Load Transfer

Technically, there is a difference between load and weight transfer, but for simplicity and the purpose of this discussion, we can consider them to be the same.

In real world cars, load transfer greatly affects cornering capability. This is because tires produce less cornering G the more heavily loaded they are. As an example, assume a car is sitting still where each front tire is capable of producing 1.00 G of cornering force. At rest, the average cornering force of the two front tires would be 1.00 G. However, in a turn, load is transferred from the inside tire to the outside tire. Because of the tires' sensitivity to load, the outside tire may now only be able to produce .96 G while the more lightly loaded inside tire may produce 1.02 G. The average G is now only .99 G which is less than the 1.00 G capability before. So load transfer actually decreases the car's cornering capability.

We have very little control on how MUCH load is transferred as it is basically set in concrete by the car design. However, we can control WHERE and how QUICKLY the load is transferred.

LATERAL WEIGHT TRANSFER:

1. Lateral Unsprung Weight Transfer--this is load due to the weight of the tires, wheels, hubs, outboard brakes, etc. Any weight located outboard of the springs is considered unsprung weight. It acts directly downward on the tires and increases in proportion to the cornering force. The only thing we can do about it is to minimize the weight of the unsprung components through careful design.

2. Lateral Sprung Weight Transfer-- This is the remaining chassis weight that is supported by the springs and constitutes the major portion of load transfer. How much sprung load is transferred depends entirely on four factors: cornering G force, chassis weight, center of gravity height, and track width. To minimize sprung weight transfer, minimize the chassis weight, lower the center of gravity height, and/or increase the track width. In reality after the car is designed, there is little we can do to affect the amount of sprung weight transfer except run with as little fuel as possible or use a lower ride height.

Although we can do little to control how MUCH sprung weight is transferred; we can totally control WHERE the sprung weight is going and how QUICKLY it transfers. Here's how it works.

There are two paths for sprung weight transfer; through the roll center via the suspension arms or through the spring and roll bar due to chassis roll.

A. Weight Transfer Through the Roll Center/Suspension Arms: This is the component of sprung weight transfer that acts through the roll center and is proportional to the roll center height divided by half the track width. These two factors are set in concrete during chassis design and there is nothing we can do about it. However, by raising or lowering the roll center height, the car designer can apportion the amount of sprung weight transfer that acts through the roll center. If the roll center is located on the ground, none of the transfer is through the roll center. If the roll center height is exactly the same as the center of gravity height, then 100% of the sprung weight transfer is through the roll center.

B. Weight Transfer Through the Spring and Roll Bar: This is the remaining component of sprung weight transfer that is not transferred through the roll center. Instead, the weight is transferred via the spring and roll bar as the chassis rolls about the roll center. If the roll center is located on the ground, 100% of the sprung weight transfer is due to roll through the spring and roll bar. If the roll center is exactly the same as the center of gravity height, then none of the sprung weight transfer is through the spring and roll bar.

What this all means is that the car designer can apportion the sprung weight transfer through the roll center or through the spring and roll bar depending on where he places the roll center in relation to the center of gravity height. Remember, however, that the total amount of weight transfer is always the same.

Now what difference does all this make if we cannot control the amount of sprung weight transfer? Here is the critical part: The amount of sprung weight transfer due to roll to each end of the vehicle is directly proportional to the roll resistances at each end. This gives us control over which outside tire the sprung weight is transferred to. This can be a confusing concept so here are some examples.

For a chassis that has a solid axle with no springs such as a chariot or cart, all of the load transfer is through the roll center and there is no chassis roll. If the front of a chassis has mythical springs with no resistance while the rear of the chassis has a solid axle with infinite resistance, then 100% of the sprung weight transfer will be to the outside rear tire. In this case, none of the sprung weight transfer will be to the outside front tire. This probably will result in the front outside tire being underworked while the rear outside tire will be overworked in relation to each other and the car will severely oversteer.

If the roles were reversed and the front axle were solid and the rear axle had zero resistance springs, then the front outside tire would be overworked and the rear outside tire would be underworked. The car would severely understeer.

Of course, we wouldn't normally design a car like this. Rather, we would like to be able to vary the front to rear roll resistance so that we can control the proportion of sprung weight transfer between the front and rear end. We can do this through the roll bars. The total roll resistance at each wheel is the combined resistance of the spring and the roll bar. As the spring resistance is usually set by other factors (such as trying to keep the tire in continuous contact with the road), that leaves the roll bar as the primary means to vary the roll resistance. The roll bar is designed to be effective only when the chassis rolls...it has no effect during dive under braking or squat during acceleration. By adjusting the roll bar resistances in relation to each other, we can apportion the sprung weight transfer between the front and rear ends and influence understeer and oversteer.

A second benefit of using the spring and roll bar to handle the sprung weight transfer is that we can control the rate at which the transfer takes place. Sprung weight transfer through the roll center is immediate while transfer through the spring and roll bar takes a certain amount of time. We can use spring dampers (shock absorbers) to control the rate at which the transfer takes place.

On the negative side, chassis roll has an adverse effect on tire camber. We want to keep the tire near perpendicular to the road at all times for best traction. So we must use really stiff springs or roll bars to limit roll. Current car design thinking is to use a low roll center which does produce more chassis roll, but also allows the use of roll bars to apportion the sprung weight transfer to control understeer and oversteer and dampers to control the rate of transfer.

LONGITUDINAL WEIGHT TRANSFER:

During braking or acceleration, weight is also transferred. As with lateral weight transfer, we really have no control over how MUCH weight is transferred, but we can control the route that the transfer takes and use that to our best advantage.

A. Deceleration/Braking: The calculation for the AMOUNT of longitudinal weight transfer is similar to the lateral transfer equation. The weight transferred depends only on the deceleration G, the total vehicle weight (sprung and unsprung), the CG height, and the vehicle's wheelbase.

  1). Unsprung Weight Transfer: Depends on the tire/wheel/outboard brake weights and deceleration rate.

  2). Sprung Weight Transfer: As with lateral transfer, the sprung weight transfer can travel through the suspension arms or through the springs/dampers as the chassis pitches down and rotates about the "side view swing arm" (SVSA) center. The SVSA is fixed by the relative lengths and angles that the front suspension arms make with each other. In theory, the designer can cause 100% of the sprung weight to transfer through the suspension arms leaving 0% through the springs and dampers. This is known as 100% Antidive and the chassis will not experience any pitch change as the car decelerates. The designer can also adjust the front suspension so that 100% of the transfer will be through the springs and dampers and none will be through the suspension components. This is known as 0% Antidive and the chassis nose will pitch down during deceleration resisted only by the springs and dampers.

It's a complex subject, but the ratio of the SVSA center's height to its distance from the front axle is compared to the ratio of the CG's height to the wheelbase. If the two ratios are the same, then this is 100% Antidive. If the ratio is zero, then this is 0% Antidive.

The front wheels typically don't receive 100% of the braking force. The Antidive calculation must take into account the portion that the front brakes contribute to the overall braking force. If the front brakes (via the brake bias selector) only receive 50% of the total braking force, then the maximum Antidive percentage will also only be 50%. Because of this, it's normally impossible to achieve an Antidive percentage greater than 50%.

In summary, the designer can control the chassis pitch angle, the amount of front suspension bump travel, and the rate of the weight transfer by varying the SVSA location, the spring rates, and dampers just as he can control the lateral weight transfer via the roll center height, springs, and dampers.

B. Acceleration: When the vehicle accelerates, weight shifts to the rear. The calculation for longitudinal weight transfer is exactly the same as before. Instead of using the term Antidive, the term Antisquat is used. Because the brakes aren't applied, Brake Bias has no effect so it is possible to achieve 100% Antisquat. Again, the designer can control the route the weight transfer uses by varying the rear SVSA center location in relation to the CG location.

Detailed Explanation of Antidive/Antisquat:

The SVSA center is defined as the point about which the suspension rotates when viewed from the side. It is fixed by the intersection of the lines along the top and bottom suspension points. The antidive force angle is calculated as the arctan of the SVSA center height divided by its length where height is measured from the ground and length is measured from the tire contact patch.

The longitudinal force angle under braking is calculated as the arctangent of the CofM height divided by the wheelbase.

If the antidive force angle is the same as the longitudinal force angle, then all of the transfer force acts along the front suspension arms, none goes through the springs, and the chassis nose will not deflect. This would be 100% antidive. If the antidive force angle is 0 as a result of the suspension arms being parallel and intersecting at infinity, then none of the transfer force goes through the suspension arms, all would go through the springs, and the chassis nose will deflect. This would be 0% antidive.

As noted before, the percentage of front brake bias limits the antidive percentage. Normally, race cars use very little, if any, antidive as their stiff springs control pitch changes well enough.

At the rear with a solid beam axle, the same principles apply for antisquat. As the brakes aren't being applied, brake bias has no effect on limiting the antisquat percentage and 100% or more is possible although rarely used on race cars except dragsters.

Dragsters make use of antisquat percentages greater than 100% by using solid beam axles and placing the SVSA center above the 100% anitsquat line. As a result, the chassis rises and the suspension lowers during acceleration. This causes two effects:

 1. As the chassis rises, the car CofMass height increases.
 2. As the suspension lowers, the wheelbase decreases.

Because weight transfer is proportional to the CofMass height and inversely proportional the wheelbase, the amount of weight transfer increases as the car accelerates. This puts more vertical force on the rear tires and they can provide more acceleration force as well.

Race cars that need to turn left and right rarely have an antisquat percentage greater than 100% as increasing CofMass height negatively affects lateral weight transfer and cornering force. 0% antidive is much more common.

For independent rear suspensions, the antisquat force angle is calculated as the the arctangent of the SVSA center height minus the rear axle height divided by the length from the rear tire contact patch. Front brake bias does not act as a limiting factor, but because the rear axle height is subtracted, it's difficult to get much rear antisquat in practice. Typical values are 25% or less.

By adjusting the SVSA centers, the designer can control what portion of longitudinal weight transfer goes through the springs and dampers just as he can use the roll centers to control the proportion of lateral weight transfer that goes through the springs, dampers, and antiroll bars.