Driving is inputs and outputs. A driver applies input to the vehicle via steering – braking – and acceleration and combinations thereof. The output is how the vehicle responds to the drivers 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. 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 – and 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.
1. Common sense says that the vehicle weight is a big factor. The lighter the vehicle the less weight transfer.
2. 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. The more steering – braking – and accelerating – the more force pushing on the vehicle – the more force the more weight transfer.
3. Height of the cg. The higher the center of gravity the more the weight transfer. This is the major reason that SUV’s and Pick Up trucks have more weight transfer that a standard vehicle.
4. 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.