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Understanding tire behaviour is key to changing the way your car performs. The tires on your car are fundamental to handling. While the characteristics of the tire will have a direct impact on handling ability, the tire can only work as well as its environment allows. This environment is defined by the static and dynamic weights of the car and how this weight is shifted at any given time.
Vertical Load is the weight applied to the tire. The dynamics of the car in motion vary this load. Chassis tuning is adjusting how and when the vertical load (weight) on the tire changes. By understanding how load changes impact tire behaviour, you can predict the effects of changes.
Traction is the tires capability to adhere to the road surface. Traction impacts ALL handling characteristics of your car, acceleration, deceleration (braking) and cornering. Traction determines how well or quickly a car can brake, corner or accelerate.
Load < > Traction
To understand how a car will handle, you need to understand how tire output (traction) is affected by tire input (weight).
A performance chart graphically shows that the available traction from a tire increases as the weight increases. However, the increase in traction is not directly proportional to the increase in weight. It’s a case of diminishing returns.
Using the Tire Performance Curve as a chart, you can see how the percentage of cornering efficiency decreases as vertical loads increase. Efficiency is output (traction) divided by input (vertical load). In the chart below, you can see that 140% cornering efficiency (1.40gs) is possible. At 75% efficiency, only 0.75 gs could be obtained by the same vehicle. This cornering data makes it clear that the less weight placed on a tire, the higher percentage its cornering efficiency.
VERTICAL LOAD < > CORNERING EFFICIENCY
Certain factors change the traction curve of a tire in a positive or negative fashion. Consideration must be given to these factors such as tread depth, contact patch and aspect ratio etc.
The contact patch is the area of the tread face of the tire in contact with the road surface.Â Maximum traction occurs when the tire is perpendicular to the ground, or ‘Zero Camber Angle’.
Positive Camber: when the top of the tire is leaning outwards, away from the car. This geometry reduces the tire patch when the tire is in its perpendicular position.
Negative Camber: when the top of the tire is leaning inwards toward the car, compensates for the flexing and movement (deflection) of suspension components resulting in zero camber when traction is required.
The Circle of Traction graphically shows the balance of traction between the possible uses, being Accelerating, Braking or Cornering. Any tire, has a limited amount of traction. The more traction directed to any function, the less that remains for the other uses.
If you’re not quite sure you follow to this point, here are three classic real time examples of how you as a driver will or have experienced this traction limit.
Let’s say your car normally under steers in corners. If you get on the gas too soon, or too hard, you are going to find your car changes to over steer in a heart beat. The traction required by the acceleration, is now lost to use for cornering, the rear end (in a rear wheel drive car), is going to want to slide out resulting in over steer.
A classic example, familiar to everyone that knows how to get the pedal to the prairie. If you perform a wheel spinning start, ALL available traction is being used (lost) to acceleration. The result, ZERO traction remains available for cornering or control, which can result in a fish tail.
Last but not least, over braking going into or through a corner will quickly result not just in under steer, but a ‘no steer’ condition as all traction is taken by the braking attempt, leaving nothing for the cornering, and you find yourself sliding in a straight line headed for the rhubarb.
These are extreme transisions in traction application simply to make it clear there IS a limit to the traction available and the more used for one aspect, the less there is for the other.
The discussion of g-force often leaves people dumb struck, yet, it is a simple term used to describe a simple force. The term g-force simply refers to the force equal to gravity. You will see g-force used to described the forces exerted most commonly in cornering but also in acceleration and braking. For example, a 2000 pound car generating 2000 pounds of cornering force would be generating 1-g load. The beauty of using g-force is that it makes all vehicles equal, by eliminating the need to recognize individual vehicle weights. If the same 2000 pound car could generate 1500 pounds of cornering force, this would be 0.75g. (3/4 of its weight) Yet, a 3000 pound car, generating the same force of 1500 pounds cornering force, is only generating 0.50g (1/2 its weight). Simple right!
g = 1.225 x R
R = Radius of circle in feet
T = Time required to complete circle in seconds
Here’s a sample using the following values
200 foot circle provides a 100 foot radius
11 second lap time
g = 1.225 x 100
11 x 11
g = 122.5
g = 1.01
With these basics in hand, you can move onto the next level.