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Weight Transfer

A fundamental topic in any discussion about handling is weight transfer. If you do not fully understand weight transfer, you will not be effective in understanding how to adjust the car for maximum handling performance.

Given a certain car weight, there is a certain amount of mechanical downforce applied to each tire. As we stated in the tire traction article, this downforce impacts the grip potential of the tire. While a car is braking, accelerating, or cornering, the effective mechanical downforce on, and therefore the grip of, the tires is constantly changing.

These changes are referred to as "weight transfer." Of course, the weight of the car isn't changing, or moving about the car, but the forces on the tire contact patches are changing due to inertia and momentum. If you were to have a set of scales under the tires while driving, you would see what appears to be a constant changing of the weight at each tire, hence the name.

What does Weight Transfer Do?

Referring to the figures, we have illustrated a street car weighing 3000 lbs, and with a typical FWD street car's weight distribution of 60% front and 40% rear. We'll assume the car's side to side weight distribution is equal. We see that when standing still, the front tires have 900 lbs of weight load, and the rear tires have 600 lbs each.

Anytime the car's direction changes through braking, accelerating, or cornering, each tire will experience a gain or loss of mechanical downforce, such as the examples illustrated. This weight transfer has significant impact on traction. Unfortunately for us, the net sum of the traction of the four tires does not stay equal. What is lost from the unloaded tires is not entirely transferred to the loaded tires. Overall there is a loss in traction.

Because of this, race car design, and some of the modifications you make to your street car, are designed to minimize weight transfer. It cannot be eliminated, but it can be reduced. The more it is reduced, the more traction is retained. By reducing the increase in load on the loaded tires, we can reduce the work they have to do. By reducing the loss of load on the unloaded tires, we retain the traction they can provide.

How can Weight Transfer be Minimized?

Contrary to what you may be inclined to believe, the amount of weight transfer is not altered by springs, shocks, anti-roll bars, etc. Weight transfer is a result of inertia and momentum. These suspension components cannot change that. What these components can do is impact how much the suspension moves in response to the load change, and how quickly the load transfers to the tire contact patches.

The amount of weight transfer is dominated by the vehicle's weight, location of the center of gravity, wheelbase, and track, and the amount of force applied during braking, accelerating, and cornering.

Weight transfer is a function of the vehicle's weight and the forces acting on that weight. Reduce the weight, and ou reduce the product the of the forces involved.

The center of gravity is the fulcrum point through which the vehicle's weight is multiplied by dynamic forces. In particular, the higher the CG point is, the greater the effect of the forces. Reduce the CG height reduces the product of the forces and vehicle weight.

The longer the wheelbase and wider the track in relation to the height of the center of gravity, the more resistance the car has to weight transfer. They behave as counteracting lateral levers to the vertical lever of the center of gravity point.

Another important concept of controlling weight transfer besides minimizing it, is to control where it is transferred. Where weight transfer occurs is related to the static weight distribution of the car, the roll couple distribution of the car, the height of the roll center of the car, and the slope of the roll center in relation to the ground plane.

Roll couple distribution is the relative roll stiffness between the front and rear of the car, and the left and right of the car. In cornering, the front of the car may roll less than the rear of the car. This has impact on how the weight transfer is distributed.

The roll center is the line through which the vehicle rolls. It is not necessarily parallel to the ground. Weight distribution, and roll coupling distribution can create a roll point at the front of the car which is lower to the ground that the roll point of the rear of the car. This creates a sloped line. The angle of this line has influence on how much weight is transferred, and where it goes.

Vehicle Weight Distribution

Modifications which reduce vehicle weight and the location of the center of gravity impact the amount of weight transferred and where it is transferred. Reducing a vehicle's total weight reduces the amount of weight transfer. Redistributing that weight front & rear, or side to side will change how that weight transfer is distributed among the four tires. This affects the individual mechanical loading and therefore grip of the tires.

Removing weight from the car reduces the work the tire must do, and improves grip. Balancing the weight evenly in the car provides an even distribution for balanced response to dynamic changes. Intentionally biasing the weight distribution to a specific side or quarter of the car might be advantageous for the net results of grip under dynamic conditions. (You might want to shift weight to the right rear of the car at a track with a lot of high speed right turns to reduce the load of the left front tire, and increase the load of the right rear tire for more balanced grip during cornering).

Static weight distribution can be changed by physically moving objects within the car (relocating the battery, removing items from the car, etc.). It can also be changed by altering the ride height of individual corners of the car. Lifting a corner alters the CG location and in effect, increases weight distributed to the opposite coner. This is done primarily through coil-over shock & spring setups and by spacers.

Relocating the Center of Gravity

Relocating the CG to a more favorable position can also reduce weight transfer. Without getting into the engineering of it all, the location of the center of gravity acts as lever handle. We know from basic physics that a lever can be used to increase force and work. If the center of gravity is very high, there is essentially a long lever in the car. During braking, accelerating, or cornering, the G forces are amplified by this lever created between the CG point and the tire contact patches. The further apart they are, the greater leverage, and the greater the weight transfer.

With a given car, you can't change the CG location dramatically, but you do have some ability to affect the center of gravity enough to make major improvements to the car's handling performance. If you're willing to sacrifice some comfort, convenience, and looks, you can subtract and relocate weight to affect the front to rear and the side to side weight centers. You can also alter the CG height by lowering the car with lowering springs, lower sidewall tires, and to a smaller degree by adding removing, or moving weight in the car.

Once you have selected your car, there's nothing you're likely going to do to change the wheelbase or track width. You might increase track width a little with wider wheels though.

Weight Transfer and Suspension Components

We said that springs, shocks, etc. cannot change the amount of weight transfer. What they can change is the rate of the weight transfer, and the impact weight transfer has on the suspension geometry caused by dive (braking), squat (accelerating), and body roll (cornering).

The rate of weight transfer impacts the responsiveness of the car to driver inputs. The faster the weight transfer, the quicker the response. This allows the driver to have greater control of the car. However, a faster weight transfer requires greater skill of the driver. Smoothness and quicker reaction sensitivity to the tire traction are needed. It turns out that shocks have the largest impact on rate of weight transfer. The stiffer they are, the faster the tranfser.

The impact of weight transfer on suspension geometry has to do with maintaining as large and flat a tire contact patch as possible. When the body rolls, dives, or squats as a result of weight transfer, the geometric relationship of the suspension components to the body and the wheel changes the shape of the contact patch. For the unloaded tires, the patch size will be reduced. This effect must be minimized. Changes in shocks, springs, anti-roll bars, and wheel alignment are made to maximize the tire contact patches of all tires during the dynamic changes of weight transfer.

In the next articles which cover the major components, we'll define what those components do, and how they can be manipulated to control the effects of weight transfer and the changing tire contact patch size.

Read Next Article (Tire Pressure)

Tags: Performance Tuning, High Performance Suspension, Lowering Springs, Coil Over Shocks, Sway Bar

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The top figure shows a vehicle with the weight as measured at each tire while the car is at rest (The grey represents the tire, the black patch is the actual tire contact patch with the road).

During braking, weight transfer increases the load at the front tires and reduces the load at the rear tires. Accelerating of course would be the opposite.

During cornering, the loads for each tire could be quite different. The goal of all "handling modifications" for your car is to optimize the the corner weights and tire contact patches to be as even as possible during cornering (assuming relatively equal left and right turns on the road course).

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