PART 3: Perfecting Your Car Control: Trail Braking and "Zero Steer"

Two Sides of the Same
Chassis Rotation Feeling

In Part 1 of this series, "How Do You Feel What the Race Car is Doing?", we showed you how your primary sensitivity in controlling the race car is your ability to feel and respond to the oversteer rotation motion.   

Your highest sensitivity for balance is feeling the rotation always in the oversteer direction. (There is no equivalent feeling for understeer. Driving the car to the understeer limit of grip feels wrong and is slow anyway.) 

If we have a well set-up fast race car, the expectation is that we will be continuously driving the race car at the oversteer limit of grip. As described in Part 1, you can manipulate the balance of the car to reduce understeer and then push until you feel you are at the oversteer limit of the rear tyres. (Some steady state understeer is needed in the setup  to give the race car sufficient stability - to make the car at least somewhat capable of self correcting small oversteer events.) 

In fact, Max Verstappen recently said, "I've never experienced a fast car which has understeer in my life in any category." Of last years car (the RB 18) that apparently did have some understeer, he said, "A car cannot be fast with understeer."

In this article, I am using the examples of "trail braking" in the corner entry and what's known as "zero steer" in the corner exit to demonstate how the oversteer feeling for balance can be your primary control mechanism. 

Just a quick recap from Part 1: As you turn the race car into the corner, there is a tiny abrupt tweak of the chassis in the oversteer direction as the rear tyre slip angles go from zero to peak slip angle and maximum cornering grip. This is the racing driver's primary feel for the balance of the car , a message direct from the tyre contact patch.

As a result, the race car adopts this drift angle, or nose in attitude, relative to the direction of travel, known as the vehicle slip angle or body slip angle. (You can see the body slip angle in photos of race cars appproaching the apex of the corner.)

This chassis rotation happens due to elastic twist between the contact patch and the rim, as the rear tyre slip angles build up. (Nothing to do with the tyres losing grip.)

Trail Braking and Rotation 

There is a lot of confusion amongst racers as to how rotation occurs when trail braking into the corner and whether weight transfer might be a factor in helping the car to turn.

In the dynamic situation, when turnng the steering wheel, there is a rotational force turning the car into the corner known as the yaw moment. 

To describe what is happening, it is helpfull to describe increasing or decreasing grip requirement at the tyres as increasing or decreasing tyre slip angle (or just "slip angle"). It is important to note that a tyre adjusts it's slip angle to support the required lateral grip, not the other way round.

On turn in, we have inceasing yaw moment. As a result, slip angle is increasing at the front tyres, followed by the increasing slip angle at the rear as required to stabilize the car, i.e. rear slip angle effectively acts to remove  or reduce yaw moment.

So, driving at or around the oversteer limit of grip, i.e. with increasing rear slip angle, front slip angles must be steady or decreasing. Otherwise the car will spin out.

It't all about feeling what the rear of the car and the rear slip angles are doing. We don't have to consider the weight transfer, except to say that reduced weight on the rear will reduce the peak grip available at the rear somewhat.

Our attention should be on the rear of the car where, as we have established, it is the generation of rear tyre slip angles that causes the chassis rotation he driver can feel.

The Phases of the Corner 

The corner entry and the corner exit are the so-called “transitions” or "transients" as opposed to the mid corner where the race car is said to be in "steady state".

Corner Entry – transitioning from straight ahead to cornering, increasing rate of turning, increasing yaw rate (measured in degrees of rotation per second.)
Mid Corner -  fixed yaw rate (even if only for an instant at the point where corner entry finishes and corner exit starts).
Corner Exit – transitioning from cornering to straight ahead, reducing rate of turning, reducing yaw rate.

Blending the Controls in the Transitions

Racing drivers naturally blend the controls to smooth the transitions in corner entry and exit. They are balancing the race car without necessarily being consciously aware of it.

By blending the controls, they combine lateral and longitudinal grip at the tyre contact patches, as per the traction (or friction) circle diagram, giving better overall grip at the limit.

That said though, I think it is the insights around blending the controls and the resulting influence on the balance of the race, that should be foremost in our thinking.

In both the corner entry and the corner exit, the racing driver can modify the speed and balance of the race car by Trail Braking in the corner entry and Zero Steering in the corner exit.

Trail braking: (Description below is an overview - not comprehensive.)
Braking while turning into the corner. Works for slow and medium speed corners. To reduce understeer, trail brake more (without going in too hot with too much brake and steering). If the car is too pointy (oversteer) then only limited or no trail braking possible.   

Zero steer: 
Exit the corner with reducing steering whilst accelerating and still turning. The balance of the race car is such that you catch any imminent oversteer without having to go past centre with the steering wheel and get to the throttle quicker, reducing drag at the tyres and balancing the car perfectly for a super-fast exit.

Summary

The techniques of trail braking and zero steering are co-dependent. By trail braking effectively, you maintain the race car balanced and on the limit in the corner entry. In so doing, you present the race car to the corner exit with just the right degree of feel for oversteer so you can “get the steering out of the car” (reduce steering but not past the straight ahead), and get to full throttle early as possible.

What’s involved here is the racing driver’s little recognised ability to feel fast-approaching oversteer in advance and so maintain very fine control over the balance of the race car.

The skill of the racing driver is in continuously adjusting the balance towards the oversteer direction to maintain the race car driving at the limit of grip throughout the corner. You don’t have to think about your speed - whether you can go faster – just manipulate the balance of the car to avoid any understeer and push until you feel you are at the limit of grip of the rear tyres. That will be the fastest you can go for the line you have chosen in the corner.

Join Us at the Racing Car Technology Car Control Workshop 2023 (Click the Link)

 In the workshop, you'll learn a complete end to end explanation of car control and the mind set you need. Gain confidence in your ability. With this mindset and knowledge on how to improve your skill, endless improvement of your car control ability is within your grasp.

Car control is not complex or hard to follow. But you do need to understand the basics - the yaw rate, the yaw moment and the body slip angle, and how all of these are related to the tyre slip angle

You’ll have access to everything we have developed in relation to car control, including our current car control online training course, "The Physics of Car Control and Race Car Handling".

As a racing driver, by understanding the mechanism behind your car control ability, you can optimise your performance and build confidence in your ability, both in driving the race car and in making improvements to the car set-up.