Sisteme AWD, care e cel mai bun?

Being the master of 4-wheel drive, Audi always insists to use the most effective system despite of pricey. Its Quattro 4WD system uses a pure mechanical LSD, Torsen differential.
Torsen, means "torque-sensing", was invented by an American company calls Gleason Corporation. Its slip-limiting ability is implemented by cleverly using worm gears / worm wheel pair. This pair has a special characteristic : driving torque can be transfered from worm wheel to worm gear, but not reverse. Otherwise they will be locked up. It is such characteristic limit slip.
The left hand side picture explain how the Torsen differential works. In normal cornering, i.e., no tyre slip in any wheel, Torsen differential provides the same function as a normal differential. The addition of worm wheel / worm gear pair does not affect speed difference between output shafts. For instance, if the car turns left, the driveshaft to right wheel runs faster than the differential housing, while the driveshaft to left wheel runs slower than the differential housing. The speed difference between left and right worm wheels can be exactly matched in the synchromesh gears. Note that the worm gears / worm wheels pair do not lock up because torque is transfered from worm wheels to worm gears.
When one of the wheels, say the right wheel, loses traction due to poor road surface or whatever reason, the worm gear / worm wheel pair get into effect. At the instant just before they become effective, one must know that by the basic differential theory no torque will be sent to the left wheel, which is with traction. Instead, all the torque will be sent to the spinning right wheel. Then, the fast-rotating right worm wheel will drive its worm gear, through the synchromesh and drive the left worm gear. Now, do you still remember the basic characteristic of worm gear / wheel pair ? Well, when worm gear drives worm wheel, they will be locked up. As a result, the left worm gear and right worm gear are actually locked together, thus wheels on both side will rotate at the same speed and get the car out of the lose of traction.
Characteristic of Torsen-equipped 4WD
Except the first generation Quattro system that appeared in the early Quattro coupe, most of the subsequent Quattro systems used Torsen differential in center and rear axles. This is rather expensive. However, Torsen-equipped 4WD has many advantages. First of all, its pure mechanical parts react almost instantly to tyre slip. Secondly, it provides linear lock-up characteristic. Thirdly, it is a strictly permanent 4WD system. In normal condition, torque split between front and rear wheels is 50 : 50 ( other ratio is also possible, depends on the pitch of worm gears. ).
Apart from Audi, nearly no other car makers adopted Torsen LSD mainly because of cost problem. Toyota's rally ace, Celica GT4 is one of the few exception. It used Torsen in the rear axle. This might be part of the reason why it was so expensive over competitors.
Who use it :

o Audi Quattro system
o Toyota Celica GT4 ( rear differential only )
o Hummer

Viscous Coupling differential
Viscous Coupling center LSD is commonly used in many simple 4WD system. One of the earliest examples was Volkswagen's Syncro system, which is still using in today's Golf and Audi A3.
Inside a viscous coupler as shown in the right hand side picture, there are many circular plates positioning very close to each other. Both drive shafts connect to roughly half of the plates in an alternating sequence as shown. The sealed differential housing is fully contain of a high viscosity liquid, which has a strong tendency to "visco" those plates together.
In normal condition, front and rear axle run at roughly the same speed so the plates and viscous liquid are relatively stable to each other. When tyres slip occurs in one of the axle, that means the alternating plates run at different speed, viscous liquid will try to visco them together. As a result, torque is transferred from the faster driveshaft through the liquid to the slower driveshaft. The greater the speed difference, the larger the torque transfer. As a result, limited slip function is implemented.
Characteristic of Viscous Coupling center differential
Note that Viscous Coupling LSD is a speed-sensing device : under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD.
Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For rear-wheel drive cars such as Porsche 911 Carrera 4 and Lamborghini Diablo VT, this is not a real problem, as I have already mentioned the desirability of controlled oversteering. However, for other front-wheel drive-based cars like VW Golf and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage.
The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero.
To cure this problem, most manufacturer varies the final drive ratio so that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95 : 5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD.
It might be less effective than Torsen system, but it is certainly the cheapest, so we can find it in many mass production 4WD cars.
Who use it ?

o Volkswagen Syncro
o Lamborghini Diablo VT
o Porsche 911 Carrera 4 (993) and 911 turbo (993)
o Volvo 850 AWD

Viscous Coupling Differential Lock
While Torsen 4WD is too expensive, Viscous Coupling LSD is part-time only, most 4WD cars, including the rally aces Celica GT4, Subaru Impreza, Mitsubishi Lancer and Ford Escort RS Cosworth adopted another kind of center differential - basically it has a regular center differential which distribute torque to front and rear under normal condition, an additional Viscous Coupling Differential Lock provides anti-slip function when needed.
Characteristic of this system
The Viscous Coupling Differential Lock is virtually the same as what we have learned earlier, therefore it also has slightly delay and non-linear characteristic. However, in reality this might not be as serious as we thought, otherwise it would be impossible that all the top rally cars use it. Moreover, Viscous Coupling Differential Lock system is lighter and cheaper than Torsen system, while having superior effectiveness over the part-time Viscous Coupling LSD system.
Who use it for center LSD :

o Lancia Delta Integrale (with Torsen in rear axle)
o Ford Escort RS Cosworth (with Viscous Coupling LSD in rear axle)
o Mitsubishi Lancer GSR, Eclipse GSX and 3000 GT VR4. (with Viscous Coupling LSD in rear axle)
o Subaru Impreza and Legacy manual versions (with Viscous Coupling LSD in rear axle)
o Toyota Celica GT4 (with Torsen in rear axle)
o Bugatti EB110

toate diferentialele transfera puterea cumva... ideea este cum o face. RS-urile au probabil LSD si pe punti.

The RS4's quattro 4WD system is also heavily modified. The center Torsen differential is now biased 60:40 in favor of the rear wheels, although 100 percent of the engine's available torque can be sent to either axle at any time as needed. There's a hypoid gear differential on both axles to distribute torque to the wheels and the whole system is backed up with a two-stage stability control system that even wipes the brakes rotors for you in the wet.

Asa ca Subaru AWD e mai bun ca Haldex pentru ca e permanent.

sa fim seriosi... vrei 4wd iti iei un 4wd, nu surogat cu electronice. asta inseamna punte fata/spate, diferential central, reductor, mrl-uri, blocante fata/spate. astea or sa bata oricind sistemele electronice, asta in cazul in care este un sofer in masina.

awd-ul te ajuta doar iarna si doar teoretic. mult mai buna e o masina echipata cu pneuri de iarna si cu esp decit un awd si pneuri normale.

pe uscat AWD ajuta demarajul numai la motoare cu puteri peste 180-200CP, fata de FWD, ca fata de RWD ( la aceeasi greutate si putere) nu prezinta nici un avantaj, ba chiar e un dezavantaj la viteze peste 40-50km/h ( pana pe la 440km/h AWD are un avantaj. la masini cu putere peste 4-500CP AWD e cam obligatoriu, desi M5 cu 500CP si RWD "begs to differ".

･ One way L.S.D. specially suited for front wheel drive cars and 4WD car front axles.
Activates under acceleration, and acts as a true LSD when the throttle is off. A great chance for improved lap times.
A CUSCO original design.
･ 1.5 way LSD
Activates under acceleration while the understeer is less than on the 2 way LSD.Recommended for drivers having difficulty with 2 way LSD understeer and one way LSD braking.
･ Two way LSD
Recommended on the rear axle for rear drive cars. Activates on both acceleration and deceleration. Recommended for those who
prefer hot driving action and big angle drift.

Corect, dar aici se vorbea despre AWD nu 4WD - adica sistemele care asigura cea mai mare stabilitate pe sosea cand alergi, nu in offroad.

Aceeasi problema: AWDul nu e doar pentru iarna / noroi / offroad, are alte scopuri decat 4WDul...

@f1anatic, eu vorbeam exclusiv de masinile de peste 500 cp de care a amintit Iulian, nu am zis ca AWD-ul nu are avantaje ci doar ca nu e deloc obligatoriu awd-ul pe masini de peste 500 cp.