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Tire Pressure

The average road car does not have a lot of adjustability to help maximize handling.  But, every car’s handling can be improved through tire pressure tuning.  Professional race teams have resources dedicated to the management of their tires who rely on extensive experience and sophisticated tools to achieve their goals.  On the other hand, the average driving school student armed with a little knowledge, a tire marker or chalk and a pressure gauge can go a long way towards optimizing tire pressures.

Before we start, we need to understand what is happening with the tire when it is loaded.  You can easily see when a car is parked that the tire squishes a bit wider where it touches the ground.  What we are seeing is the tire deflecting and changing the shape of the contact patch.  In principle, the bigger the contact patch, the more grip a tire will generate.  The tire also acts as a spring and the combination of tire structure and pressure affect its spring rate.  So, when we adjust air pressure to help us fine tune the balance of the car in a corner, we are actually adjusting both the contact patch shape and tire spring rate.

When the car is cornering, weight is transferred to the outside tires.  This means that the tire is loaded both vertically and laterally, changing the shape and size of the contact patch.  By changing tire pressure, we are effectively trying to optimize the balance between the spring characteristics of the tire and suspension to maximize the area of the contact patch.  Simple but elusive …

To get started, take a good look at your tires.  Most high performance tires will have a small arrow between the blocks where the tread meets the sidewall.  This is the “sweet spot” to which the tire is intended to roll if it is to provide its best possible performance under heavy cornering loads.  Some tires have a circumferential ridge separating the tread from the sidewall to represent the same “sweet spot”.

You need to start your work with a “cold” tire adjusted to 3 to 5 psi below the car manufacturer’s recommendation.  This is because as the tires heat up on track, the pressure will rise by 6 to 10 psi, bringing you closer to where you need to be. 

The best way to do this is to set your pressures the night before the event to 4 to 6 psi below the manufacturer’s recommendation.  Then, in the morning, before setting off, adjust the pressure down to the desired setting.  This only takes a few extra minutes but will establish your cold and only truly repeatable setting for the day. 

Before your first run, use the tire marker or chalk to make a wide mark on your tires highlighting the “sweet spot”.  Take note of your tire pressures at all four corners.  Check your wheel nut torque and take your car out for a few of laps.  Keep in mind that your tires need to heat up before your car will handle predictably.

When you come in, take a look at the mark you put on the side of your tire.  If the mark on the sidewall scuffed off further than the “sweet spot”, the tire sidewall rolled over too much and your tire pressures are probably too low.  If the mark on the sidewall is still there and above the small arrow, then the tire sidewall did not roll over enough and you can lower your tire pressures.  Adjust your pressures up or down making small 1 to 2 psi changes (equally to both the left and right side) and let things cool down.

Repeat the steps above for each of your sessions trying to keep your driving as consistent as possible.  Within a few runs, you will find that your handling will be come more consistent and predictable and the day goes on.  You have now found your good baseline.  The next morning, when the tires are cold, check and note your “cold” pressure.  This is your new baseline.  From here, you can start to fine-tune the handling by adjusting tire pressures individually and in pairs. 

If the car is understeering in a corner, you will want to lower the front tire pressure, thereby increasing front grip by reducing the front tire spring rate and increasing its contact patch.  You can also reduce rear grip by increasing the rear tire spring rate and decreasing its contact patch.  If the car is oversteering in a corner, you will want to increase rear tire grip by reducing the rear tire air pressure.

Always try to increase grip on one end of the car that is sliding, rather than reducing grip on the end that has grip.

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