, (full name Torsen traction
), is an automotive
part, a limited slip differential
. It was invented by American
Vernon Gleasman and manufactured by the Gleason Corporation.
Torsen is a contraction of Torque-Sensing.
TORSEN and TORSEN Traction are registered trademarks of JTEKT Torsen North America Inc (formerly Zexel Corporation, formerly Gleason Power Systems).
Torsen differentials can be utilised in one or more positions on a motor vehicle:
- centre - used to apportion appropriate torque distribution between front and rear axles on a four-wheel drive vehicle.
- rear - used to apportion appropriate torque distribution between left and right sides in rear axles. This may be on either a rear-wheel drive or four-wheel drive vehicle.
- front - used to apportion appropriate torque distribution between left and right sides in front axles. This may be on either a front-wheel drive or four-wheel drive vehicle.
As an example, on a four wheel drive vehicle, it may use either one, two, or three Torsen differentials.
Types of Torsen
There are currently three types of Torsen differentials.
- The original Torsen T-1 (Type A) uses crossed axis helical gears to increase internal friction. The Type I can be designed for higher torque bias ratios than the Type II, but typically has higher backlash and the potential for Noise, Vibration, and Harshness (NVH) issues, and requires a precise setup/installation.
- The later Torsen T-2 (Type B) uses a parallel gear arrangement to achieve a similar effect. There is also a specialist application of the T-2, known as the T-2R (RaceMaster).
- The latest Torsen T-3 (Type C) is a planetary type differential, in that the intended torque split is not 50:50. The Type C is available as single or twin version, the Torsen twin C differential has front and center differential in the same unit.
The Torsen T-3 will be employed as the centre differential in all future non-Haldex Traction Audi models with quattro four-wheel drive, current applications: 2008 Audi S4 (B7), Audi RS4 (B7), and Audi Q7. Alfa Romeo uses Torsen C twin differential in Alfa Romeo 156 Crosswagon Q4 and then in 159, Brera and Spider Q4 models. Also Toyota uses a Torsen T-3 in the center differential of the Toyota 4Runner and Lexus GX470, with manual locking feature, and GM has a Torsen T-3 centre differential in the (non rear drive) Chevrolet TrailBlazer SS.
How they work
The Torsen differential works by supporting a torque imbalance, the maximum ratio of torque imbalance being defined by the Torque Bias Ratio (TBR). When a Torsen has a 3:1 TBR, that means that one side of the differential 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 output side of the differential 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 output side of the differential.
NOTE: The TBR should not be confused with the uneven torque-split feature in the planetary-type Torsen III. The planetary gearset allows a Torsen III center differential to distribute torque unevenly between front and rear axles during normal (full traction) operation without inducing wind-up in the drivetrain. This feature is independent of the Torque Bias Ratio.
Torsens in front and/or rear axles
When attempting to turn with a torque sensitive differential, the outer wheel will need to rotate more quickly relative to the differential, and the inner wheel will rotate more slowly 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.
When a Torsen differential is employed, the slower-moving wheel always receives more torque than the faster-moving wheel. The Torsen T-2R 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.
Notable Torsen users
One of the most famous uses of Torsen differentials are in many of the various Audi quattro
models, notably excluding the A3 & S3
(which have transverse-mounted engines
and use Haldex Traction
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 Lancia Delta Integrale, as well as the 1999-2002 model Pontiac Firebird and Chevrolet Camaro, had a Torsen differential.
The HMMWV or Humvee uses two Torsens, front and rear, with a normal manually lockable center differential (NVG242HD AMG transfer case) in the center.
Center and rear
Front and rear axles
Front axle only
Rear axle only