A conventional open differential splits torque equally between two wheels — which is exactly the problem when one wheel is on ice. Whatever torque the slipping wheel can hold (near zero) is all the gripping wheel gets. Limited-slip designs fix this with clutch packs, viscous fluid couplings, or electronic braking. The Torsen (torque-sensing) differential solves it mechanically with nothing but gears.

Invented by Vernon Gleasman in the 1950s and named for "TORque SENsing," the Torsen uses worm gears paired with spur gears to exploit a property covered in an earlier lesson: worm gears can drive spur gears easily, but spur gears struggle to backdrive worm gears due to the shallow lead angle and high friction. This asymmetry is the entire mechanism.

Inside the housing, each axle shaft ends in a worm gear (the "worm wheel"). These are meshed with worm pinions oriented perpendicularly, which are in turn coupled to each other through spur gears. Under normal driving, everything rotates together and torque splits evenly. When one wheel starts to spin faster than the other, that wheel's worm gear tries to backdrive its worm pinion — and the friction resists. The resisting force gets transmitted through the spur gear couplings to the opposite side, forcing torque toward the slower (gripping) wheel.

The critical spec is the Torque Bias Ratio (TBR): the ratio of torque delivered to the high-traction wheel versus the low-traction wheel. A typical Torsen Type 1 has a TBR of 2.5:1 to 5:1. If the slipping wheel can hold 100 lb-ft, a 3:1 TBR sends 300 lb-ft to the gripping wheel — 400 lb-ft total instead of the open diff's 200.

The catch: Torsen units multiply torque, they don't create it. If one wheel is completely airborne (holding zero torque), TBR × 0 = 0. This is why Audi Quattro systems (which used Torsen center diffs for decades) still add electronic brake intervention — braking the spinning wheel gives the Torsen something to multiply against.

Rule of thumb for selection:

• Type 1 (parallel-axis worms): highest TBR, most aggressive lockup — rear axles, off-road
• Type 2 (perpendicular worms in pockets): smoother engagement, lower TBR — front axles where steering feel matters
• Type 3 (planetary): asymmetric torque split (e.g., 40/60 front/rear) — center differentials in AWD

Compared to clutch-pack LSDs, Torsens need no maintenance, no friction modifiers in the gear oil, and no wear-related TBR degradation. The tradeoff is cost, weight, and the airborne-wheel problem.