Tire pressure observation

I DO NOT want to start another 5 page debate on tire pressure, just want to state something I have noticed. Got my car a week ago and the tires were set to the high- 39/44. I did a search and found some of the guys down at 33/39. I tried this and liked it better, but, I have noticed that the tire tread is not totally making contact with the ground still. More so the fronts than backs. I went to 32/38. The backs contact patch seem better but not where I would like it, and they still feel pretty hard! Anyway, the thing I am asking is, does anyone look at the footprint of their tires, or the contact patch? My fronts, even at 32, are not having the full width of the tread in contact with the ground. I can see this contact point when I pull into my garage, the floor is dusty, and my tires are amor all'd, and I can see how much of the tread is touching the ground, and how much is not. I realize the tire is hot at this point and tire pressure will obviously be higher then when cold. Am I way off looking at my tire inflation in this manner? As I said, I dont want to start anything, but I am gonna experiment some more till I see the type of contact I am satisfied with, and see how the ride and handling is, without gettin too low on the pressure. I dont track the car, but DO get moving when the oppertunity arises

Then you have to know how the car-manufacturer determines the advice pressure for cars. I have been busy with that since 2007 and learned mijself Excell to make spreadsheets for it. I use the same formula that the European car- and tyre- manufacturers do. got hold of a copy and worked it out. Translated a few from Dutch to English . http://cid-a526e0eee092e6dc.skydrive...0tyre-pressure in this map the one for re-calculating when other then original tyres. But can also be used to se how low you can go savely with the original tires. The high values you give are probably those for maximum load upto maximum speed of the car. For normal use with front-wheele drive the front stays the same and the back can be mostly even lower then the front. And this is all only for radial tyres. But diagonal tyres are seldomly used nowadays. If you have questions ask them here, and I am prepared to make an example, if you give me the needed data, asked on the first sheet when opening the spreadsheet.

I'd like to get the answer to this one too... I keep mine at 36/44 based on what I was reading here... Mine is a DD with an occasional spirited romp (actually every morning at 5 am), so if there's a better idea from the real techies here I'm interested in hearing it. Rudy

Some interesting technical research and conclusions a couple of previous posts up. Maybe this is the exact scientific method to determine pressures. But there is more to it than that. I'll admit that the calculated or factory recommended pressures are a good starting point, and if they work for you, fine. Don't mess with success. I learned in nearly a decade of motorcycle roadracing that correct tire pressures can be more important than anything else. Pressure affects rolling resistance, turning ability or resistance, traction, braking, compliance of suspension, top speed, and combinations of all the above. If the pressures are wrong, the machine won't work as it should. For racing motorcycles, tire pressure was set cold at a rate determined from previous experience. The information used included ambient temperature, road surface aggregate, track temperature (sunny or cloudy) track configuration (short linked turns vs long sweepers and straights) horsepower and weight of machine, rider experience. During a practice session a rider would come into the pits and pressures checked. A 10% rise from cold to hot was an indication the starting pressure was correct. Rider feedback was exchanged and any adjustments made. On a street car many variables exist and I would recommend the factory pressure for starters. Adjust from there for comfort of ride and handling. If my tires are the recommended pressures, everything seems to work well. As the pressures get lower, tramlining on the freeway occurs and steering becomes a bit heavier but traction in the canyon twisties is a bit improved. Everything becomes a tradeoff. Learn your car and you will find the correct pressure.

One thing someone forgot to mention. What about Nitrogen in the tires? I used this in my old car on a 17' rim which seemed to work perfectly. The nitrogen in the tire lasted longer than air and its slightly lighter than air also. I would think more for a track event Nitrogen is the way to go. Race cars, and mainly airplanes have been using for years.

I've kept mine at 36/44 also. One thing jumped out at me in your post - your tires are Armor All'ed all around? That has to be bad for your tread/grip - did you mean just the sidewalls?

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Just the sidewalls! But when I put the stuff on, the applicator goes onto the edges of the tread. Naturally it wears away in minutes of driving, but due to my lack of sweeping my garage floor, its dusty and makes it very easy to see what I stated, when I pull back in after a drive.

Tires run different on the road or track with sidewall pressure and what not. For a faster warming up of the tire run lower pressure which causes more flexing which heats the tire. What you have to do to find the ideal presure is adapt it to your driving style. Kind of tough when mainly driving on the street. Contact patch changes during acceleration or braking in turns and thats where the best view of a contact patch would be. So you just gotta test and test on the road till you are happy. Better gas mileages are at higher tire pressures.

do remember your camber, if its around -2 deg.(and a lot are) then nearly one third of your tires not touching the ground more obvious on rear as its a longer (wider) tire.

Bigrob don't take what your tire looks like driving slow on the garage as what the stance and contact patch is at speed. The suspension changes with down forces and applied power and braking. That's when contact patch is important. When turning is an important period which goes way far away from whats going on rolling slow.

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I agree! That makes perfect sence as does the camber adj. I just wanna experiment a little and see what I find. Something that makes me curious as well is the fact that my C5 Vett, a car of similiar weight as my Carrera, called for 30 lbs all around with similiarly sized tires and wheels, granted my Vett had a front mounted motor and rear mounted tranny for better weight distrubution, but this is a huge difference in pressures.

Maybe the engine being out past the rear axles on a porsche figures as more weight means more air pressure. Then with more weight you get more traction and forces down on the tires cornering. The added forces of corning is possibly another reason fo added tire pressures. I kind of think going with taller rim sizes adds another reason for going with higher pressures. Not for just protecting the rims from damage with bumps but to keep a more uniform shape. Hard thing is tire manufatures are getting pretty fancy in belting and side wall constructions now of days so it can go on and on. Porsche does have a rating for tires on being good to use with their rear engined cars. All in all the results wanted are what would need to be tested from actual conditions you want to have results in. I need a get out of jail free card for me and the car out of the police yard for that The front is a light front so at least don't be shy going lighter in pressures up there.

Think of the tires as 4 bags of air that supports the weight of a vehicle. The recommended pressures have more to do with how much weight each bag supports than anything else. remember I said "RECOMMENDED"

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There is a better way to get a general idea what the tires are doing while the car is being driven. Clean the Armor All off and mark each tire with a stripe of white shoe polish or chalk or similar substance. Caveat: white shoe polish is great for autocross or the track because it doesn't come off unless rubbed off by the track, but the downside is that it's not easy to clean it completely off your tires. Then go out and drive the car. When you come back into the garage the areas of the tire where the shoe polish rubbed off is making contact with the road. Ideally the tire should be making contact with the road on the tread and on the shoulders (in hard cornering) but not on the sidewall. If the tire is overinflated, there will be shoe polish still on most of the shoulder i.e. the tread is bulging out like a balloon. If the tire is underinflated, the tire will roll over in hard corners and rub the sidewalls. Also, if the tire is underinflated it flexes more, which generates excessive heat, which can make it feel greasy. Adjust the tire pressure, re-mark the tires and take it out for another drive. Repeat as necessary. Here is good explanation with pictures: http://otodiy.blogspot.com/2009/11/t...r-maximum.html

I think we can make more than 5 pages of this Just experiments with the pressures but make sure you test them when the tires are warmed up. If you can't find a good point for the pressures at least you will know which tires if any is loosing air and needs attention

The recommended pressures for your car are the baseline assumption Porsche used to calibrate its suspension. As part of the deal, those pressures will generally put the most tread on the ground for an OE width tire, though there is obviously some latitude there. You can riff on these pressures, to your taste. The results are pretty predictable. AFAIK, lower pressures are used at the track to accommodate what happens when the tire gets hot. Nobody's tire gets that hot on the street. The benefit of nitrogen is that it's more temperature stable. A tire filled with nitrogen will remain at its set inflation pressure over a wider range of temperatures. A tire filled with air will gain or lose about 1psi per ten degrees of ambient temperature, give or take. That's also been my experience. I don't personally find the rest of the nitrogen sales pitch persuasive, but I can't disprove it. My .02, anyhoo...

Experimenting is always interesting....however, if you search for previous posts, there is a history of people trying lower pressures, enjoying the softer ride, then wishing later that they hadn't when they bent a rim on a pot hole. Finding optimal pressures at track events makes the most sense as the pavement is usually in decent condition and you're dealing with a limited temperature range & conditions. My guess is that if the factory numbers were off, we would have seen a long history of reports of improperly worn tires. We haven't -- so I'm guessing that (assuming relatively stock suspension), the factory numbers are trustworthy for street conditions.