The Detroit Truetrac is a parallel axis planetary helical gear limited-slip differential. It uses the planetary gears to bias torque to the wheel with more traction when the other starts to spin. It's unique as a limited slip because it acts like an open differential until wheel speed differentiation occurs. In fact, the Truetrac is basically an automatic limited slip. Other limited-slip differentials use clutch packs to achieve the limitations in axle speed differentiation. These clutch packs are always applying force, and the clutch-packed units are acting as limited slip differentials 100 percent of the time. These clutches can require servicing from time to time. The helical gear Truetrac requires no servicing because it has no clutch packs.