normally, the pressure and sidewalls resist this force, but when the tire is underinflated, the sidewall will roll under the car and will roll on the sidewall. (In addition to all the not-so-great-things this does to your wheel and tire, you also lose grip and can have an accident.) When racing on street tires, people will even add additional pressure! You have a sports car. Sure, it's got a lot of luxury attributes as well, but it's still a sports car. This is one of them. Properly inflate your tires and learn to love the response Bear in mind this may only be written on one side of the tire, so you may have to jack it up to get around to the backside to read it on all 4 wheels.

Try keeping the rears at the recommended tire pressure. Then dial the fronts down to about 2 less than recommended. I found this to work well. Remember that we have a rear engine vehicle, so it is hard to compare to BMWs. I had the same problem when I first got my 996.

Just curious...is this your first Porsche?

Did you post pitcures ??

I have those Toyo tires on my BMW 530i and they are very loud. When I bought my C2 in June last year I installed 2 new Pilot Sport Ribs on the front and was amazed at the drop in road noise.

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Thanks for all the comments, guys. Well, at least most of them I'm gonna bring 'em back to Les Schwab and have them check the balance, first. I'll try to find out, but I suspect they're not that old. Great feedback. I am wondering about the comparison between the Toyos and PS2s. I maybe have been spoiled with my PS2 experiences?? I knew I was gonna get some flack on these comments! I love the sports car ride and feel, don't get me wrong, but I think something's not quite right. No, that wasn't me. I'm in Washington state. I have a while ago, but I repost. Always like to show it off.

As requested, here's some pics: rhs_2.jpg PassengerSide_2.jpg lhs.jpg Attachment 351558 RearThreeQuarter.jpg It's a 2002 base 996 with the following equipment / options: Seal Grey Metallic Black Full Leather Advanced Technic Package Power Seat Package 18" Lt Alloy Carrera Wheel Wheel Caps with Colored Crest Porsche Stability Management Supple Leather front and rear 3 Spoke Str Whl - Carbon/Leather Porsche Crest in Headrest Flared Rocker Panels Brake / Shifter Carbon Aluminum Rear wing (don't have the official name for this one)

Wow, nice! FYI; I run a little lower in pressure than Porsche recommends. From talking to the dealer and even PCA, it is not that uncommon. For me I am running 32F and 40R.

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Thanks! I think she's gorgeous, but I'm obviously biased! The car was basically brand new when I bought it, too, with only 15K miles. I have 19K on it right now. Thanks for the pressure info. i need to find a happy medium. Think I'll try close to what you're running.

BimmFiniti, when I first switched from my 540i, I had the same issue too. The recommended psi for the Pcar was too harsh for me. I was riding on 33/41 for a while. Now, having gotten used to the ride, I'm riding on 35/43. And I'm sure different tires will have different ride qualities as well.

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I found the date stamp on the outer sidewall of both right tires (mid-year 2006 for RH front & rear); no date stamp whatsoever on either of the left tires. Is this legit? I read the tirerack info. posted by C4CRNA and that seemed to say that all tires should have some sort of date code visible on both side-walls.

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The datestamp is probably on the right side of the left tires, which makes it difficult to read 99% chance it's mid-06, too. My tires (Dunlops) are only stamped on one sidewall, btw. I can't say it's something I've really been looking for on other tires, so I don't have much basis for comparison for yours. Regarding tire pressures, the narrow front tire, by rational physics, should have a higher pressure than the rear tires so that the ground pressure is consistant. Porsche specs out pressures designed to provide nice safe understeer. Rapid wear in rear tires seems to indicate stock rear tire pressures may be a bit higher than necessary. If you raise pressure in the front and reduce it in the back, you may gain performance, but it may also increase tendency to oversteer. You have PSM, so I wouldn't worry about it too much. Not to mention oversteer is WAY more fun than oversteer!

Tire brand makes a significant difference in the ride noise, handling, wear, etc. I picked up a second set of rims recently with Continentals on them and they are by far some of the worst tires I have ever experienced (besides Dunlops back in the 70s). Some of the BEST tires I have ever used are Bridgestones. Now that you have a Carrera, spend a few bucks and get good rubber on it. It will completely change the way the car behaves.

Mount a set of 17s if you want a "comfort" ride.

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I keep mine at spec for 18" 36/44. I don't think it is wise to deviate from spec I run conti's for summer and pirelli's for winter. I have 18" on my 2004 996 C2 and the dealer consistently uses pressure spec for 17" - low pressure. The Conti's do ride softer on low pressure but they are definitely squirrely at speed. The pirelli's are actually much noisier at low pressure. I have the M030 suspension and I am still impressed with how well the car rides, especially cruising at highway speed+ for long distances. Then again, I really wasn't looking for soft and quiet.

I'm probably going to get yelled at but I run my PS2's cold at 36 lbs (all around) as this offers a nice neutral balance to the car. With rear pressures at 40 plus the car pushes tremendously (on Autocross courses) and I suspect that the factory intentionally sets spec at this to provide a setup more conducive to understeer. I've found that at this pressure, my wear pattern is right up to the edge of the tread (where the arrow indicator points to the edge of the side block). Even at extreme corning load, I'm not getting any rollover. I've had no ill effect with this pressure and the ride and NVH is none existent. Of course, keep in mind that with this set up, you get quite a bit of TTOS so you have to be used to this. I run these pressures on the road and they have been fine as well. I suspect that track pressures might be different as you may want a bit more air in them so that you get a little more push to be on the safe side and corning load is higher, at the track, so you will want to avoid rollover with higher pressures.

I run 39 and 32. Like it....

Your P-car is beautiful - very nice example. I believe your wing is simply the OEM wing from Porsche. Best of luck sorting our your tire issues - post your results when finished.

Are you guys all talking PSI hot? or is there a mix of hot and cold? Numbers seem high for cold pressures, but that's not based on any facts, just initial opinion

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What kind of alignment does your car have? What suspension? I have a "little" negative camber in my tires, which causes a bit of road noise after the tires are broken in. I also use cheap tires as the camber tends to eat them pretty fast. This also causes a little noise in the cabin. Also, keep in mind, 911s aren't luxo-barges with a soft ride and 500 pounds of sound deadening. They are actually on the loud and uncomfortable side when compared to most family cars. [not bone jarring or deafening, but definitely not a couch on wheels]. At heart, they are more sports car than luxury car. -td

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No one ever refers to tire pressures "hot." It would be impossible as temperature affects pressure substantially. Pressures are based upon a 'standard day.'

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That's what I thought, but then I saw this post: Thought maybe the 996 did things differently

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I suspect the same thing. Why else would Porsche spec out a smaller tire with a lower pressure on the front, if not to decrease grip vs the rear? Tire wear is a big indication of this, too- everyone wears out the rear tires in classic "overinflation" style, with fronts lasting twice as long. I can see why they do this (993 lessons learned), but with PSM, they really shouldn't have to anymore.

I have 18's and love the ride - very firm and comforting at fast speeds and fill them to spec as per the manual. If I wanted a softer ride I'd buy a Caddie. Oh, wait...

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Thanks! Appreciate it. I really love the combination of performance, looks and driveability (although you might not think so given this thread!). You're right that it is the OEM wing. I just don't have the offical name for it: everything else was on the original window sticker; the wing was added afterward. I'm not quite sure what I'll be doing yet with my setup. Maybe something's wrong, maybe I'm just expecting more of a BMW ride, maybe I need different tires? Stay tuned...

OK, now you've done gone and confused a rookie! If 36 front and 44 rear sets up a lot of understeer, that should mean there is a lot less grip in front and a lot more grip in the rear. Doesn't it then make more sense to correct this by giving the front more grip (i.e. raise the front pressure towards 44) rather than taking grip away from the rear (i.e. lowering the rear pressure towards 36)?

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Wait...doesn't more air pressure create less rubber grip, and less air = more grip???

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What suspension are you running? I have stock suspension and 18's with PS2's and the ride is way better than any BMW I have had or driven (and I've had a chance to drive many ranging form E30-to E92 and everything in between). My 996 is easily as comfortable as my 98 323is with normal (non sport suspension). BMW's especially the sport suspension package, are always over damped on compression which gives them a harsh initial hit even though they have relatively low spring rates. How old is your suspension? Your dampers could be shot which is giving you the bad ride. Mines a bit on the old side and I still find the car controlled and comfortable. It' has a bit too much body roll for my tastes but considering it's stock it's not bad. I suspect something else is afoot here.

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Generally speaking, the lower the air pressure and bigger the contact patch, the better. (Tire rubber has a dynamic coefficient of friction that decreases with pressure; if you push on it twice as hard, it grips more, but not twice as much.) Other factors come into play, though, such as sidewall stiffness and tire deformation, so simply bleeding air doesn't work out all that well. Optimum tire pressure is on a curve with a sweet spot. Too little air, and tires lose grip. Too much air, and tires lose grip. (Generally, too much air is less detrimental than too little.) This changes when you compare cornering grip to accelerating/braking grip. And from one model tire to another. Logic dictates Porsche optimized the front tire pressure for braking, and rear for cornering, to ensure nice safe understeer. Which is nice and safe for street use, but not so good for performance handling.

Just a quick update. I took my car into Les Schwab today and they did find the left front wheel was imbalanced. I'm running my tires @ 33/42 now, close to the 36/44 that's recommended. * I think I might be able to go to 36/44 with PS2s, but not with the T1Rs. * Around town - which has always been fine - is still OK. * On the highway... well, we'll see. I'll update next time I take a significant highway drive to see if the wheel balancing has improved the situation.