I understand how all-wheel drive functions in my Evolution IV but I'm clueless how it works in my urS6 :(
This is what I know:
1. Torsen designed all-wheel system
2. limited slip rear diff
3. slightly different between years ('92-'95.5)
I searched but did not find requested information.
12-10-2008, 07:57 PM
torsen center diff, 50:50 split, up to 75%
open rear diff with manual lock
open front differential, no lock
1997 S6 uses generation IV, open rear diff with electronic differential lock
12-10-2008, 08:34 PM
As per Wikipedia:
There are currently three types of Torsen differentials.
1. 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.
2. 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).
3. 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.
12-10-2008, 08:41 PM
As per: http://wikicars.org/en/Quattro
Quattro Generation II
Starting from 1988 on older generation Audi C3 platform and Audi Quattro turbo coupe until the end of their production and on new generation B3 platform (1989-1992) Audi 80/90 Quattro, B4 platform (1992-1995) Audi 80, Coupe Quattro, Audi S2, Audi RS2, C4 platform (1991-1994) Audi 100 Quattro, Audi S4, earlier C4 platform (1995) Audi A6/S6.
What: Permanent all wheel drive.
Torsen center differential, 50/50 split, automatically locking with up to 100% of torque transfer to either axle.
Open rear differential, manually lockable via switch on center console located next to handbrake. ¹
Open front differential, no lock.
¹ - ABS disabled when locked, automatically unlocks if speed exceeds 25 km/h (15 mph)
How: When rear differential is manually locked the car will not move if one front wheel and both rear wheels lose traction, but this is valid if all wheels are on the ground. Note that due to the constructive feature of the Torsen (TORque SENsing) differential if no torque is sensed on one axle the Torsen works as open differential e.g. if one front wheel is raised in the air the Torsen differential will not lock, all power is transferred to the wheel in the air and the car will not move.
Starting from 1996 on Audi A4/S4/RS4, Audi A6/S6/Audi_RS6, Audi A8/S8 with both manual and automatic transmissions. Also on VW Passat B5, where it was initially referred to as syncro, but by the time it reached US soil, it had been rechristened 4motion. Also used on the Volkswagen Phaeton and sister vehicles; also the Volkswagen Touareg where they utilize separate transmissions, PTU's and front axles. Manually locking rear differential replaced by Electronic Differential Lock (Difflock imitation, detects wheelspin via ABS sensors and applies brakes to spinning wheels thus transferring torque via open differential to another wheel which has more traction). EDL works at speeds up to 40 km/h (25 mph), on more powerful versions such as S4/S6/S8 - up to 80 km/h (50 mph). In addition to EDL, Audi A8/S8 limits engine rpm when excessive wheelspin occurs.
What: Permanent all wheel drive.
Torsen center differential, 50/50 split, automatically locking with up to 100% of torque transfer to either axle. Some recent vehicles [2006+ B7 RS4] have a 40/60 front/rear torque split rather than 50/50
Open rear differential, Electronic Differential Lock.
Open front differential, Electronic Differential Lock.
How: In on-road conditions the car will not move if all four wheels lose traction. Torsen effect with one wheel in the air will not happen on quattro IV because Electronic Differential Lock will apply brakes to spinning wheel and Torsen differential will transfer torque to rear axle.
In off-road conditions (wheels in the air and an obstacle restricting the vehicle from moving forward) the car will not move further when one front and one rear wheels lose traction. The reason of this behavior is that Electronic Differential Lock is not a replacement for mechanical differential lock and it is not able to transfer enough torque to another wheel. The car will end up spinning one front and one rear wheel with crackling EDL trying to stop them from spinning.
12-11-2008, 07:15 PM
The standard torsen torque split ratio is 2:1. The axle with grip can only get a max of 2x the torque supplied to the slipping axle.
This can be changed with a torsen unit with a higher ratio. Stasis engineering has a 4:1 unit that can send more torque to the front or rear, but still not 100%.
12-12-2008, 06:12 AM
Already addressed the Torsen in Wiki on this forum. There can not be 'locking' to 100% either axle on a torsen. WRT the urS4/S6 it is a T1 unit (Torsen all applications thru 2000 S4) known as the University Special. They are all 78:22 = 3.5TBR units. Audi lists them at 3.0TBR, and Torsen calls them 2.5TBR applications.
Stasis does offer a higher torque bias ratio unit (up to 4:1), but I have proposed that is not better for a street or occasional track car, and hardly worth the money unless all other chassis components are optimized. 95% of drivers and 99% of cars couldn't isolate the differences. And in a non optimized street car, more torque shift is not equal to "better" car control. Changing diff fluid in the torsen does affect TBR.
The rear systems on the urS cars is Pre-95.5 a locking rear diff with auto unlock (15mph), the 95.5> torsen cars us Electronic Differential Locking (Traction control thru braking) up to 40mph.
Scott aka "Torsen Boy" J
12-12-2008, 12:49 PM
The torsen ratio for the UrS is 2:1, but I have read that the max torque split is 75/25%. This calculates to a 3:1 ratio.
Am I missing something in how the torque split is calculated from the ratio?
The Stasis website states that their 4:1 unit can send up to 88% of the torque to the front or rear. A 4:1 ratio should max out at 80/20 by my calculation.
I agree that a higher ratio torsen center diff is of little use to most street and even track cars. The only way you would see any benifit is if you have enough power to spin the front wheels on corner exit when the front inside tire is unloaded.