Weight Transfer Part 3 - It's all about the ROTATION


Weight Transfer Part 3:  How Does the Driver Control the Race Car?

This is the final part in our “Weight Transfer” series.  See Part 1 and 2 home page /blog

Summarizing  what we discussed in Parts 1 & 2:

Nowadays, “weight transfer” thinking is commonly used to describe what the racing driver is doing.  The driver is said to manage or control the weight transfer.  The “rate of weight transfer” is considered important.  And as discussed in Weight Transfer Part 2, the driving coach Rob Wilson talks weight transfer almost exclusively when he describes what he is teaching to drivers.

Talking “weight transfer” with respect to race driving is a fairly recent phenomenon.  Most of the authors of books on handling don’t talk about  weight transfer  in the context of the driver controlling the car.  

My major sticking point with weight transfer thinking is that drivers cannot feel weight transfer.  Weight transfer is just weight, or vertical load coming off one or more tyres, and being transferred to others not something you feel.

However, as a result of weight transfer, the car will roll and pitch.  Could roll and pitch be the mechanism that allows drivers to feel the weight transer?

Roll and Pitch Motions as Indicators of Weight Transfer

It may be that if you are driving a road car on a race track that the roll and pitch motion will be of value to you.  You could well be taking cues from these motions in controlling the car.

But do you need them? 

Go karts don’t have suspensions, and therefore don’t pitch and roll on the suspension at all.  No one is suggesting that karts are too hard to drive, and that a prospective driver should learn in a car first, so they can feel the pitch and roll.  Similarly, modern formula cars, such as Formula Ford and FIA Formula 4, are super-stiff in roll, and pitch only a little.  Yet any competent driver can hop in one of these cars and happily drive it at speed around a race track.

However, many production cars in racing do roll and pitch quite noticeably.  And it is true roll and pitch motions will slow down the response of the car, potentially making it more manageable. 

But overall, it works out that race car designers wish to restrict roll and pitch to improve the performance of the race car.  The performance outcome for the car is paramount, and there is no counter argument that the driver needs to feel roll and pitch.

We don't have a situation where the driver says I need to soften out the car a little bit so I can feel it.  Even in the wet there is no need to let the car roll more, or whatever, like we thought in the old days. 

Today, we are almost totally concerned about balance.  We look to change the balance of the car whenever it is perceived that the car is too nervous (oversteer, too agile) on the one hand, or on the other hand, could turn in better (understeer, too much stability).

If we get too pre-occupied with weight transfer, we can get stuck on the idea that race car handling is based on some very complicated combination of the grip at the four tyres. 

Fortunately handling in reality is much simpler than that.  There are no mysteries.

It’s All About the Rotation

Racing drivers generally, especially professional drivers, recognize feeling the motions of the car as being important – most drivers like to be strapped firmly into the seat, for instance, to aid sensing.

The feedback for the driver comes from the motions of the car resulting directly from the forces acting at the tyre contact patch.  

The key sensing for the driver to control the race car is this subtle rotation of the body of the car, while the tyres continue to grip the road, 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. An indication how hard you are cornering - are you on the limit?  Or could you 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?

Rotation: In the graphic, we show firstly cornering with the steering wheel fixed, and secondly with the steering wheel in motion.

  1. Rotation of the car around a fixed point - the turn centre inside the corner:

    Here’s our race car traveling in a constant radius corner.  The Driver holds the steering wheel in a fixed position.  We’ve got the lateral G force of cornering pushing sideways on the driver (the red arrow pointing at the turn centre.) 

    As our car rotates further in the corner, the driver does not feel any rotation – only the same lateral G force of cornering.  The car has rotated in the corner, but there has been no rotation of the car around its own axis.

  2. Rotation of the car around a vertical axis through the centre of gravity:

    In the sketch, you see the lateral forces of cornering at front and rear axles, FF and FR.  These forces transpose to the centre of gravity, giving us the lateral force of cornering (FF + FR) or lateral G. shown in diagram 1. 

    When we turn the steering wheel, we create a lateral force at the front axle, FF, which is quickly followed by a balancing force at the rear axle, FR.   There is a time delay, known as the "phase delay".

    While the steering wheel remains in motion, there is this small turning moment at the front of the car, turning the car further into the corner.  It's represented by the small red arrow in the diagram.  But it only continues while the steering wheel moves and the rear rotation is catching up with the front rotation.  Once the steering wheel stops, and rear has caught up, the rotation feeling goes away.

Rather than “controlling the rate of weight transfer”, it can truly be said the driver controls the rate of turning, meaning the rate at which you are building up cornering force, based on your perception of the rotation.

In our on-line training course, "A Physical Understanding of Race Car Handling" , we study the detail of what is happening here, together with our other key diagram showing the slip angles at the tyres. 

These simple, yet powerful explanations are exclusive to us.  You cannot read about this in any books on handling,  Neither have we "mind engineered" this stuff.  It's supported by the science - all based on vehicle dynamics principles.

Feeling the Rotation – Driving on the Road

You’re driving your road car through a series of corners.  Driving at roughly constant speed, and cornering fast enough, you can feel the lateral G building up in corner entry, maintaining during the mid-corner and letting go in corner exit.  Lateral G is greater for the tighter corners, less for the more open corners.

In corner entry, it might take a second or more turning the steering wheel, followed by another time period, with the steering wheel held steady (the mid corner).     

In the mid corner with the fixed steering, it doesn’t matter how hard you drive, you won’t feel any rotation.  You’d be forgiven for thinking there is no rotation to be felt in cornering.   

However, with the steering wheel in motion, there is always some rotation of the car on the tyres in the corner entry.  But on the road, as a rule, the motion won’t be sufficient for you to feel it.  (The rate of rotation needs to be around 3 degrees per second or more, for the driver to feel it.)  

If you make a quick movement of the steering wheel on the road and feel the response of the car, you can feel the rotation.  In racing, where you’re building up cornering force from zero to maximum lateral grip in corner entry, the rate of rotation will generally be enough for the sensitive driver to feel it.

I think the implications of this are huge.  If you take this as a starting point, then you can build a complete mindset about handling and suspension set-up, starting from what’s happening at the tyres, right through to the suspension set-up and vehicle response. 

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