Creative Car Control Handbook
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Basic Dynamics of Vehicles
Advanced Handling »
We have seen that grip changes dramatically with vertical loading of the tyre and that self-aligning torque changes with slip angle. We have the ability to feel the slip angle at the front of the car because we are connected to the front wheels by the steering wheel.
As we enter a corner, the car understeers slightly and we feel the steering torque increase up to a peak. This is the point of maximum grip at the front of the car, turning the wheel further has little effect on the turn in. As we generate grip at the front (and develop lateral movement of the chassis), the rear tyres develop grip and they settle in to pushing the rear of the car round the corner.
If the rear grip is exceeded the chassis moves laterally to the outside of the corner and this affects the slip angle at the front of the car (just as the front affects the rear as lateral movement of the chassis takes place). The difference here is that we can detect the rear lateral movement in the steering feel. This means that we can feel grip at the rear as it is translated to the steering feel.
The net result is that we can feel frontal grip on the entry to the corner and we can feel rear grip in the transient and especially on the exit if the vehicle goes into oversteer. When the vehicle is pointed where we want it to go we do not need steering (it causes drag and slows the response of the front of the car down) so we need to get rid of steering as soon as possible. If the vehicle oversteers getting rid of steering is crucial.
We have all developed a model of where to point the vehicle on the way into a corner and it works very well. When we start to oversteer the front wheels are thus pretty much pointed in the right direction for the front to go where we want it. At that point we can start to remove steering input and the first thing that happens is that the slip angle is reduced to zero (even though the front wheels are pointed away from centre position we can have a zero slip angle at the tyre). At the same time steering torque reduces to zero as the steering is removed.
It transpires that if the rear of the car is sliding laterally the slip angle that this induces in the front tyre generates a corresponding self-aligning torque in the tyre that can be detectable as a torque in the steering wheel.
Allowing the steering wheel to move with the torque induced in this way will keep the front wheels pointed in the direction that they were originally pointed. The steering wheel may move a considerable amount in these circumstances. Letting go of the steering wheel altogether will allow maximum freedom of movement of the steering wheel in response to self-alignment torque generated in the front tyres by lateral movement of the rear of the vehicle.
Letting go altogether also provides for minimum interference with the process of keeping the front wheels pointed in the right direction (i.e. the original direction that they were pointed in before oversteer set in).
NB. I am not suggesting that we all go round letting go of the steering wheel at every opportunity but as a learning tool it is very important to recognise that the steering torque is capable of moving the wheel appropriately. This can then be translated into a steering technique that pays exquisite attention to steering wheel torque in response to grip near to the limit of adhesion. This in turn provides the basis for ultimate car control.
As the lateral movement of the rear of the vehicle increases several things happen or may in combination;
We have seen that steering torque provides information regarding slip angles and thus grip at both ends of the vehicle. We have seen that tyre performance (and grip) is dramatically affected by vertical loading. The combination of optimising grip for steering and weight distribution is a relatively simple concept - more grip at the front comes with more weight on the front. Lifting off the throttle provides a more "pointy" unstable vehicle.
The problem with all this is compounded by the basic vehicle requirements for stability at high speed and instability to turn corners at moderate and lower speeds. We have to manage the way in which we drive to accommodate these needs.
High speed cornering is thus very different from low speed cornering and it is the weight distribution that makes all the difference to stability. High speed corners are taken generally on throttle for maximum stability with weight to the rear minimising the possibility of high speed oversteer (at the same time bearing in mind the possibility of breaking traction with too much throttle in high performance vehicles). Speed needs to be controlled accurately prior to corner entry in high speed corners so that vehicle trajectory is set up to be able to negotiate the corner without having to lift off (with its inevitable effect on vehicle stability) in the middle of the corner.
Low speed corners are generally taken off-throttle with maximum steering performance being the important criteria and this gives rise to instability of the rear which has to be managed with accurate steering and careful throttle application.
Optimising the use of brakes is an interesting area. At high speeds on good road conditions the brakes are generally incapable of generating enough power to lock the wheels. At lower speeds this is normally not the case. Maximum braking effort at high speeds will result in best stopping, as the speed reduces so too does the requirement to extract maximum performance from the brakes and less brake pressure is required to avoid locking the wheels.
Best practice would suggest us braking at threshold (the point at which the brakes start to lock). This means high brake pressures at high speeds reducing as speed falls off. In low grip situations we have the possibility of locking wheels more easily and we have to allow for this in our operation of the braking system.
Braking is optimal when the vehicle is flat and level that is not cornering or on a weird camber. Braking generally happens in combination with other hazards or events and provides transient phases between these events. It is thus important to bear in mind what happens after the braking and phase one activity into the next. ABS systems do not permit better braking. ABS systems allow a degree of control in cornering and braking combinations. They do not provide as much ultimate performance as a well driven well set up car. In general ABS systems permit chassis engineers to optimise braking performance for the general population.