Basic Dynamics of Vehicles

Understeer and Oversteer

When we drive round corners at increasing speed there will be a loss of grip at one end of the vehicle or the other. When this happens the vehicle is sliding at one end more than the other. i.e. there is a larger slip angle at one end than the other. All cars are tuned by the chassis engineer to slide at the front slightly more than the rear. This is called understeer. When a vehicle has larger slip angles at the rear than those at the front the vehicle is described as oversteering.

The fundamental difference between the two manifests itself in the event of a crash. The understeering driver sees what he crashes into whereas the oversteering driver who goes off backwards generally doesn't.

The consequences for the driver making best endeavours to maintain control are very different; understeer requires the driver to generally increase steering input (true and effective up to a point). The oversteering driver needs to reduce steering input to maintain control.

In practice the experience of driving understeering vehicles is one of comfortable safety because the steering model we all have of turning the wheel to negotiate a corner is honoured by the dynamics of the vehicle. In other words, if the corner gets tighter we can turn the wheel a bit more and the vehicle turns in more. As we approach the limit of adhesion we get plenty of warning that the steering is becoming less and less effective. Even though our model becomes non-linear it doesn't fail altogether.

Driver's experiences of vehicles that oversteer are more complicated. All vehicles understeer to some degree as they turn in otherwise we would have no apparent steering control. As the oversteering vehicle nears its performance envelope our steering model fails completely. The steering becomes too effective and the vehicle turns increasingly too much. Ultimately the vehicle could spin in or off the road. The driver goes through a process of comfort as the initial understeer model works fine but as the vehicle starts to oversteer the experience of the driver changes as the steering model fails completely and the means to safe negotiation of the bend transcends into turning the steering wheel in the counter intuitive direction - opposite lock. This is generally experienced as unexpected at the same time it is often experienced as very scary indeed.

The fact that oversteer is ever considered as a dynamic option is a bit of a mystery to most people who simply want to drive safely from A to B and who only want one steering model that is simple, reliable and safe to operate; they don't care very much about the ability to zig or zag so long as it gets the job done they are happy to adapt their driving to suit.

Sporty drivers are attracted to vehicles that point effectively and faithfully with a sharper steering model at speed. They do not like their vehicles to turn sluggishly with slow steering response because they do not feel in control of the situation. The vehicle will not zig or zag in a satisfactory way for them.

This leads to a compromise which tends towards oversteer, the balance of the vehicle is tuned to provide the driver with the ability to maximise performance from all the tyres. Rear grip tends to match front grip more closely under normal operating conditions. As the performance envelop is reached this gives rise to the possibility for the driver to upset the balance more easily in the direction of oversteer.

It also leads to ever increasing sophistication in chassis design, augmented by ever sophisticated electronic systems which manage and manipulate the response of the vehicle on behalf of the driver to maintain a "simple" steering model.

Sophisticated sporty drivers notice the electronic interventions and generally dislike the intrusion into their control model.

Manufacturers sell to all markets and often provide various options and levels of intervention for their clientele.

Power Induced Oversteer

The use of power in a rear wheel drive car leads to a particular variety of oversteer called power induced oversteer. The rear tyres lose adhesion because they cannot provide enough traction to drive the vehicle forward and this has the effect of destroying most of the lateral grip afforded by the rear tyres.

The recent hike in engine power output provides plenty of opportunity to induce power slides especially when there is no traction control system or when it is switched off.

Managing Power slides

Power induced oversteer is fun and dangerous because it is difficult to control effectively (fun) and it induces the "oversteer" steering model combined with the absolute need for precise throttle control.

In the case of power oversteer the steering model changes dramatically. It is not enough to simply move the steering wheel in the right direction to point the vehicle appropriately; it is mandatory to control the throttle position to manage the amount of traction and thus the lateral grip at the rear and the dynamic weight distribution.

Overall balance of the vehicle is achieved by manipulating steering to point the front of the vehicle and the throttle to point the rear. Throttle has a dominant effect upon the dynamic weight distribution which in turn affects the steering response.

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