This paper describes the addition of tandem-drive axles and tractor suspension dynamics to a digital computer model of a tractor-semitrailer truck. The extended model provides 22 degrees of freedom for the vehicle. Two degrees of freedom are included for the motion of each tractor axle; vertical tire flexibility, tandem-axle suspension jacking, and tandem-axle roll steer are also included in the extended model.The features of the previous vehicle model (based on the work of Mikulcik) are retained in the extended model. These features include nonlinear equations for translation, yaw, pitch, and roll of both the tractor and the semitrailer (except as these motions are constrained by the fifth wheel), wheel rotation dynamics, and antiskid brake control. The model also includes a simulated “driver” which specifies the steering angle and the air pressure applied to the brakes.The extended model is compared with the prior model by performing comparison tests involving simultaneous cornering and braking. The model is then used to investigate the effects of nonhorizontal torque rods at the tandem axles on the behavior of the vehicle.