A Viscous Coupled Differential is a limited-slip design that resists wheel speed difference using shear in a silicone fluid between interleaved plates rather than mechanical clutch friction. One set of plates is driven by each output shaft, and when the two sides rotate at different speeds the fluid’s shear resistance transmits torque toward the slower wheel, with the transmitted torque rising as the speed difference and fluid temperature rise. The action is smooth, progressive and quiet, and it requires no preload or maintenance, which is why it appears in road cars. The drawbacks for competition are that it only acts once slip is already occurring, that its behavior drifts with temperature, and that the unit cannot be rebuilt or tuned the way a clutch-pack differential can.