Load transfer

In automobiles, load transfer is the imaginary "shifting" of weight around a motor vehicle during acceleration (both longitudinal and lateral). This includes braking, and deceleration (which is an acceleration at a negative rate). Load transfer is a crucial concept in understanding vehicle dynamics.

Often load transfer is misguidedly referred to as weight transfer due to their close relationship. Load transfer is an imaginary shift in weight due to acceleration, in which mass inertia causes a torque to appear whose forces are the tyres' traction forces at road level, and the equal but opposed force of the mass inertia located at the centre of gravity (CoG) where the arm is the distance from the road surface to the CoG. The difference is that weight transfer involves the "actual" (small) movement of the vehicle CoG relative to the wheel axes due to displacement of liquids within the vehicle, whereas load transfer is conceptual. All result in a redistribution of the total vehicle load between the individual tires.

The major forces that accelerate a vehicle occur at the tires' contact patches. Since these forces are not directed through the vehicle's CoG, one or more moments are generated whose forces are the tyres traction forces at pavement level, the other one (equal but opposed)is the mass inertia located at (CoG) and the arm is the distance from pavement surface to CoG. It is these moments that cause variation in the load distributed between the tires. Often this is interpreted by the casual observer as a pitching or rolling motion of the vehicles body. Although it is interesting to note that a perfectly rigid vehicle without suspension that would not exhibit pitching or rolling of the body would still undergo load transfer. However, the pitching and rolling of the body adds some (small) weight transfer due to the (small) CoG horizontal displacement with respect to the wheels axis suspension vertical travel and also due to deformation of the tyres i.e contact patch displacement relative to wheel.

Load transfer affects traction available at each wheel to accelerate a vehicle in any direction. "Ideally", for a given vehicle load more total traction will be available if the load is shared equally between all the tires, than if any single tire carries more of the load. This tire characteristic is attributed to a phenomenon known as tire load sensitivity.

Lowering the CoG towards the ground is one method of reducing load transfer. As a result load transfer is reduced in both the longitudinal and lateral directions. Another method of reducing load transfer is by increasing the wheel spacings. Increasing the vehicles wheel base (length) reduces longitudinal load transfer. While increasing the vehicles track (width) reduces lateral load transfer. Most high performance automobiles are designed to sit as low as possible and usually have an extended wheel base (length) and track (width).

See also

* Car handling

References