(054) CONTACT PATCH & VERTICAL LOAD (6 of 13) Intelligent Racing, Minimal Spending: The Definitive Introduction To Racing Concepts


Welcome to part six of this racing concepts series. Remember: the most dangerous words in the English language are “I already know that.”

Ready? Here we go.

The only thing connecting your car to the ground is the tire contact patch. Obviously, your car has four of them–one on each tire.

And, as we talked about in the previous post, finding a perfect tire pressure (the one that allows the entirety of the tires width to contact the driving surface) is the key to maximizing traction.

Consider this: the shape of your contact patch will remain relatively consistent.

However, if you can increase the useable surface area of the contact patch inside that shape you will have more traction, because more rubber on the ground creates more grip.

This is exactly why tire selection plays a critical role going faster. Let’s use autocross for an example.

There are generally three tire choices a driver can make before racing.

  1. The driver leaves the factory production tires on the car.While this is a cheap short-term solution, his stock tires (unless the car came with performance summer tires) have been specifically designed to perform well in any situation.This means that the tires are good at shedding water, and are hard enough to withstand several thousand miles of use.While this combination sounds like a dream come true, factory tires generally make poor racing tires.

    Because of their ability to shred water, they have many recesses cut into tread–and every recess is a small area in which no rubber is touching the pavement.

    This means that the factory tires, when subtracting all the grooves from the total surface area of the tire, produce a very small contact patch.Factory tires aren’t usually designed to withstand the high operating temperatures of racing.

    They often deteriorate quickly, chunking rubber and disintegrating when their optimal temperatures have been exceeded.

    This is why I feel that the factory tire is a short-term solution–because you will be buying more very soon.

  2. The driver can purchase a set of extreme performance summer tires. This is a great option for the racer on a budget.This set of tires can be driven to the track, raced on, and driven home without many issues.But, because these are still intended for road use, they must have some ability to channel water.This means that, like the factory tires, the recesses in these tires still have to be subtracted from the overall potential contact patch.The overall result is better, but still isn’t optimal.

    On a more positive note, performance summer tires can usually be purchased in a relatively soft compound, making them grippier than factory tires.

    Along with more contact patch, this is their best quality.

    Performance summer tires are the best choice for Street class autocrossing.

    3. The driver’s third option is to purchase a purpose-built set of purpose-built racing tires.

    These tires aren’t designed to be driven on the road, so there is no need for the tires to channel water.

    These tires have minimal recesses (or none) and are designed to maximize the potential contact patch of the tire.

    In addition, these tires will most likely be a soft compound since they don’t need to withstand thousands of travel miles.

    Not only do they have an excellent contact patch, they are stickier and will produce even more grip.

    And it gets even better. Soft compounds come up to operating temperature very quickly, allowing better initial performance during the short runs found in autocross.

    Obviously, purpose-built tires are the best option and possibly the single greatest upgrade you can make to a car.

    If you have the money (and your rulebook allows it) I strongly suggest acquiring a set.

    NOTE: The difference between factory tires and purpose-built tires equates to something like 2-3 seconds per autocross run.

    And while this may not seem like much on the surface, 2 seconds can easily be the difference between the winning car and the 10th car.

Vertical Load

Vertical load is the weight applied on a tire from a force that is perpendicular to the driving surface (Figure 15).


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Example: Imagine a static racecar with an overall weight of 2000 lbs.

If the car is perfectly balanced then each tire will support 500 lbs (2000 lbs total weight / 4 tires = 500 lbs each).

In other words, the weight of the car is transferred through the suspension components, through the hubs, through the wheels that the tires are mounted on, through the tires, and onto the ground.

In this example, each tire is experiencing a vertical load of 500 lbs.

As a vehicle changes speed or direction it is experiencing weight transfer. As weight shifts to various positions on the vehicle, the vertical load on each tire will also change.

There are many factors that can change vertical load.

  • Driver input. As a racecar driver applies the brakes at the end of a straight, his car will experience a forward weight transfer.With more weight at the front of the car, the front tires will have a higher vertical load and the rears will have lower.The same is true for accelerating and steering, respectively.
  • Aerodynamic downforce. As a car equipped with a large rear wing accelerates down a straight, the air passing over the wing deflects upward.As we know from Newton’s third law, “every action has an equal but opposite reaction.”The reaction in this case is a physical downward force. The faster the car goes, the more downward force the air creates.This means that at higher speeds the rear tires will experience a higher vertical load– effectively increasing potential traction.This is also true of splitters or any other aerodynamic device.
  • Elevation change. As a vehicle crests the top of a hill and the road surface begins to lower, upward momentum will temporarily cause the vehicle to weigh less.Provided that none of the tires leave he ground, all four tires will briefly experience some lower value of vertical load that is significantly less than normal.This is because the vehicle’s overall weight, relative to normal conditions, is lower.During this moment traction is significantly decreased because the vertical load has been relieved.The same theory, although opposite in nature, can be applied when a vehicle starts up a hill from a flat surface, or finishes a hill onto a flat surface.

As you can see, vertical load is constantly in flux.

Depending on what the vehicle is doing in the moment, such as accelerating, braking, corning, a combination of the three, cresting a hill, or creating downforce through speed, the tires are continuously experiencing various vertical loads.

And, therefore, constantly varying levels of potential traction.


NEXT WEEK: Co-efficient of Friction and Slip Angles

The next part in this series is only a week away. Go back and read through this again. Make sure you truly understand what you just saw, because the series progressively builds as it continues.

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