Weight transfer

In automobiles, weight transfer (often confused with load transfer), refers to the redistribution of weight supported by each tire during acceleration (both longitudinal and lateral). This includes braking, or deceleration (which can be viewed as acceleration at a negative rate). Weight transfer is a crucial concept in understanding vehicle dynamics.

A) CENTER OF GRAVITY
Weight transfer occurs as the vehicle's center of gravity (CoG) shifts during automotive maneuvers. Acceleration causes the sprung mass to rotate about a geometric axis resulting in relocation of the CoG. Front-back weight transfer is proportional to the ratio of the center of gravity height to the vehicle's wheelbase, and side-to-side weight transfer (summed over front and rear) is proportional to the ratio of the center of gravity height to the vehicle's track.

Liquids, such as fuel, readily flow within their containers, causing changes in the vehicle's CoG. As fuel is consumed, not only does the position of the CoG change, but the total weight of the vehicle is also reduced.

By way of example, when a car accelerates, a weight transfer toward the rear wheels is said to occur. An outside observer can witness this as the car visibly leans to the back, or "squats". Conversely, under braking, weight transfer toward the front of the car will occur. Under hard braking it is clearly visible even from inside the car as the nose "dives" toward the ground. Similarly, during changes in direction (lateral acceleration), weight transfer to the outside of the direction of the turn occurs.

B) TRACTION
Weight transfer causes the available traction at all four wheels to vary as the car brakes, accelerates, or turns. For example, because of the forward weight transfer under braking, the front wheels do most of the braking. This bias to one pair of tires doing more `work' than the other pair results in a net loss of total available traction. The net loss can be attributed to the phenomenon known as tire load sensitivity.

An exception is during positive acceleration when the engine power is driving two or fewer wheels. In this situation where all the tires are not being utilized weight transfer can be advantageous. As such, the most powerful cars are almost never front wheel drive, as the acceleration itself causes the front wheels' traction to decrease. This is why sports cars always have either rear wheel drive or all wheel drive (and in the all wheel drive case, the power tends to be biased toward the rear wheels under normal conditions).

C) ROLLOVER
If (lateral) weight transfer reaches the tire loading on one end of a vehicle, the inside wheel on that end will lift, causing a change in handling characteristic. If it reaches half the weight of the vehicle it will start to roll over. Some large trucks will roll over before skidding, while passenger vehicles and small trucks usually roll over only when they leave the road. Fitting racing tires to a tall or narrow vehicle and then driving it hard may lead to rollover.

D) MOMENT OF INERTIA
One other means of applying force to the tire contact patches is a result of the moments of inertia. Measuring the moments of inertia in a vehicle is quite difficult, but is routinely done by many organizations. A lower moment of inertia means the vehicle's mass is concentrated more closely around the center of mass. A higher moment is the opposite. Imagine a car sitting on a rotating platform positioned so its CG is directly above the rotational axis. Now push sideways on the front tires and the car begins to rotate on the platform. The forces are only applied at the end of the vehicle that is doing the steering.

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