Weight Transfer

Quite possibly the most important factor in a car’s reaction to driver input is the position of the weight on the four wheels. The balance of this weight can determine if the car will understeer, oversteer, spin, or perform perfectly.

     When a car is at constant velocity, the weight is evenly distributed between the four tires. During acceleration, weight will shift from the front onto the rear tires. During braking, the weight shifts from the rear onto the front wheels. During cornering, the weight shifts from the inside tires to the outside tires. Thus, if you were accelerating while turning right, the least weight would be on the front right tire, and the most weight would be on the rear left tire. Tires with less force on them will lock up faster and lose more traction than those with more force. It is important to note that actual mass does not shift in the car. Instead, the suspension causes the body to tilt, and apply the force differently.

          A car with a lower center of gravity will transfer less weight, as will a car with a stiff suspension. A car with a high center of gravity or with a loose suspension will transfer more weight. This is why SUVs are so prone to rolling. Their CG is high, and in a sudden change of acceleration and/or direction, can transfer too much weight.

          The value Lf is the force on the front wheels, and Lr is the force on the rear wheels. Lf + Lr always equals the weight of the car. In our 1455-kg car, which weighs about 14260-N (1455*9.8)  braking at 1-G will cause about 14260-N of braking force. If we also know the car’s wheelbase (2.54-m) and the height of its center of mass (.51-m) we can find the amount of weight transferred. The equation to find the Lf and Lr is as follows:

                                    Lf = d (weight) – Fh/w

                                    Lr = (1-d)(weight) + Fh/w

In this equation, F is the acceleration force. When accelerating, F would be positive, and when braking F is negative. The value for d is the fraction of weight in the front, when the car is stopped. In our example we assume a 50/50 weight ratio.

          As an example, let’s say our car is braking at 1-G. We can easily find the new weight distribution:

                                   Lf = .5(14260) – (-14260)(.51) / (2.54)

                                    Lf = 7130 + 2863

                                    Lf = 9993-N

 

                                    Lr = .5(14260) + (-14260)(.51) / (2.54)

                                    Lr = 7130 – 2863

                                    Lr = 4267-N  

    So, in a one G braking maneuver, 2863-N will transfer from the rear to the front. In terms of kilograms, the front now weighs 1020-kg, and the rear weighs 435-kg. One G of braking causes a serious transfer of weight. If the driver attempted to turn, he would have huge oversteer with all that weight on the front tires.