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

Uncategorized Sep 12, 2024

Your Feel for Chassis Rotation in the
Corner Entry AND the Corner Exit, the so-called "transitions"


In my article Rotation - Your Primary Sense of Feel in Controlling the Race Car we showed you how your primary sensitivity in controlling the race car is your ability to feel and respond to the oversteer rotation motion of the chassis, whether that is the tiny initial rotation of the chassis due to tyre slip angle or the better recognized rotation due to oversteer. 

To be sure, 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. 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 in the fast corners - to make the car at least somewhat capable of self-correcting small oversteer events.) 

Max Verstappen recently said, "I've never experienced a fast car which has understeer in my life in any category." Of his 2022 F1 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 demonstrate how the oversteer feeling for balance can be your primary control mechanism in both the corner entry and the corner exit. 

Just a quick recap on the advance warning you get from the rotation due to tyre slip angle:

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 approaching the apex of the corner, or when you are following another race car.)

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.)

The chassis rotation due to slip angle acts as a warning of oversteer about to happen, BEFORE the rear tyres DO start to slide.

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.

Whether or not there is improved grip at the front when trail braking does not matter to the driver. Not something the driver can feel.

Our focus should be on what we CAN feel - whether the car is rotating faster or slower while grip is building up at the tyres in the corner entry, before the car takes a set in the corner:

Rotating too fast? The car could go into oversteer
Rotating too slow, or can't feel any rotation? The car could be understeering, going too slow, or you have used incorrect trail braking technique (or really bad set-up).

 Our attention should always 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 the 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 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 is 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. Zero steer is best described in print in Paul Gerard's book  "Optimum Drive".

Summary

Your feel for rotation allows ultimate control of the race car in both the corner entry and the corner.

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 recognized 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.

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Be amongst the first racing drivers to learn these new techniques and the race car dynamics confirming rotation as your primary feel in both the corner entry and the corner exit.

Join my new on-line live training program, "The Science of Race Car Driving and Suspension Set-Up" before it is sold out for the first co-hort training.