Handling performance refers to the way in which the vehicle/tire responds to the many demands of the driver, such as steering, accelerating and braking. Handling is crucial both in terms of safety, for the stability of the vehicle, and driving pleasure.
As far as the driver is concerned, handling is above all measured by the extent to which the vehicle responds to the driver's actions. Everything should run smoothly, consistently and without any surprises.
For car manufacturers, handling is one of the major aspects of performance. Linked as it is to safety and driving pleasure, performance in this area is a deciding factor for tire approval. Indeed, it is one of the performances most commented on by professionals (opinion leaders and at press events), as well as by the final customer.
Handling development is particularly complex as it depends on a range of factors at three different levels, namely:
• the vehicle (geometry and weight distribution),
• the elements involved in ground/vehicule contact (tires, steering and suspension),
• the driving assistance systems (stability control and the steering axle).
The tire plays a key role in road holding because at the end of the chain it ensures the transmission of the efforts between the vehicle and the ground to maintain the trajectory defined by the driver.
The handling performance of tires is assessed through a number of tests (both objective and subjective, on dry ground, wet ground and on machines).
In order for a vehicle to remain on its path when it comes to a bend, a force needs to be generated that is equivalent, yet opposite to the centrifugal force that would send the vehicle off course.
This lateral force has to be generated by the vehicle’s four tires to counter the centrifugal force.
Distortion of the tread blocks in contact with the ground generates a lateral force. Body slip – the angle between the direction of the wheels and the path followed by the vehicle – enables the tire to distort the tread blocks when cornering, thereby generating the required lateral force. In concrete terms, this means that when cornering, a vehicle's tires are over-steered in relation to the path being followed.
Using the steering wheel, the driver turns the front wheels of the vehicle, creating body slip in the front tires. The lateral force generated by the front axle begins to turn the vehicle. This change in behavior on the part of the vehicle then causes body lean in the rear axle. The vehicle will thus continue to turn around its centre of gravity. Turning the wheel using intuition and successive estimations, the driver finds the required steering angle for the front wheels and thus generates the front and rear lateral force required to keep the vehicle on its path.
A vehicle is over-steered when the rear tires have a greater slip angle than the front tires. This means that in extreme cases the vehicle can go into a spin.
Instability of this kind definitely needs to be avoided as it is very difficult to regain control using driver reflexes.
A vehicle is under-steered when the front axle has a lower slip angle than the rear axle. When cornering, the path followed by the vehicle therefore tends to be less tight than required and, in extreme cases, the vehicle continues straight ahead. In this situation, however, the vehicle remains stable and the driver’s natural reflexes (to slow down and/or turn the wheel more) are generally enough to ensure full recovery.
For obvious safety reasons, all vehicles on the market are designed to under-steer. And over-steering vehicle would by its very nature be unsafe to drive.
It is however worth noting that even under-steering vehicles are occasionally susceptible to extreme over-steering, depending on driving conditions and driver behavior. The fact that a professional driver can control over-steering is due to the intrinsic nature of the vehicle to under-steer before it loses grip