How our AWD works

[RJS5689]

So this is my first vehicle with AWD and I just wanted to make sure im understanding the mechanics and how they all work together. Our vehicles have a rear and center differential and an open up front. Standard split is 50/50 (unless you are 5EAT.) Now, when slippage occurs, the center differential is responsible for moving power front to rear? At the rear end, the differential is responsible for sending power to the wheel with the most grip, rather than the wheel with the least resistance, and up front, the wheel with the least resistance will spin because the front end is open? Do I have the basics correct? If you have both the right wheels on snow, but the lefts on pavement and you mash it what would happen mechanically? Would the rear send power to the left wheel while the VDC brakes the inside right wheel? And what about up front, would the wheel on the snow try and spin but VDC slow it down? Does it have a way of sending power to the outside left wheel? I really feel dumb having to ask these questions as someone who is an "enthusiast" but it's good to know how things really work.

[NSFW]

Different models have different differential schemes. My '05 has viscous couplings for the center and rear diffs, and an open diff up front, and a 50/50 front/rear torque split, but I think the spec B is different.

 

A differential is primarily responsible for allowing its output shafts (be they left/right or front/rear) to rotate at different speeds. That's nice when you're turning on dry pavement, less nice when you're on something slippery. Open diffs kinda suck in the latter scenario, as they'll cheerfully let one wheel zing along while the other wheel remains stationary.

 

The thing that makes non-open diffs better is that they will bind the output shafts together, to some degree. People talk about 'sending torque' like there's something fancy or intelligent going on, but really it's just acting less like a diff and more like a solid axle. There are different mechanisms for avoiding open-diff-syndrome, but that's really all it comes down to.

 

The VCs in our cars don't bind completely, and IMO they don't bind nearly enough, but they do try.

[RJS5689]

Different models have different differential schemes. My '05 has viscous couplings for the center and rear diffs, and an open diff up front, and a 50/50 front/rear torque split, but I think the spec B is different.

 

A differential is primarily responsible for allowing its output shafts (be they left/right or front/rear) to rotate at different speeds. That's nice when you're turning on dry pavement, less nice when you're on something slippery. Open diffs kinda suck in the latter scenario, as they'll cheerfully let one wheel zing along while the other wheel remains stationary.

 

The thing that makes non-open diffs better is that they will bind the output shafts together, to some degree. People talk about 'sending torque' like there's something fancy or intelligent going on, but really it's just acting less like a diff and more like a solid axle. There are different mechanisms for avoiding open-diff-syndrome, but that's really all it comes down to.

 

The VCs in our cars don't bind completely, and IMO they don't bind nearly enough, but they do try.

 

Yea, the Spec.B uses a viscous center and torsen rear.

 

I know what you mean about an open diff. I used to love doing "one wheel peels" at the strip.

[Bolksey85]

Yea, the Spec.B uses a viscous center and torsen rear.

 

I know what you mean about an open diff. I used to love doing "one wheel peels" at the strip.

I'm sure your spider gears did, too.:lol:

[carrata]

If you have both the right wheels on snow, but the lefts on pavement and you mash it what would happen mechanically?

 

What I've noted over the last few days (except my right tires were on sheer ice, and left were on dry pavement). I'm running Nokian Hakka 4 studless:

 

From a stop with VDC on:

Traction control will pulse the brake on the slipping tires (in this case the front right, and to a lesser extent the rear right) but will also limit throttle - which invariably limits the output to the 2 wheels that HAVE traction. Personally, I find this intrusive.

 

From a stop with VDC off:

TC still seems to limit the slippage on the right tires, but does not limit throttle input. So, the rear LSD (in our case, Torsen) gets to do its job and the power gets applied to the wheel with traction.

 

Mind you, through turns it's a whole different ball game, as VDC does way more than just Traction Control....

 

my observations are just from a stop...

[Impulse]

OP, from your scenario:

 

from what i understand, if either front wheel starts to spin, the shaft going to the front wheels from the center diff will begin to spin faster (open diff). this, in turn, will cause the center diff to start to "lock up" and will use the slower spinning rear shaft to slow down the front shaft. at the back, the right wheels will have less resistance, so they will try to spin. The viscous/torsen diff in the rear will already have the left and right wheels coupled, so the left wheels are already preventing the right wheels from spinning wildly out of control. because of this, the rear shaft does not rapidly spin up out of control like the front one will.

 

basically, ideally, in your situation. power will get transfered to the rear left wheel and you will basically be "1-wheel drive". im not 100% sure how the VDC works, but it would probably brake the front right wheel, reducing the load on the center diff b/c the front shaft wont be spinning wildly out of ctrl, dumping power into the wheel on the snow.