Tire pressure part 2

Hi Gents, I bought a book recently entitled "Porsche 911 Performance Handbook" by Bruce Anderson. Second edition. I found an interesting section about tire pressure and thought I'd reproduce it here. It's interesting because it points out some facts which are quite opposite to what many of us may be thinking. Here goes: Tire Pressures Porsche recommends tire pressures that work as a compromise between handling and tire wear, but because of the wide range of potential driving conditions worldwide this is a challenge. When modern, low profile tires are run under inflated at higher speeds they will probably show excessive wear in the center. This may run counter to your experience with older, conventional , cross-bias tires, which wore on the outside when they were under inflated. Wide, under inflated, low profile tires will wear in the center due to carcass flex caused by centrifugal force. At 65 mph the centrifugal force is approximately 250 g, and at 125 mph the force is about 1025 g. A German driving a 911 on the Autobahn at 125 mph is more likely to have the centrifugal-force-induces wear problem than we are here in the top-speed-restricted United States. Check your owner's manual for Porsche's recommendations for tire pressures for different tire and wheel combinations. I suggest that you start with these inflations, watch your tire wear, then customize the pressures for your 911 based on your observations and experiences. You might actually want to buy a tire tread depth gauge and check your tire wear across each tire's road surface every 1000 miles or so. If you have a newer 911 with the 16, 17, or 18 inch wheels and experience wear in the center with Porsche's recommended pressures, but you never drive more than 70-75 mph, you might want to try reducing the pressure by a few pounds. At the reduced speeds here in the United States you probably will not need such high pressures to compensate for centrifugal forces. In fact, some of the tire manufacturers do recommend 36 psi inflation for the 265/35ZR18 rear tires on the Carrera 4S instead of the 44 psi inflation recommended by Porsche. Competition Tire Pressures Bob Strange, BF Goodrich test engineer and race driver, has a great deal of experience with radial tires on Porsches and was willing to share some information that he put together for one of BF Goodrich's ads. In the ad, Bob explains: The question most people ask me about their tires is how much air they should put in the. Of all the factors that can directly affect your tires' performance, inflation pressure is one of the most important, and sadly, one of the most neglected. The fact about inflation pressure is that the amount of air you put in your tires affects the way in which the tread comes in contact with the road. By varying the pressure, you can vary your car's performance. Over and Under: Over inflation and under inflation each have their own problems. When tires are over inflated, they absorb less of the shock of driving and that puts extra strain on suspension systems. Also, because the extra pressure lifts the shoulders off the road, traction can be substantially decreased. On the other hand, under inflation can make tires sluggish and unresponsive. Our tires are designed to have the proper balance of rolling resistance, cornering, and resistance to hydroplaning when correctly inflated. What is the right pressure for your vehicle? For the every day driver, the proper inflation numbers are specified on your vehicle and in your owner's manual. For all cases, the pressure should never be below the vehicle manufacturer's recommendation or above the maximum branded on the tire sidewall. For light truck tires, pressures are generally higher because they have reinforced sidewalls to carry additional loads. The Pressure of Competition When racing, the rules of inflation pressures change. In autocross, pressures can run much higher than for everyday driving. The higher pressure quickens steering response and can increase cornering and traction. Off-road racers run with lower pressures to provide a smoother ride, more bruise resistance, and increases traction thanks to the tires' larger footprints. In competitive situations, inflation pressures are often used to compensate for oversteer and understeer. Lower front tire pressures and higher rear tire pressures generally increase understeer, while higher front tire pressures and lower rear tire pressures increase oversteer. Check the Chalk The most common method of finding the right tire inflation pressure for a competitive vehicle/tire/track combination is the chalk test. Chalk the outside shoulder and sidewall area in two or three places around the circumference of the tire, and, after each race or practice session, check how much the chalk has been worn away. Ideally, the chalk should be worn away within 1/4 inch of the point where shoulder rib and sidewall meet. If the chalk remains on the tread/shoulder rib, the tire is over inflated or has too much negative camber. If all the chalk is worn away, the tire is running on the sidewalls and is under inflated or has too much positive camber. When either situation occurs, add or subtract air in 2 psi increments or modify the camber. Re-chalk the tire and try again until the chalk shows your tires are properly inflated. The chalk method is good enough for rough estimates, but the method most professionals use to precisely set the right inflation pressures uses a pyrometer- a thermometer for your tires. After a practice session or a race, a needle-like device is inserted in the center of the tread and in each shoulder, and the temperature is read. An over inflated tire runs heavier on the center of the tire rather than on the shoulder, so the center will be hotter. An under inflated tire runs heavier on the shoulder, so the shoulder will be hotter than the center. Material above quoted from p214-p216 of the Porsche 911 Performance Handbook, second edition, by Bruce Anderson. Cheers, Ken

Thanks for the info Ken. I've been riding 34 psi in the front and 38 in the rears on 18" rims. I had like a 100 mile drive today so I pumped up the tires to the recommended factory settings of 36f/44r to see if I liked the ride. I found it makes for a lot harder, less forgiving ride. You seem to hit bumps alot harder. I didn't really get to test things out as far as cornering goes. For my street driving, I prefer 34f/38r so I'm going to let some air out!

ken this is the most enlightening conribution yet to our long discussions regarding tire pressure. in particular the centrifugal force considerations at autobahn speeds vs u.s. speeds should be taken into consideration <img src="graemlins/c.gif" border="0" alt="[ouch]" /> thank you <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" /> <img src="graemlins/xyxwave.gif" border="0" alt="[bigbye]" />

I agree excellent information! Thanks!

Most of this information is a bit dated for the current crop of track tires. The chalking or shoe polishing the sidewall is no longer a very good indicator of inflation as sidewalls are now much stiffer and shorter. Then this advice was written (mid to late 80's) most of us were running 50 series (aspect ratio) tires or higher which actual had some "give" in the sidewalls. A 35 series just doesn't fall over very much regardless of pressure.

Our sidewalls may be stiffer, but I would think the cetrifugal force information still applies. If we are running lower speeds here in the US, that may be why Porsche recommends high pressures. Cheers, Ken

[quote]Originally posted by Palting: <strong> I may have misunderstood you Ken, but it seems that in the US, at the speeds that we drive relative to Europe, the pressures should be LOWER. Here's a quate from the first post: "If you have a newer 911 with the 16, 17, or 18 inch wheels and experience wear in the center with Porsche's recommended pressures, but you never drive more than 70-75 mph, you might want to try reducing the pressure by a few pounds. At the reduced speeds here in the United States you probably will not need such high pressures to compensate for centrifugal forces. In fact, some of the tire manufacturers do recommend 36 psi inflation for the 265/35ZR18 rear tires on the Carrera 4S instead of the 44 psi inflation recommended by Porsche."</strong><hr></blockquote> Just got back from a 2 hour drive, and those expansion joints can be gruesome at 85-90. Maybe I can still use the lower pressures.

palting i am now using 34/38. ride quality and grip are much better. viken was right as usual! the comments above regarding centifugal force provide a good rationale for using lower pressures in the u.s. now do i dare go any lower? i doubt it <img src="graemlins/nono.gif" border="0" alt="[nono]" />