4WD v.s. AWD [question]

i seem to have believed that most all wheel drive systems are ff most of the time and really the only way the rear wheels get power is when the front wheels are slipping, e.g. when the front wheels are spinning in the snow. however, the change happens almost instantaneously, because the gears in the center differential would now actually have something to lock into.

think of it this way. when you are cruising down the highway in your alltrac, the front wheels are moving at basically the same speed as the rear wheels, therefor both sets of gears (front+rear) are spinning at the same speed, so the transfer case doesn't put any power to the back wheels because they are already spinning at the same speed as the front.

however, when the rear wheels, and in turn, the rear "gears", are not spinning, but the fronts are, this causes a difference in gear speed inside the center diff, which makes the gears lock together, front to rear, sending power to the driveshaft.

it's hard to explain but basically awd is only putting power to the rear wheels if there is a difference in wheel speed, like when the front wheels are stuck in the snow. awd is good because if the rear wheels are spinning, like when stuck in snow, the front wheels will the be locked to the rear in order to pull the car out of a ditch.

awd to me is 2wd until you start slipping, the switches to 4wd, all automatically and unnoticable, which really gives the appeal to manufacturers and people. no buttons or levers to deal with. awd also, because it is basically 2wd, seems to get better gas mileage on the highway because it is both a lighter duty system and because there is no really noticeable power sapping of weight of the driveshaft because the driveshaft itself is actually more being turned by the rear wheels moving, rather than power being sent to it.

4wd is actually sending power, not just compensating for differences in wheel spin, to both the front and rear wheels at all times when 4wd mode is selected. because of this, 4wd trucks that are in 2wd mode will not automatically lock both sets of wheels together.

i hope i explained it clearly enough. because of what i think i know, i consider the 86-87 celica gt4's with the center diff locking switch in the cabin 4wd while i consider the 88-on alltracs awd because of the viscous coupling center diff

 

Not quite - It's not a set of gears that are able to engage/dis-engage

The viscous diff is constantly engaged - think of it as an LSD coupling.
If the speed difference between the front and rear gets too much the viscous coupling (V/C) will start to burn.

It was a serious problem on the early WRC cars who did a lot of "drifting" around corners and caused many retirements in 1988/9.
The problem was solved by switching to XTRAC gearboxes and I think the V/C was eliminated completely

Even dis-engaging the coupling doesn't help - ie on a 2 wheel dyno you have to be very carefull not to overheat the V/C

Later models don't even have the dis-engage mechanism, Toyota realised it was a waste of time

 

After reading this again I had to google viscous coupling, what an invention it is and I quote :-

A viscous coupling is a mechanical device which transfers torque and rotation by the medium of a viscous fluid. It is made of a number of circular plates with tabs or perforations, fitted very close to each other in a sealed drum. Alternate plates are connected to a driving shaft at one end of the assembly and a driven shaft at the other end. The drum is filled with a dilatant fluid, often silicone-based. When the two sets of plates are rotating in unison, the fluid stays cool and remains liquid. When the plates start rotating at different speeds, the shear effect of the tabs or perforations on the fluid will cause it to heat and become nearly solid because the viscosity of dilatant fluids rapidly increases with shear. The fluid in this state will effectively glue the plates together and transmit power from one set of plates to the other. The size of the tabs or perforations, the number of plates, and the fluid used will determine the strength and onset of this mechanical transfer.

This type of device essentially differs from fluid couplings such as torque converters by using the viscosity of the medium to transfer torque, rather than its momentum. This makes it potentially useful even on very small scales. It tends to have less cooling. The torque transmitted is sensitive to the difference in speeds of the input and output but is almost independent of their common rate.

Viscous couplings are used as the center differential in some four-wheel-drive (4WD) vehicles such as the Toyota Celica GT-Four, and also as a limited slip differential (LSD) in rear axles. They offer a cheaper way to implement four-wheel-drive than technologies like the mechanical-transfer Torsen differentials used by Audis.

One of the first mass-produced viscous couplings for a permanent 4WD off-road-capable vehicle was used in the AMC Eagle, produced from 1980 to 1987. The AMC Eagle's central differential was single-speed (no low range option) and used a thick viscous fluid coupling for quiet and smooth transfer of power to the axle with the greatest traction, on wet or dry pavement

Volvo, Subaru, Range Rover, Vauxhall/Opel and many others have used viscous couplings in their drivelines at various times, now mostly superseded by more complicated electronically controlled devices.