Driving force distribution control system for 4WD vehicle

A drive system for a vehicle having a prime mover, primary driving wheels and secondary driving wheels, comprising:

• a transfer for transmitting a driving torque from said prime mover to said primary driving wheels and secondary driving wheels, said transfer comprising a transfer clutch for varying a driving torque distribution ratio which is a ratio of a driving torque transmitted to said secondary wheels to a driving torque transmitted to said primary wheels,
• means for sensing a primary wheel rotational speed of said primary wheels,
• means for sensing a secondary wheel rotational speed of said secondary wheels,
• means for sensing a vehicle speed of said vehicle,
• means for sensing a turning radius of said vehicle corresponding to a steering angle of said vehicle,
• means for actuating said transfer clutch to vary said distribution ratio in response to a control signal, and
• control means for determining a speed difference between said secondary wheel speed sensed by said secondary wheel speed sensing means and said primary wheel speed sensed by said primary wheel speed sensing means, and determining a secondary wheel driving torque from said speed difference by using a characteristic which causes said desired secondary wheel driving torque to be varied with an increase in said speed difference, and adjusting said characteristic in accordance with said turning radius sensed by said turning radius sensing means and said vehicle speed sensed by said vehicle speed sensing means,
• wherein said control means increases said actual secondary wheel driving torque gradually with an increase in said speed difference when said speed difference is smaller than a reference value which is determined in accordance with said turning radius and said vehicle speed, and increases said actual torque steeply with an increase in said speed difference when said speed difference is greater than said reference value, and
• wherein said reference value is equal to a product between a result of subtraction of a predetermined speed value from said vehicle speed, and a rate which is inversely proportional to the second power of said turning radius if said vehicle speed is greater than said predetermined speed value, and said reference value is equal to zero if said vehicle speed is smaller than said predetermined speed value.