The few square inches where the tires contact the track surface are the key to everything that happens in racing. These small areas are known as the contact patch. Cornering forces and braking forces are transmitted through all four contact patches, and power is transmitted through the rear contact patches. The most effective chassis setup makes optimum use of the contact patches.
The construction of the tire, including the way the cords are wound into its carcass, as well as the nature of the rubber compound and the shape of the tread, if any, determine the amount of force that the tire is able to generate under any given conditions.
The shape of the tire's contact patch and the distribution of vertical loads on this patch determine the amount of force that the tire is able to generate.
Tire pressure affects the way the vertical loads are distributed on the contact patch, as does camber. (More about these below.) Ideally, the loads will be evenly distributed across the contact patch. If either or both edges of the tire are loaded less lightly than the rest of the contact patch, available grip will be reduced. If the middle part of the contact patch is less lightly loaded than the edges, available grip will be reduced.
A small-diameter, wide tire will have a short, wide contact patch and will tend to be more affected by camber and pressure; a large-diameter, narrow tire will have a long, narrow contact patch and will tend to be less affected by camber and pressure. The larger the contact patch, the more grip will be available.
As lateral forces increase, the contact patch distorts. This changes somewhat the nature of the forces which are generated. As longitudinal forces increase, the contact patch also distorts, but in a different way. Again, more about these below.
The forces acting on a tire include the vertical load induced by the chassis, longitudinal forces induced by the brakes and by power transmitted from the engine, and lateral forces induced by cornering.
Vertical loads include the weight of the chassis, plus increases or decreases in this force induced by inertial loads from bumps, crests, dips, and banked and off-cambered track survaces.
The tire is able to generate a given maximum horizontal force under any given conditions. Any combination of lateral and longitudinal forces greater than this will cause the tire to slide and the forces available will decline from the peak.
If the tire is generating its maximum force longitudinally, it will not be able to generate any lateral force. Similarly, if it is generating its maximum force laterally, there will be no longitudinal grip available. If the tire is generating a combination of longtitudinal and lateral forces at its maximum, any increase in force in one direction must be accompanied by a decrease in force in the other direction.
As a lateral load is applied during cornering, the tire generates a lateral force to oppose this load. As it generates this force, the tire begins to distort. The carcass flexes and distorts laterally. As a result, as the tire rolls, it "crabs" sideways a little each revolution.
The rubber in the tire tread (which forms the contact patch) also distorts, flexing sideways so that the contact patch is displaced laterally from its position at rest. The trailing part of the contact patch is displaced more than the leading part.
The result is that a tire under lateral load does not travel down the road in a direction parallel to the tire's centerline (i.e. perpindicular to the axle's centerline). It travels at an angle to the tire's centerline. The difference between the tire's centerline and the direction of travel is called the slip angle.
Any given tire generates its maximum lateral grip at a given slip angle, which is determined by the construction of the tire and its rubber compound. At slip angles higher than this peak slip angle, the available lateral force declines. Above a certain slip ratio, the wheel is sliding and has no directional stability.
The force the tire generates can be plotted against the slip angle; such a plot is called a slip angle curve.
Bias ply tires, such as those on the cars in GPL, tend to generate their optimum force at a relatively high slip angle, somewhere around 9 to 12 degrees. Radial tires, as used by modern road cars and many modern race tires, generate their optimum force at a smaller silp angle, somewhere around 6 degrees.
For this reason, bias ply tires tend to be more forgiving. Cars on bias ply tires tend to be more interesting to watch, because the higher slip angles at which they corner are more visible to the onlooker and more dramatic. Differences in driver technique and skill are also more apparent.
The most skilled drivers operate close to the peak of the slip angle curve as much of the time as possible. Less skilled drivers operate over a broader range, spending more time at areas of the slip angle curve - on both the near side and the far side - where less grip is available.
As a longitudinal load is applied during cornering, the tire generates a longitudinal force to oppose this load. As it generates this force, the tire's tread begins to distort. The trailing part of the contact patch is stretched, while the leading part is compressed. As a result, as the tire rolls, it slips a little each revolution.
The carcass also "winds up", or distorts longitudinally under longitudinal load, but this is a transient condition. Once the tire has wound up under a given longitudinal load, this distortion does not contribute to distortion.
The result is that a tire under longitudinal load does not travel down the road at quite the speed that its circumferance would dictate. It travels a slightly shorter distance than its circumferance. The difference between the tire's circumferance and the amount of travel is called the slip ratio.
Any given tire generates its maximum longitudinal grip at a given slip ratio, which is determined by the construction of the tire and its rubber compound. At slip ratios higher than this peak slip ratio, the available longitudinal force declines. Above a certain slip ratio, the tire is spinning and has no directional stability.
The longitudinal force the tire generates can be plotted against the slip ratio; such a plot is called a slip ratio curve.
Incidentally, GPL seems to use a simplistic tire slip ratio curve. The sensation of having a torque converter between the engine and the wheels when you ramp up the power suggests that the slip ratio "curve" is actually a straight line from zero to peak. I suspect the same is true of the slip angle "curve".
Bias ply tires tend to generate their optimum longitudinal force at a relatively high slip ratio, while radial tires generate their optimum force at a smaller silp ratio.
The most skilled drivers operate close to the peak of the slip ratio curve as much of the time as possible. Less skilled drivers operate over a broader range, spending more time at areas of the slip ratio curve - on both the near side and the far side - where less grip is available. Such drivers are getting either too much wheelspin under acceleration or are locking the wheels under braking.
If the tire is cambered with respect to the chassis, it will generate a lateral force in the direction of its camber. If it has negative camber, it will generate a force toward the center of the chassis. This force is called camber thrust.
The use of negative camber on both sides of the car generates camber thrust at each tire. On the straights, these forces balance each other out. However, in corners, the inside tires become more lightly loaded, while the outside tires become more heavily loaded. The outside tires' additional camber thrust produces a slight increase in useful lateral force, yielding an increase in cornering speed.
Camber affects the vertical load distribution within the tire contact patch and can also impact the shape of the contact patch. As lateral loads increase, the tire tends to distort, or "roll under". Depending on the suspension's camber change characteristics, the rolling of the chassis about its longitudinal axis may tend to aggravate this.
The result is that the inside edge of the tire tends to become more lightly loaded as cornering loads increase. The inside edge of the tire may even pick up off the track, losing contact with the track surface entirely.
However, if the chassis is set up so the tire has negative camber when the chassis is static, this will compensate for the distortion of the tire and the rolling moment of the chassis. In the static state, the outside edge of the tire will be lightly loaded and the inside edge will be heavily loaded, but as the lateral loads go up and the chassis rolls, the outside edge will be loaded more and the inside edge will be loaded less.
If the camber is optimal, both edges of the tire will be loaded approximiately equally when the car is cornering at the tires' maximum grip. This will be reflected in the tire's temperatures after doing several laps on the track. The temperature on the inside edge should equal that of the outside edge.
Note: In real-world cars, it may be better to have temperatures on the inside edge somewhat higher than those on the outside edge, but this does not seem to be the case in GPL.
When a tire is displaced from its natural direction of travel - when it is operating at a slip angle greater than zero - it generates a torque about its vertical axis tending to rotate the tire back to its natural direction of travel. This force is called the aligning torque.
The aligning torque is responsible for some of the self-centering effect of the steering wheel at speed. (Caster is responsible for most of the remaining self-centering effect.)
The aligning torque increases as the slip angle increases, up to a point. As the tire nears its peak-grip slip angle, the aligning torque begins to decline. Past the peak, the aligning torque becomes smaller until, when the tire is sliding, there is very little aligning torque.
The result is that as the front tires reaches their maximum grip, the steering wheel will begin to "go light". As the rear tires reach their maximum grip, the tail will begin too feel slippery, or "loose". The skilled driver senses these inputs from the tires and uses them to help operate the car near the optimum slip angle.
The tire's capability to generate horizontal forces increases in proportion to the vertical load to which it is subjected. As the vertical load goes up, so does the lateral and longtitudinal force available.
However, the horizontal force capability does not go up in direct proportion to vertical load. Horizontal force capability increases at a slightly lower rate than vertical load.
For example, a tire subjected to 200 pounds of vertical load might be able to generate 200 pounds of lateral force. If the vertical load is increased to 400 pounds, its lateral capability might increase to about 390 pounds.
This fact is important when it comes to balancing the car. More about this below.
Tire pressure affects the shape of the contact patch. Ideally, the pressure should be such that when the tire is hot, the contact patch is subjected to equal vertical forces across the width of the tire.
If the tire pressure is too low, the middle part of the tire will be more lightly loaded and the edges will be more heavily loaded, resulting in less than optimal grip.
If the tire pressure is too high, the middle part will bulge out and unload the edges of the tires, which will also result in reduced grip.
The tire pressure increases as the tires get hot. With optimal tire pressure, when the tires are cold, the edges of the tire will be heavily loaded and the middle of the tire will be lightly loaded, but as the tires heat up the forces will become balanced across the tire, resulting in a middle tire temperature that is roughly equal to the average of the tire's edge temperatures.
The nature of the tires' reaction to increased vertical load is of critical importance to balancing the car so that it neither understeers or ovesteers excessively.
If one end of the car is gripping more than the other, the race engineer can increase roll stiffness at that end of the car relative to the other end. This will increase the vertical load on the outside tire and reduce the vertical load on the inside tire at that end.
The additional load will cause the outside tire on the newly stiffened end to become slightly less efficient and the inside tire to become slightly more efficient. However, since the inside tire is generating less than half of the overall grip at that end, the net result will be a slight decrease in overall grip at that end.
The other end, which has been softened in roll relative to the formerly grippier end, will experience the opposite. Its outside tire will now see less vertical load and the inside tire will see more vertical load. The outside, more heavily loaded tire will become more efficient, which will increase the overal grip at that end.
The net effect is that the formerly grippy end of the car will become less so, and the other end will become slightly more grippy, moving the overall grip of the car closer to a balanced state.