The Influence of Torque and Speed Sensitive Differential Characteristics in a Front Wheel Drive Vehicle During On-Limit Manoeuvres

Abstract

AbstractPassive Limited Slip Differentials (PLSD) are a well-established means of improving the traction limitation imposed by the open differential, and achieve this by transmitting a bias torque from the faster to slower rotating driven wheel. This torque bias is typically proportional to the differential input torque (torque sensing) or the speed difference between driven wheels (speed sensing). In the motorsport environment however, there exist devices which are able to bias torque through both methods simultaneously, but to date, remain unexplored with respect to their influence on handling and the differential models required to study them. Plate (Salisbury) type and Viscous Coupling (VC) differential models are formulated, then combined into a Viscous Combined Plate (VCP) model and used as a basis for handling characterisations through typical cornering scenarios. An 8 degree of freedom (DOF) vehicle model created in the MATLAB/Simulink environment is parameterised around a front wheel drive (FWD) saloon racing vehicle. Path preview steering control is used to give a robust means of comparing the necessary driver inputs to maintain a particular racing line, whilst reaching the lateral acceleration limit of the vehicle. At low lateral accelerations (<5 m/s2) VC, VCP and locked differentials were shown to increase levels of understeer most, as the torque bias of a plate differential is proportionally less due to the minimal throttle input required to maintain vehicle speed. At these low speeds, any torque bias acts to reduce initial turn-in yaw rate response. At higher lateral accelerations (>5 m/s2), inner wheel tyre saturation ultimately limits the maximum speed through a corner. The degree of torque bias that a PLSD can provide to the outer wheel will delay the onset of inner tyre saturation, and increase the maximum lateral acceleration limit. In raw performance terms, this means a locked differential is up to 0.05 s quicker through a constant 50 m radius corner when compared to an open differential equivalent.KeywordsVehicle dynamicsLimited slip differentialViscous couplingTorque sensitiveSpeed sensitive