Contact patch compliance is a metric to understand the level of lateral deformation in a suspension system when subject to a lateral cornering force at the tyre contact patch. It is measured in millimetres of lateral contact patch deflection per kilo-Newton of lateral force, mm/kN. Some suspension designers prefer to use its inverse, contact patch stiffness; both are given in RACE reports. The contact patch compliance reported is a ‘projected’ contact patch compliance from the wheel centre and it does not include any lateral wheel or tyre stiffness. It is purely the contribution of the suspension system to the overall contact patch compliance.
Contact patch compliance is one of the key contributors to the steering connection and turn in response of a vehicle. Compliance at the contact patch contributes to a delay in the build-up of lateral cornering force. Low levels of contact patch compliance (or high contact patch stiffness) improve steering response and steering connection. Both front and rear contact patch compliance play their part. On initial turn in the rear suspension contact patch compliance is key as the vehicle yaws around the rear axle and the vehicle starts to turn. As the cornering forces build mid-corner and the vehicle leans on the outside front suspension, the front contact patch compliance becomes the dominant factor. With low front contact patch compliance any additional mid-corner inputs from the driver are felt as a vehicle response without delay.
In production vehicles, delivering low contact patch compliance often comes with a requirement for stiff suspension bushes which is very often in direct conflict with NVH (Noise, Vibration and Harshness) requirements. The reality is that in many ordinary vehicles, more customers will notice poor noise and vibration performance than will notice poor steering connection or steering response. As a result, many production vehicles are very often compromised from a steering point of view.
The typical range of contact patch compliance seen in mainstream production cars would be between 1 – 2.5 mm/kN. Sports cars would typically be lower with some down to 0.5 mm/kN. Getting below 0.5 mm/kN would require ball joints or very stiff bushes and would only be seen on track orientated cars or very focused road cars.