Torsen

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Torsen traction, an automotive part adding differential, was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. Torsen is a contraction of TORque SENsitive.

There are currently three types of Torsen differentials. The original Type I Torsen uses crossed axis helical gears to increase internal friction. The Type II Torsen uses a parallel gear arrangement to achieve a similar effect. The Type I can be designed for higher torque bias ratios than the Type II, but typically has higher backlash and the potential for NVH (Noise, Vibration and Harshness) issues, and requires a precise setup/installation. The Type III is a planetary type differential, in that the intended torque split is not 50/50. The Torsen III will be employed in all non-Haldex Quattro in the future, current applications: Audi S4 manual transmission, Audi RS4, Audi Q7, Alfa Romeo 159 Q4, Alfa Romeo Brera Q4. Also Toyota uses a Torsen III in the center differential of the Toyota 4Runner & Lexus GX470, with manual locking feature, and GM has a Torsen III center differential in the (non rear drive) Chevrolet TrailBlazer SS.

The torsen differential works by supporting a torque imbalance, the maximum ratio of torque imbalance is defined by the TBR (Torque Bias Ratio). When a torsen has a 3:1 TBR, that means that one side can handle up to 75% while the other side would have to only handle 25% of applied torque. During acceleration under asymmetric traction conditions, so long as the higher traction side can handle the higher percentage of applied torque, no relative wheelspin will occur. When the traction difference exceeds the TBR, the slower wheel receives the tractive torque of the faster wheel multiplied by the TBR, any extra torque remaining from applied torque contributes to the angular acceleration of the faster wheel.

When attempting to turn with a torque sensitive differential, the outer wheel will need to move forward relative to the differential, and the inner wheel will move slower than the differential. Friction in the differential will oppose motion, and that will work to slow the faster side and 'speed up' the slower/inner side. This leads to asymmetric torque distributions in drive wheels, matching the TBR. Cornering in this manner will reduce the torque applied to the outer tire, leading to possibly greater cornering power, unless the inner wheel is overpowered (which is easier to do than with an open differential). When the inner tire (which has less traction due to weight transfer from lateral acceleration) is overpowered it angularly accelerates up to the outer wheel speed (small percent wheel spin) and the differential locks, and if the traction difference does not exceed the TBR, the outer wheel will then have a higher torque applied to it. If the traction difference exceeds the TBR, the outer tire gets the tractive torque of the inner wheel multiplied by the TBR, and the remaining applied torque to the differential contributes to wheel spin up.

This can lead to vastly changing cornering characteristics, and a torque sensitive differential requires expert driving skills to catch the change from enhanced outer rear traction to reduced outer rear traction to avoid rear oversteer. This has led to problems in center differential applications, ie. Audi Quattro 'Spider bite', in that when cornering more power goes to the rear, and with Audi Quattro's front wheel drive like weight distributions 60/40, the rear inner wheel loses traction allowing for a forward torque shift. This changes the cornering balance and sets up an understeer, oversteer, understeer oscillation. Audi actually eliminated this by adding a second torsen differential to the rear axle in the Audi V8. As a cost cutting move, the torsen was removed and this spider bite will be re-remedied by the Torsen III center differential on the Audi Q7.

When a torsen differential is employed, the slower-moving wheel always receives more torque than the faster-moving wheel. The Torsen IIR-racemaster is the only Torsen to have a preload clutch. So even if a wheel is airborne torque is applied to the other side. If one wheel were raised in the air, the regular Torsens would act like an open differential and no torque would be transferred to the other wheel. This is where the parking brake 'trick' can help out. If the parking brake is applied, assuming that the parking brake applies even resistance to each side, the drag to the airborne side is 'multiplied' through the differential and TBR times the drag torque is applied to the other side. So the ground side would see (TBR X drag torque) minus drag torque, and hopefully that can help restore progress either forward/backwards. In Hummer/HMMWV applications, there are both front and rear torsen differentials, so use of the main brakes will operate this 'trick' on both axles simultaneously.

One of the most famous uses of the Torsen differential is in many of the various Audi quattro models, notably excluding the A3 and TT. It is also used, for example, in the Toyota Supra and Toyota Soarer (or Lexus SC430), the B5 platform revision of the Volkswagen Passat 4motion (based upon the Audi A4), and some versions of the Mazda MX-5/Miata

The HMMWV uses two Torsens, front and rear, with a normal manually lockable center differential (NVG242HD AMG transfer case) in the center.