The Control Authority of Passive and Active Torque Vectoring Differentials for Motorsport Applications
Limited Slip Differentials (LSD) have traditionally been shown to offer significant improvements in traction and vehicle stability [1 , 2 ]. In the motorsport environment however, during high lateral accelerations, the grip level of the inner driving whee
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Abstract Limited Slip Differentials (LSD) have traditionally been shown to offer significant improvements in traction and vehicle stability [1, 2]. In the motorsport environment however, during high lateral accelerations, the grip level of the inner driving wheel can be compromised and force the LSD torque bias to switch directions. Thus initial corner entry understeer, snaps to mid-corner oversteer as the inner wheel saturates. Torque Vectoring Differentials (TVD) offer greater flexibility and provide a system which can both improve vehicle stability and agility [3, 4]. The result is a more predictable car, which is more linear in its response to steering inputs. To date, such systems have required costly hydraulic or electromechanical control systems and as a result, are banned in all but the highest echelons of motorsport [5]. This paper investigates the potential of a passive torque vectoring differential strategy which apportions torque depending on lateral acceleration levels. The resulting objective is to decouple the torque transfer direction from the driven wheel speed difference and redistribute torque to the driven wheel with most grip. Differential models for a passive LSD (PLSD), active and passive TVDs (ATVD, PTVD) are presented, then combined with a 8 Degree Of Freedom (DOF) vehicle model to evaluate their influence on handling behaviour. The models are constructed in the Matlab/Simulink environment and parameterised around a RWD saloon racing vehicle. The PLSD was shown to reduce the yaw rate response of a vehicle during turn in and was more uncontrollable at the limit due to driven wheel tyre saturation. The ATVD control F2012-C04-002 A. Tremlett (&) F. Assadian D. Purdy N. Vaughan Cranfield University, Cranfield, Bedfordshire, UK e-mail: [email protected] A. Tremlett A. Moore M. Halley Xtrac Limited, Berkshire, Thatcham, UK
SAE-China and FISITA (eds.), Proceedings of the FISITA 2012 World Automotive Congress, Lecture Notes in Electrical Engineering 193, DOI: 10.1007/978-3-642-33744-4_30, Springer-Verlag Berlin Heidelberg 2013
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strategy was able to modify vehicle handling balance, both improving its linearity and extending the point at which control could be maintained. The PTVD was shown to increase vehicle agility by improving initial yaw rate response and extend the linear handling region. However, careful consideration must be given to the magnitude of the torque transfer at higher lateral accelerations, since lateral and longitudinal tyre forces are strongly coupled in this region. For performance applications, this paper brings to light the need to optimise passive differential torque transfer around tyre force characteristics.
Keywords Vehicle dynamics Limited slip differential differential Yaw moment control
Torque vectoring
Nomenclature Ap empirical differential friction factor Fp differential preload force (N) Nl,r left and right ATVD clutch speed ratios Rr mean contact radius between ramp and crosspin (m) Ro,i outer and i
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