Technical Series: Part 2: The ever present phenomena of Understeer and Oversteer

In Part 2 of this series, as continuation of Part 1 where we studied the handling and balance of a race car, we will look at two of the most important factors affecting the drivers ability to eek out highest levels of performance. These phenomena’s are so critical to a racing car (irrespective of what series you look at) to achieve its maximum potential, that for race engineer, it is vital to understand how it affects the drivability of the car and in turn, how can it be countered. The points of discussion we are talking about are Understeer and Oversteer.

Now during a race weekend, you would have heard drivers complain about their car either suffering from understeer or oversteer. Such is the impact of these two physical phenomenas that it can decide the fate of the entire championship. Anything in excess makes a car difficult to drive. And this is particularly true for Understeer and Oversteer, as it directly affects the ability of the driver to extract the true potential from the car.

Any successful championship winning car has to negotiate the corners in the fastest possible manner. Understeer and Oversteer are conditions which have the potential to thwart this and ruin what would otherwise be a fast car. Success also depends on how well these two aspects can be countered, both by making small but significant changes either to the car setup or driving style. An established driver finds ways through driving skills to counter these aspects which enable him to extract the maximum around corners.

However, it is also true that most often than not, since these two aspects are so difficult to eliminate completely from the car, drivers tend to have preferences of which one they would rather have on their car. And usually it is common to have an Understeering car rather than an Oversteering one. Why? Let us understand better as to why.

So what is Understeer and Oversteer?

Understeer: Simply put, when a car wants to turn in the corner, but hesitates to do so (the front gives way before the rear and continues straight), this is when we call the car to be an Understeering car. It also means that front tires are not able to provide the necessary grip as compared to rear tires.

Oversteer: Inversely, when a car turning in a corner, turns so much (the rear gives way before the front) that it spins instead of continuing around the corner, this is when we call the car to be an Oversteering car. It also means that the rear tires are not able to provide the necessary grip as compared to front tires.

In short, both Understeer and Oversteer affect the performance of the car during “Cornering” and are a result of external forces acting on the car coupled with various setup implications and grip levels.

Now it is not difficult to understand which of the above two would cause the most difficulty in achieving the best possible lap time, but in reality, both are detrimental and need to be avoided as much as possible.

So how are these concepts introduced in the first place?

It is understood that Understeer and Oversteer are concepts which are only introduced during cornering. Hence it is important to understand the effects of cornering and the various forces acting on a car/its wheels during cornering.

Before we look into what is understeer and oversteer, it is critical to understand the concept of slip angle. (Figure A) shows a car travelling in a straight line until a horizontal side wind force is applied to the centre of the car. The side wind force pushes the car off-course. The car proceeds at some angle to the original direction of motion. As per laws of physics, the tire grip forces (Fyf & Fyr) resist this wind force. This naturally causes distortion of the contact patch of the tire which results in angular deviation. The difference between the direction of motion and the longitudinal axis of the tire is defined as the Slip Angle (a-alpha).

The slip angle increases with side-force up to a point where the limit of 7-10 degrees is reached at which point, the racing tires let-go, resulting in a slide across the road surface. This is when a tire loses its ability to provide sufficient grip thus asking the driver to apply corrective steering inputs to prevent running off track.

Now before the car starts either to understeer or oversteer, there is a neutral condition (Figure B).  This condition is introduced when the driver applies quick steering adjustments. As the wind force continues, a steady state condition emerges where the car is proceeding in the right direction oriented at the slip angle (a). If a steady state condition as shown in the Figure B is achieved, where both front and rear tires have the same slip angle without steering inputs, then the car is said to be a neutral handling car.

Now for a driver to achieve this steady state condition, the driver has to turn the front wheels into the wind (Figure C). In this case, as can be seen from the way the tires have turned, the front slip angle (af) is greater than the read slip angle (ar). This is when a car is said to Understeer. The tires need to counter the centripetal force to such an extent that the driver, without any major steering inputs can turn the car in the desired direction. If the tires fail to do so, then the car deviates from the desired direction, which is as a result of Understeer.

Now when the exact opposite happens as seen in (Figure D), where the driver finds it necessary to turn the wheels away from the wind in order to control the car,  then the front slip angle (af) is far smaller as compared to the rear (ar). This results in the read tires giving way and trying to pass the front resulting in a spin. This is called as Oversteer.

So, how can you counter both these aspects?

There are series of modifications that can be made to counter these two aspects.

We will keep in mind our base image that describes the critical parameters that govern the design of a modern Formula 1 car. (Refer Part 1 for more details)

• To counter Understeer, driver can back off the throttle and slow down.
• To counter Oversteer, driver can apply opposite lock to the steering.
• Setup changes – based on where the onset of understeer or oversteer is, (reduce brake bias, reduce front ride height, increase aerodynamic downforce, adjust the tire pressure, softening the front coil spring rate, soften the anti roll bar stiffness, increase rear toe-in, reduce rear ride height)

References:

Race-Car-Design-by-Derek-Seward