Weight Transfer

Acceleration – rear weight transfer
Advantages
Disadvantage
Deceleration – forward weight transfer
Advantage
Disadvantage
Transitions in weight transfer
Steering

When driving a car, weight transfer can occur in the longitudinal (front to back) and lateral (side to side) directions. Depending on how and when the weight transfer occurs, it can be a help or a hindrance when on the road or track.

There are three ways that weight transfers can occur:

• Acceleration
• Deceleration
• Steering

Acceleration – rear weight transfer

When you accelerate, the weight of the car is thrown backwards (see Figures 1 and 2). This causes the rear suspension to compress slightly and increases the available grip at the rear tyres (see The physics of grip for more information).

Figures 1 and 2: Heavy acceleration - the darker blue tyres indicate more grip

Advantages of rear weight transfer

1. Pulling away in a rear wheel drive car

If you're trying to get a good start, rear weight transfer will be to your advantage in a rear wheel drive car, as the resulting increase in rear grip will provide you with more traction and reduce any wheelspin.

In a front wheel drive car, however, you're more likely to get wheelspin off the start, so a smooth throttle is more important.

2. Preventing oversteer in a front wheel drive car

Oversteer is caused by a lack of traction at the rear wheels, but if you accelerate in a front wheel drive car as oversteer is occurring, the resulting rear weight transfer can increase the available grip at the rear wheels and aid recovery.

Note: Oversteer in a rear wheel drive car is rarely helped by acceleration, as this will increase the traction demands of the rear wheels, and leave less available grip to oppose the sideways motion.

Disadvantage of rear weight transfer

Can cause understeer

Understeer results from a lack of traction at the front wheels. If you're accelerating into a corner, the levels of grip at the front wheels will be reduced due to the rear weight transfer, and be more prone to continuing in a straight line despite your steering inputs.

Deceleration – forward weight transfer

Cars can brake much harder than they can accelerate. Weight transfers under braking are thus more likely to affect the balance of the car (see Figures 3 and 4).

Figures 3 and 4: Heavy braking - the darker blue tyres indicate more grip

Advantage of forward weight transfer

Prevention of understeer

If you're hurtling towards a corner, turn the wheel and find yourself ploughing straight on, you've probably approached too quickly. But all might not be lost - easing off the throttle will result in a forwards weight transfer which will increase the available grip at the front wheels. Hopefully this should correct your course. If you're already off the gas, then a light dab on the brakes should help.

Disadvantage of forward weight transfer

Reduction of traction at the rear wheels

If you stamp rapidly on the brakes, you'll find that the rear wheels (in a car without any clever technology) will tend to lock up first. This is the reason almost all cars have smaller brakes at the rear. Despite this fact, the rear wheels are still the most likely to lock when using the brakes aggressively. Braking in a corner should be avoided for the same reason, as this can result in oversteer.

Transitions in weight transfer

To get the best times on the track you need to be either accelerating or braking - any coasting means you're not going as fast as you could. When transitioning between accelerator and brake you'll get exaggerated weight transfers which can further upset the balance of the car.

Figures 5 and 6 - Transitional weight transfers caused from heavy acceleration to braking

Steering

It's important to consider sideways weight transfers when cornering. If you turn into a corner progressively rather than 'chucking it in', this will transfer the weight to the outside wheels in a gradual manner. As the outside wheels provide most of the cornering effort, the resulting progressive increase in grip can help you take corners at a greater speed before traction is lost. However, if you throw the car into a corner, the sudden weight transfer can unsettle the vehicle and cause the wheels to break traction suddenly.

The moral again ladies and gentlemen is smoothness.

Figures 7 and 8: Sideways weight transfer when cornering - the darker blue tyres indicate more grip