Weight Transfer
Understanding the simple physics of weight transfer is the key to tuning cars. When stationary, the weight of the car is distributed more or less evenly on all four wheels. Where does the weight of the car go during acceleration? The rear wheels, right? Likewise, during braking the weight is transferred to the forward wheels. Also, during cornering the weight is transferred to the outside wheels. That is, in a left turn, the weight of the car goes to the two wheels on the right (when looking at the car from behind).
Simple, is it?
No, its not!
Setting up a purpose built race car, or developing a racing set up for a street car - the principles are the same. Now we will talk about setting any type of the car, not only open wheeler class. In the stock car racing aero is not as important as in open wheel designed car.
We will not take into consideration aerodynamic influences like aero balance, efficiency or aero grip but only how to reach good mechanical grip with help of weight transfer.
Key set up changes you can make on the race car are to change springs, anti-roll bars or roll centre height. You are adjusting ride stiffness (a spring change), and/or roll stiffness (springs, anti-roll bars and roll centre height all contribute). Ride and roll stiffness are key inputs in determining the understeer/oversteer balance of the car.
The weight transfer setup recognizes the importance of ride height and roll stiffness in determining a good balanced set up for the car. It applies for all cars, especially racing, sports and high performance road cars.
Your shock absorbers are considered after your ride and roll stiffness have been selected. Your shock absorbers control the tire contact with the road on bumpy surfaces, but they are not as effective as a tuning tool, because the slow shock shaft speed forces are too weak to have any effect in weight transfer tuning. The tuning effect we are after is to be able to influence the timing of the weight transfer and shock absorbers are simply to slow.
On the next pictures you can see natural Center of Gravity adjusted during design and setup process by mean of adding ballast weight (number 1), and CoG changed by weight transfer during acceleration and braking, and cornering (number 2).
Weight transfer during the squat (acceleration)
Weight transfer during the pitch (decceleration)
Weight transfer during the roll (left corner)
Weight transfer during the roll (right corner)
Here is an explanation of the reasoning behind the Weight Transfer
The total lateral weight transfer, at a given lateral g-force in cornering, is a function of the mass of the vehicle, the Centre of the Gravity and the track width. In the mid corner, we cannot influence the total weight transfer by any other means e.g. not influenced by more or less roll.
But we can influence front vs. rear lateral weight transfer, increase one decrease the other, the balance of the car by the following: Tire tests show that lateral grip increases with vertical tire load, but in decreasing increments. This is referred to as the "load sensitivity of the tires". Thus, a pair of tires more unequally loaded has fewer grips than two tires more equally loaded. It works out that this mechanism gives us an extremely sensitive adjustment for relative grip between the front and rear wheels of a vehicle.
We now show how the "roll resistance" is used to apportion the weight transfer front vs. rear. Consider the chassis of the car to be a solid object with a compliant suspension at each end. Analogy of roll resistance in a race car is this: You are carrying a sailboard along the beach with the sail up, you at one end and your friend at the other. Say there is a constant force of the wind in the sail, trying to overturn the sailboard. You and your friend apply counterforce (or resistance), so as to balance the wind force in the sail. If you decrease your counterforce, your friend must increase his counter force a matching amount and vice versa. If the force in the sail changes, either one or both of you have to change the counterforce you apply. This process is sometimes referred to as the "roll couple".
Now we can understand that:
The stiffer end in roll (higher roll resistance) will transfer more weight, purely because of the extra twist being applied to the chassis vs. the other end. The other softer end will transfer proportionally less weight.
We need a stiff chassis to be able to re-distribute tire load in this way. But this is only half the story. We have some weight transfer that goes directly via the suspension links and chassis, not via the springs (see geometric vs. elastic weight transfer below). This still happens on a car with a flexy chassis. When you fit a strut brace to your car, and get better response, this is in part because you are assisting more positive geometric weight transfer.
Through tire load sensitivity, the stiffer end looses grip and the softer end gains.
It is the difference in stiffness that counts. An increase in resistance both ends that keeps the split the same results only in less roll and no change in the balance of the car.
It is meaningless to consider what would happen if the front of the car could roll independently of the rear. The two are inter-dependent. Both ends contribute to one roll angle of the chassis.
It is the roll stiffness of the "wheel pair" that counts, the combined stiffness of right hand and Left hand springs. In roll only, there is no affect on the balance of the car with different spring rates Right&Left Hand sides, although it does affect balance in pitch and combined roll and pitch (because we are now looking at RH front and rear springs and LH front and rear springs as the wheel pairs of interest).
Total Weight Transfer is the sum of three very important components that we can calculate:
Non Suspended Weight Transfer:
Due to the component of lateral force applied by the weight of the wheels, uprights, brakes etc. For live axle, includes total axle assembly weight. We take the axle height as a close approximation to the centre of gravity, (CG), for the non suspended mass.
And two components of Suspended Weight Transfer:
Geometric Weight Transfer:
Due to the component of lateral force, applied directly at the Roll Centre (RC). Geometric WT is reacted directly through the suspension linkages, and does not induce body roll.
Elastic Weight Transfer:
Due to the component of lateral force, applied at the Suspended Mass CG, and does induce body roll. This force is reacted in the springs, anti-roll bars and shocks, and is the only one of the three components of total weight transfer that does induce body roll.
It is clear that low roll centre give little geometric weight transfer and most of the weight transfer goes through the springs (elastic weight transfer), and is therefore delayed by the time it takes for the vehicle to take a set. Conversely, with high roll centre most of the weight transfer precedes the body roll, leaving a smaller amount of weight transfer to go through the springs.
The location of roll centre heights and the affect on geometric weight transfer vs. elastic weight transfer is of importance in the set up of the car. Geometric weight transfer is a major influencer for cars of high front weight percentage and/or for FWD. Also for RWD with live rear axle. Also for current open wheelers with high downforce and little suspension movement.
In current open wheeler racing, geometric weight transfer. can be used because of the reduction in jacking affect: small suspension travel, wide track, long suspension arms to stop the roll centre height moving around so much relative to the chassis i.e. you don't get "progressive" jacking as the car rolls more. In fact, you need the geometric weight transfer. To help reduce the roll angle and suspension travel, while using less rear anti-roll bar, sometimes none at all.
So if you are going to modify the setup of any vehicle, racing or road, it is clear you need to consider weight transfer numbers.
And it's not so simple
