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Determining Tire Pressures
Submitted by SuperUser on Monday, December 22, 2003 - 1:49pm

Grip. We all want more of it. We're all looking for that perfect shock, strut, sway bar, spring, wheels, and tire combination that will result in the kind of grip that leaves others in awe. What's the most important part of the puzzle? Lets start with the basics. When the tires loose traction, the car slides. That makes perfect sense, as the tires are the intermediary between your car and the road. Let's go back a step further and look at the most basic part of the tires: the air in them. It doesn't matter if you have the best tires on the planet, if you run them at the wrong pressure they will not perform properly. But the air pressure is the simplest part of the whole picture, right? I have two questions for you: How much air pressure do you personally run, and how did you decide on that pressure? Many people run a particular pressure because someone else runs that same pressure and their car really handles. Other people look on the sidewall of the tire and air them up to whatever the maximum pressure is. Neither one of those methods is the best way to determine what's right for your car. That's because every car is different. Tire brands, sizes, wheel sizes, vehicle weight, alignment specs, and suspension components all add up to make each car unique. If each car carries it's weight in a unique fashion, then it makes sense that the best tire pressures may vary from car to car. So why is all this pressure nonsense important? Tire pressures affect tire temperature. Under inflation causes the tire casing to flex excessively, overheat, and provide less traction. Over inflation will eliminate the problem of case flexing, but it also prevents the full face of the tire from getting up to the optimum operating temperature. Before I tell you how I go about determining the optimum pressure, let me just say if you are just using your car for regular commutes back and forth to the office, use whatever pressure is stated in the owner's manual. If you do that and remember to check it every time you do an oil change you'll probably be fine. Also, if your car handles poorly because those 10-year-old struts are blown, your sway bar mounts have rusted off, or your tires are bald, you've got other things to consider. Dialing in the correct air pressure is a tool to improve the handling of cars that are in good working order. I know of two ways to "read" a tire and determine the optimum tire pressure. The best method that I know of requires the use of a pyrometer. The first reaction of some people may be, ‘Now I have to buy a pyrometer, just to air up my tires?!?!?'. I can understand that response. Buying a temperature sensing tool just to put air in a tire does seem like a bit much, but it's the best way I know to insure that I'm getting the most traction that I can out of a tire. If you spend six hundred dollars every eighteen months on street tires, but still think that a $100 or so for a pyrometer is too much, here is the two-dollar chalk method. At the track, or on your favorite curvy stretch of road, take some chalk and mark all four of your tires. Make the mark across the tread portion of the tire, around the edge, and about an inch down the sidewall (about where the lettering starts on the side of the tire). After making a few passes through both left and right hand curves, pull over and check the tires. What you hope to see is all the chalk rubbed off the tread on the tires, and almost all of it still on the sidewalls. How much is almost? If you've only rubbed off one sixteenth of an inch of the chalk on the sidewalls, that's good. If any chalk remains on the outer edges of the tread, you have too much air pressure. If the chalk is rubbed off down to the lettering on your tires, you've got way too little. That's as close as the chalk method gets. It lets you know that you're not at the extreme end of either spectrum. Pyrometers work a little differently. Accelerating, braking, cornering and even driving down a straight road will cause the tires to heat up. The tread heats up based on how the weight of the vehicle is spread out across the tires. You will find that the tread typically will not heat up evenly. If you know how the temperatures are spread across the tread, you can come to conclusions about the alignment settings, whether you are running the proper tire sizes, your sway bar settings, and whether you're running the proper amount of air pressure. To read the air pressure with a pyrometer, I would take the car out on that same track or curvy road and do a little spirited driving. As soon as I made that last turn, I would bring the car to a stop and immediately take some tire temperature readings. Cool down laps or continuing to drive after the last curve will throw the readings off. I record the temperature of the tread in three places; the outside (about an inch from the edge), the center, and the inside (also, about an inch from the edge). Now that I have recorded all the numbers, what do they mean? If the center of the tread is hotter than edges, the tire pressure is too high. If the edges of the tread are hotter than the center, the tire pressure is too low. If the inner edge of the tread is hotter than outer, the suspension has negative camber. This is not uncommon. Sports cars typically have some negative camber dialed in. If the difference in temperatures is greater than 20 degrees, you may want to have the alignment checked. If the outer edge of the tread is hotter than inner, the suspension has positive camber or toe-in. For me, that's an indication the car may benefit from a trip to the alignment rack. If you have called the tire manufacturer and determined that your tires are running below the ideal temperature range (and unlike me you actually have the driving skills to bring a tire up to that range), then the tire pressure is either too high, the tire is too wide, the springs are too stiff or the sway bar is too thick at that axle. If the tires are above the ideal temperature range, either the tire pressure is too low, the tire is too narrow, the springs/sway bars are too soft at that axle, or you really know how to drive! If the front tires are hotter than the rears, then the car is under steering. That can be a result of front springs that are too stiff, a front sway bar that's too big, rear springs that are not stiff enough, a rear sway bar that's not stiff enough, front tire pressures that are too low, the front tires being too narrow, or rear tires that are too wide. If the rear tires are hotter than front, the car is over steering. That can be from rear springs that are too stiff, a rear sway bar that is too thick, front springs that are not stiff enough, a front sway bar that's not thick enough, the rear tire pressures being too low, front pressures too high, the rear tires too narrow, or the front tires too wide. What are the goals? The ultimate setup would result in all the tires having the same temperature readings in all three spots. Rest assured, that's not going to happen. What I hope to settle for is having the center of the tread average between the inner and outer portions of the tread. Secondly, I want all four tires as close as possible to the same temperature. Thirdly, I want the temperature range between the three areas as close as possible. Achieving these three goals usually results in all four tires working as equally as possible to achieve the maximum amount of grip as I negotiate the turns. Once the tire pressures are such that the temperatures are where you want them, there will be some leeway for tuning. For example, if the car pushes (understeers) in most corners, you will be able to reduce the front tire pressures a little without throwing the temperatures off. If you can't lower the front's enough, you can increase the rear tire pressure. The reverse is true if the car oversteers. I like to adjust the pressures on the axle that is having the problem before making adjustments on the other axle. I really recommend that you buy or borrow a pyrometer. The first thing that you'll have to decide when buying a pyrometer is which type to get. There are probe type pyrometers and infrared (IR) pyrometers. If exacting tire temperatures for a track car are the sole purpose for this purchase, a probe type pyrometer is the best choice. IR pyrometers only measure the surface temperature, and don't reveal the temperatures of the rubber at the cord. The temperatures at the cord can be as high as 40 degrees F higher than the surface temperature. When a tire manufacturer supplies the optimum temperature range, that temperature is typically taken with a probe type pyrometer. Engine heat, brake heat, and air temperatures can also influence IR temperatures. That doesn't mean IR pyrometers are useless, it only means that probe types are better for this specific application. For simply determining air pressures on a street car, using an IR pyrometer is fine. If you're going to buy a pyrometer for a wider range of tasks, an IR pyrometer is usually the better purchase. IR pyrometers can be used to determine the following:

  • If your AC is blowing ice-cold air, or if it's just cool.
  • If and where your radiator is clogged.
  • How much cooler the input side of the oil cooler is than the output side.
  • When thermostats (coolant or oil) begin to open.
  • The health of your catalytic converter.
  • If your high flow/open element air filter is sucking air in a 200-degree environment.
  • If an aftermarket intercooler really works better than the factory one. Cockpit temperatures
  • Brake disc temperatures
  • Track temperatures
Using a pyrometer to determine optimum tire pressure won't necessarily result in the kind of grip that pulls chunks of asphalt from the road in your wake, but it does allow amateurs like us to make educated guesses about suspension tuning. This is a lot better than just buying parts and hoping things get better.

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subject:
Wow
author:
date:
April 23, 2004 - 7:44am
Wow, I learned more about tire pressures and the effect it has on my tires and driving in this short lesson than in my 35yrs of driving. Thanks for the knowledge.

No trees were harmed in the transmission of this message.
However, a rather large number of electrons were temporarily inconvenienced
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subject:
They fill it with Nitrogen ga
author:
No Rotor
date:
July 27, 2004 - 10:40pm
They fill it with Nitrogen gas to keep moisture out. Nitrogen is also much more stable
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subject:
What about
author:
No Rotor
date:
December 31, 1969 - 4:00pm
I have heard about BMW and porsche guys filling their tires with pure nitrogen gas? What does this do?
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subject:
No title supplied
author:
No Rotor
date:
December 31, 1969 - 4:00pm
test
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