Weight Transfer Part 1 - It's Not Doing What You Might Think

Uncategorized Jun 15, 2018

Racing drivers often talk about weight transfer – weight transferring forward under brakes, rearward under acceleration and from inside wheels to outside wheels when cornering.

Weight transfer concepts are hugely important in suspension set-up.  Weight transfer across the car in cornering, is the basis of the most important tool we have in suspension set-up  – using weight transfer distribution between the front and rear axles to adjust the balance of the car.  But the driver is unaware of the weight transfer distribution itself - only the feel of the car resulting from the balance adjustment.

Weight transfer is the result of acceleration, braking or cornering.  If you accelerate, brake or corner harder, you transfer more weight. 

For weight transfer to be useful to the driver in controlling the car, the driver would need to feel the weight transfer, or something related to it.

Weight transfer in pitch and roll is just weight transfer coming off one pair of wheels, and adding to another pair.  It's evident the driver can't feel the weight transferring. 

However, most racing drivers quite reasonably are of a view that pitch and roll motions are important feedback to the driver in controlling the car.  My intention in this 3-Part series is to show you that our perception of the motions of the car directly related to what's happening at the tyres, is far more important.

What Can the Driver Feel?

It’s accelerations (or motions) of the car that the driver feels. Yes, roll and pitch motions are felt by the driver.  But the most useful motions for driver control of the vehicle happen as a direct result of forces acting at the tyre contact patches. 

(Roll motion and pitch motions of the car can be equally felt by the driver, but are not directly related to forces at the tyres.)

The driver perceives accelerations resulting from the linear forces of longitudinal acceleration and braking and the lateral force of cornering.  These are substantial forces on the driver’s body, easily felt - the same G-forces we record on the data logger.

In cornering, the linear (straight line) force of cornering is felt on the drivers body as the lateral G building up in corner entry and letting go in corner exit.

There's also this small rotation of the car about its own vertical axis, a rotation the driver can feel under the right conditions.  It's the most important feedback the driver gets.     

Due to the tyre slip angles building up in corner entry, the body of the car rotates slightly on the tyres, creating the nose in attitude of the car we see in cornering.

This subtle rotation is an indication to the driver of the rate of turning the corner, the rate cornering grip is building up.  It's the "rotation" the driver can feel under the right conditions:
1. Rotating more than expected.  Counter steer may be required.  You can get this warning before the tyres lose grip.
2.  Rotating less than expected.  You can push harder.

This key sensing mechanism is instant feedback on what is happening with the grip at the tyres.  What other sensing could there be that allows you to respond in the blink of an eye when the car wants to step out under power, for instance?

Pitch and roll happen as a result of the forces of acceleration, braking and cornering acting at the centre of gravity, causing weight transfer forward or back, or across the car. 

If the centre of gravity was at ground level, there would be no weight transfer.  But seeing the centre of gravity is obviously somewhere above ground, there must always be weight transfer.  (Even so, a lower car, lower centre of gravity is always better, subject to satisfactory suspension geometry and suspension travel.  The lower car will always corner faster.)

The pitch and roll motions of the car are the result of weight transfer.  Unlike the motions of the car due to the forces at the tyre contact patches, they are only indirectly linked to the grip at the tyres. 

Could the pitch and roll motions be useful in controlling the car?  We can say with certainty they’re not essential – a go-kart or roll stiff formula race car does not roll at all on the suspension – yet the driver can control the vehicle perfectly well.

In Weight Transfer Part 2, we'll look at the way the language of weight transfer is used in race driving - with the "Formula 1 drivers coach", Rob Wilson.