Creative Car Control Handbook
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The Fundamentals
Basic Dynamics of Vehicles »
The handling performance of a car is a function of the car, the driver's ability, and the performance of the tyres. The driver's performance is crucial. It's also the most flexible and easily adjustable element of the three.
Driver behaviour and thus performance wholly depends upon the beliefs and values held by the driver. An appropriate set of beliefs concerning the way in which tyres perform can play an important role in the realisation of optimum performance.
Modern high performance tyre construction is an inaccessible black art maintained by a small number of secret sorcerers beavering away in the world's tyre research laboratories. There are, at the same time, inescapable similarities in construction, performance and use that are the basis of a useful model that drivers can use to gain valuable insight into optimising their performance.
The tyre is constructed around the bead, which is effectively a steel hawser that runs circumferentially round the inside of the wheel rim. There are two such beads, one fitted to the inside wheel rim, the other on the outside. The beads are connected together by very strong man-made fibres, (typically nylon, rayon, Kevlar etc.) The weave of these fibres takes them round one bead, round the tread section and round the other bead. The fibres are not actually radial to the bead but set at angles of typically 2-9 degrees from true radial.
The woven carcase is augmented by various types of man-made rubbers in the sidewall and tread areas. These form the basic structure of the tyre, preventing air from leaking and supporting the rest of the structure.
The section under the tread of the tyre is generally fitted with a steel or fibre reinforcing circumferential belt which provides dimensional stability and puncture resistance. Dimensional stability is important if the gearing and speedometer functions are to remain constant and excessive tyre growth at high speed as used in dragster tyres (having no belt) would result in damage to the bodywork of road vehicles.
The tread itself is moulded onto this carcase. Modern high performance tyres are often constructed with asymmetric treads which have different profiles for the inside part of the tread compared with the outer regions. Typically the outside of the tread is made up of large tread-blocks which have more resistance to wear and less ability to clear water from the road surface. The inside tread tends to have more channels for the water to drain into and these water channels lead to larger "canals" which store the water which is squeezed across the tread block as it makes contact with the road surface.
The wheel drives the tyre by rotating the bead which in turn causes the forward facing fibres in tension, dragging the tread in the direction of the drive. When braking occurs the forward facing fibres relax and the tension is applied to the rearward facing fibres dragging the tread in the opposite direction.
Tyres don't go exactly where they are pointed unless they are going in a straight line.
Slip angles relate to the degree of angular distortion which results from the twisting of the bead relative to the footprint.
If you consider a plan view of the wheel and tyre combination, the wheel can move relative to the footprint. It does this when we turn the steering wheel. The wheel is turned by the steering mechanism and the tyre distorts slightly, this results in the wheel pointing in a slightly different direction to the footprint. The amount of angular distortion between the direction of the wheel and the direction of the footprint is the slip angle. When the tyre is distorted in this way it generates a side force which is what tends to push the vehicle round a corner. Generally increasing the slip angle results in more cornering force being generated by the tyre. And there are always limits. Slip angles vary from tyre to tyre and rarely exceed 15 degrees before the tyre starts to slide and smoke.
As drivers we generally claim that we are in charge of turning the wheel. This generates slip angles at the front of the car, and we know that slip angles are the only way of generating side forces that push the fronts of our cars round corners.
Most of us never consider what pushes the rear of the car round corners.
Since it must be a slip angle that causes a force to be generated there must be something that turns the rear wheel relative to the rear footprint.
This turns out to be the chassis.
The front of the car moves laterally in response to steering input from the driver, this causes the chassis to rotate around some point rearwards and it is this lateral movement that generates a slip angle in the rear wheels. This is the slip angle that generates the force that pushes the rear of the vehicle round the corner.
One of the most important areas in understanding tyre (and thus vehicle) performance concerns the sensitivity of the tyre in respect of vertical loading. When we push down on an object which is sliding we expect the sliding force to increase, or the sliding to slow down (from time to time we all stop things moving by pushing down on them.)
It's the same with tyres; push down more and you get more grip. In general this is true. At the same time, there is a law of diminishing returns, as we increase vertical loading on a tyre it can generate more grip up to a point when the tyre becomes "saturated". This is due in part to the way in which the tyre keys into the road surface and also to the way in which the tyre material adheres to the road surface.
When a tyre carries a light load the grip changes are very large with changes in vertical load, this sensitivity decreases to zero as the tyre nears its maximum loading (saturation).
Taking an extreme case of a motorcycle accelerating it is possible (indeed it's a regular event) that the front wheel lifts clear of the ground as the effect of inertia causes all of the weight to shift off the front wheel onto the rear. In this instance the front wheel has no ability to generate a side force to effect cornering (because it's off the ground).
Porsche 911 GT3 RS has extreme weight distribution with static loadings near to 70:30 rear to front. The rear wheels are excessively wide and are capable of very large cornering loads when suitably loaded in a vertical sense. High vehicle acceleration rates result in large vertical loadings, which afford high degrees of traction. Braking into corners has the opposite effect; as weight shifts forward rear loadings become so low that the vehicle has very little rear grip and oversteer is prevalent in tight corners near to the limit of grip. Nice if you want to turn and not nice at all if you're not able to control the slide.
Managing dynamic weight distribution near to the grip limit is crucial and understanding the importance of timing and subtlety of throttle application is vital to success.
When we distort the tyre by applying a slip angle the tyre fights back; in plan view this is a torque. Engineers call this the "self-aligning torque" of the tyre. The self-aligning force generated in the tyre can be detected at the steering wheel. In practice we are able to feel slip angles and changes in slip angles (even if we are not calibrated to know what they are precisely, we can easily detect changes in slip angles.)
When we apply torque to the steering we feel the slip angle at the front tyres. When the slip angle reaches its maximum this coincides almost perfectly with maximum self alignment torque and the point of maximum grip. If the grip under the tyre decreases as a result of surface change the tyre will react by aligning itself more with the wheel and this can be detected in the feel of the steering wheel as the self-aligning torque reduces. The classic case of this is when the driver drives a wet roundabout too fast - the steering "goes light" meaning the self-aligning torque is reduced because the tyre cannot sustain a large slip angle because the road surface has insufficient grip to hold the tread at a large slip angle.
NB. This is not the end of the story about feeling front slip angles in moving vehicles and we need to read on to gain a more comprehensive understanding of what goes on.
When we get to a corner we turn the steering wheel. This generates a slip angle at the front tyres, which in turn develop a side force that thrusts the front of the vehicle onto a new course. The process takes a little time to develop full grip and this time (the relaxation time of the tyre) is generally of the order of a quarter of a second. So a quarter of a second after we apply our slip angle (using the steering wheel) the front tyre has optimised its level of grip for that slip angle.
The steering can only apply a slip angle to the front wheels and the rear wheels don't know what's going on at the front until the chassis starts to move laterally. When this happens the lateral chassis movement causes the rear wheels to generate a slip angle in the rear tyres.
This in turn takes time to develop into grip, it takes a quarter of a second before grip is developed fully. Since the rears follow the fronts the response time for the whole vehicle to develop its grip is half a second. A long time in the life of an emergency manoeuvre.
Drivers tend to operate very aggressively in times of stress and in general they tend to move the steering wheel far too far during the initial phase of a corner (especially in an emergency) because they don't get the instant response they want.
We have already seen that it is possible to impose larger slip angles than the optimum by steering too much, this often happens to excess with novices especially on race tracks and in emergencies. It gives rise to excessive tyre wear and unresponsive vehicles.
Less is invariably more.