Driving is inputs and outputs. The student applies input via steering, braking and acceleration and combinations thereof, and the output is how the vehicle responds to their inputs. It may be an over simplification but it's what is happening. The output all starts with the weight transferring to the tires, which, in turn, puts weight on the tire contact patch (more on the contact patch later). If there is excessive weight on the tire patch it can causes the driver to lose control.
The Results of Weight Transfer
Acceleration
If we could put scales under the front and back wheels of the car when the gas pedal is depressed we would see that the weight on the rear scale increases and the weight on the front scale decreases. During acceleration, weight is transferred from the front to the rear of the car. This additional weight in the rear presses down on the car's suspension, affecting tire contact patch in the rear. If too much weight is applied, the rear tires can lose adhesion.
Braking
Once more, the car is on our imaginary scales. The driver applies the brake, shifting weight onto the front end of the car. This time, the front-end weight increases and the rear-end weight decreases. In this case, if too much weight is shifted forward, even with ABS it is possible to lose steering control.
Steering
If the wheel is moved to the right, weight is transferred to the left; if the wheel is turned to the left weight will go to the right; once more, by way of the suspension and onto the tire contact patch.
Therefore one could say that a vehicle is a weight transfer machine. When a control is moved weight is moved from front to rear or from side to side. Any of these actions can exceed the tires' limit of adhesion, causing loss of control. The contact patch can lose adhesion two ways. With too much weight applied to the patch or not enough weight being applied to the contact patch. The characteristics of weight transfer onto the suspension and how those transfers affect the limit of adhesion is a major determining factor in just how the car feels to the driver.
There are four things that determine the amount of weight transfer, hence the amount of weight pushing down on the contact patch.
- Common sense says that the vehicle weight is a big factor. The lighter the vehicle the less weight transfer.
- The force acting on the car. The one thing the driver does control is the amount of weight pushing on the vehicles center of gravity. As we discussed in previous articles the more steering, braking and accelerating, the more force pushing on the vehicle; the more force the more weight transfer.
- Height of the center of gravity. The higher the center of gravity the more the weight transfer. This is the major reason that SUVs and Pick Up trucks have more weight transfer that a standard vehicle.
- Track width and wheel base. When braking, the shorter the wheelbase the more the weight transfers. When cornering, the shorter the track the more weight transfer.
Weight Transfer and Rollovers
A combination of these points is how the National Highway Traffic Safety Administration (NHTSA) determines a vehicles rollover probability. The criterion used by NHTSA is called the Static Stability Factor (SSF). The SSF is the ratio of track width to center of gravity height. The equation is T/2H where T = Track Width and H is the Height of the Center of Gravity. The higher the number the less likely the vehicle will roll. As an example a Crown Vic has an SSF of 1.51 and a Ford Expedition has a SSF of 1.16. You can find the numbers for most all vehicles on the NHTSA web site. As an EVOC instructor, if you are conducting training programs with high Center of Gravity vehicles, it would be advisable to look up the SSF number.
The Tire Contact Patch
All vehicles are supported by a cushion of air contained in four flexible rubber tires. If you could place a car on a glass floor and look at it from below, you would see four patches of rubber, most folks are surprised at the size of these four patches depending on the vehicle, as each patch is a little smaller than a hand, touching the glass. These are the only points of contact between the vehicle and the road. Each of these four small patches of rubber is known as the contact patch. It is these four patches that create the traction which makes the vehicle go, stop and turn. It is these four patches that send the feedback back to the student, and it is these four patches the student has to manage. Consider them to be the source of the information needed to control the vehicle.
How Much Rubber Do You Have?
If you want to know the size of the contact patch, jack up your training vehicle, put ink on the bottom of the tire, and let the car down on a piece of paper (we use finger print ink). Have someone keep their foot on the brake as you let the vehicle down, it keeps the tire from rolling, and lower the tire on the paper. The tire will leave a mark on the paper that represents the tire contact patch. The paper should be outline paper dived in one inch blocks, it makes measuring the contact patch easier.
Once done you have a picture of the tire contact patch, and can easily measure the size of the patch in square inches (width of the patch times the height of the patch). In our vehicles, police package Crown Vics with P225/60R16 tires with 32 PSI, we have 36.75 square inches of contact area giving us a total of 147 square inches of rubber touching the road. This makes a good training and or handout for the students.
