all wheel drive

[Paul Pedersen]

I understand that you can get any split you want through the
center differential. But wouldn't this show up in the gear
reductions stated in the service manual ? The WRX's specs
show different gearing front and back (balanced by the center
differential) but the Sti's specs are equal front and back.

 

[Patrick Fisher]

I understand that you can get any split you want through the
center differential. But wouldn't this show up in the gear
reductions stated in the service manual ? The WRX's specs
show different gearing front and back (balanced by the center
differential) but the Sti's specs are equal front and back.

No. Why would they? The only reason some cars utilize different ratios is to
allow a continuous, small transfer of torque to one axle when the center
coupling is viscous. In a viscous coupling, no torque is transferred until
there is some slip. If the ratios are the same, then generally there will be
no slip, and the torque goes to one axle (the front). As soon as the front
wheels started to spin, torque would suddenly be transferred to the rear as
the fluid heats up and thickens. The last thing you want is sudden torque
transfer to the rear in a low traction situation. So they "pre-tension" the
coupling by having different final drive ratios. It "seems" like the rear is
always slipping, so there's almost no "play" in the coupling. Torque
transfer under slip conditions is smoother.

In any other diff setup, different final drive ratios will just cause the
diff to melt - you'll be constantly fighting the diff's slip-limiting action
by causing perpetual "slip".

 

[BlueSTi]

All I can say is the STi handles like a rear wheel drive until you
really nail the throttle in a corner. My previous AWD vehicles seemed
to handle more like a FWD.
The STi feels more like a "push" than a "pull".

BlueSTi
"Scary-Fast"

 

[Yousuf Khan]

No. Why would they? The only reason some cars utilize different
ratios is to allow a continuous, small transfer of torque to one axle
when the center coupling is viscous. In a viscous coupling, no
torque is transferred until there is some slip. If the ratios are the
same, then generally there will be no slip, and the torque goes to
one axle (the front). As soon as the front wheels started to spin,
torque would suddenly be transferred to the rear as the fluid heats
up and thickens. The last thing you want is sudden torque transfer to
the rear in a low traction situation. So they "pre-tension" the
coupling by having different final drive ratios. It "seems" like the
rear is always slipping, so there's almost no "play" in the coupling.
Torque transfer under slip conditions is smoother.

I think there is some confusion here. The viscous coupling in Subaru's 5MT
centre diff is there just to aid a fully mechanical gear-based differential.
The two ends of the diff are usually spinning at exactly the same speed
(when travelling in a straight line), and they don't really have a speed
differential until the car starts turning. The viscous coupling then puts
some resistance on the two ends of the diff to prevent slippage. It only
takes effect during turns.

Yousuf Khan

 

[Dominic Richens]

I think there is some confusion here. The viscous coupling in
Subaru's 5MT centre diff is there just to aid a fully mechanical
gear-based differential. The two ends of the diff are usually
spinning at exactly the same speed (when travelling in a straight
line), and they don't really have a speed differential until the car
starts turning. The viscous coupling then puts some resistance on the
two ends of the diff to prevent slippage. It only takes effect during
turns.

I would go futher than that. The center differential is an open
differential with the two output shafts connected in the middle by a visous
coupling. Even without the viscous coupling, you still have an AWD vehicle;
i.e. equal torque to all four wheels, which is fine so long as all four
wheels have traction.

Where the VCD comes in is when you are stuck in the snow/mud with one wheel
with no traction (i.e. zero torque - hence all wheels have zero torque).
The VCD heats up and essentially stops the differential from being a
differential (i.e. locks up).