AWD vs. 4WD - can someone edjimecate me?

[cannonball]

So! What's a 4x4 then? Aside from a dismissive term for a land rovery, jeepy type gas guzzler. 4wd? AWD? depends who the marketing people are targeting.

 

You had to throw the old "4X4" term in their didn't ya?

[nixon]

You had to throw the old "4X4" term in their didn't ya?[/

 

Old? It's brand spanking new in blighty!

[Martinjmpr]

So! What's a 4x4 then? Aside from a dismissive term for a land rovery, jeepy type gas guzzler. 4wd? AWD? depends who the marketing people are targeting.

 

4x4 is a military term that dates back to at least WWII and I think there may have been uses of the term as early as the 20's.

 

First number is total number of wheels

 

Second number is number of driven wheels.

 

The type of system or T-case is irrelevant. It was simply used to classify types of vehicles. Even today you will hear military people refer to some of the 2 1/2 and 5 ton trucks as "6x6's"

 

So, bottom line anything with 4 wheels on the ground and 4 wheels driven is a "4x4."

 

Note: Some people (and even car dealers) get the relationship between total wheels/driven wheels backwards and will refer to a 2wd truck as a "2x4." A 2x4 is a peice of lumber, a 4x2 is a 2wd truck.

[otis]

Ok, so a few years ago (i think), you could buy a ford explorer with either 4wd or awd. Seeing as there were 2 different systems for the same car, it would no longer be a semantics issue. So why SHOULD one particular customer buy an awd over a 4wd (or vice versa) if they were to buy this suv?

[cannonball]

Ok, so a few years ago (i think), you could buy a ford explorer with either 4wd or awd. Seeing as there were 2 different systems for the same car, it would no longer be a semantics issue. So why SHOULD one particular customer buy an awd over a 4wd (or vice versa) if they were to buy this suv?

 

99.9% of the Explorers SHOULD be AWD at best since none of them will ever see the need for locking the center diff. Then you would have that LSD effect all the time. Snow is the biggest threat at this point, and the AWD should be adequate for most situations to get up and going in the slick stuff. IMO ice/ snow mix is the best recipe for fun with the subie and far exeeds a 4WD in the handling department.

(note: see my definition of AWD and 4WD from my earlier post)

 

A realistic reason to have a 4WD would be if you had a heavy boat that you had to get in and out of the water on a steep landing.

Truth be known I don't really know how the AWD Explorers work. Without researching I would assume that it is RWD until slippage is detected.

[Juan]

Watched the Service Tech spin one rear wheel on my AWD '05 OBW AT while it was up on the rack. Both rear wheels spun at the same rate as he searched for the stem to air up the tire. The front wheels didn't move. I asked him why the front wheels didn't spin as well. He told me that they would if the wagon had a manual tranny in it.

 

Does this mean that both rear wheels, at least, are always under power with respect to an AT?

 

Juan.

[cannonball]

Watched the Service Tech spin one rear wheel on my AWD '05 OBW AT while it was up on the rack. Both rear wheels spun at the same rate as he searched for the stem to air up the tire. The front wheels didn't move. I asked him why the front wheels didn't spin as well. He told me that they would if the wagon had a manual tranny in it.

 

Does this mean that both rear wheels, at least, are always under power with respect to an AT?

 

Juan.

 

If both rear wheels spun the same direction then it means you have a rear LSD.

[zyewdall]

Watched the Service Tech spin one rear wheel on my AWD '05 OBW AT while it was up on the rack. Both rear wheels spun at the same rate as he searched for the stem to air up the tire. The front wheels didn't move. I asked him why the front wheels didn't spin as well. He told me that they would if the wagon had a manual tranny in it.

 

Does this mean that both rear wheels, at least, are always under power with respect to an AT?

 

Juan.

 

As I understand it, the automatic transmission subarus are front wheel drive with rear wheel assist, just like the honda someone mentioned. The rear wheels don't engage unless the computer senses the front wheels turning faster than the rear ones (or unless you hotwire around the duty C solenoid to lock the center clutch on it). The manual transmission has viscious couplings between the front and rear sets of wheels -- if it starts slipping, the fluid gels up and sort of locks the front to the back.

[Juan]

The wheels spun in the same direction. So must have LSD.

 

NOW: When I romp on it in the snow, ALL tires spin. Prior posts seem to suggest that this is not how Subaru AWD works ie one, two (maybe three) tires may spin, but not ALL at the same time. Am I mistaken as to ALL four wheels spinning at the same time? I turn around and look at the tire footprints after a "burnout" in the snow, and it sure looks like a true AW burnout.

 

Juan

[Setright]

Try it again with both right wheels on snow and the lefts on clean tarmac. The right wheels will spin most of the power away.

 

More pronounced on the front if the rear LSD is working.

 

It's not a question of "true 4wd", it's about balance between traction from wheel to wheel. All four wheels have drive going to them, it's a question of whether they can put it to use.

[cannonball]

The wheels spun in the same direction. So must have LSD.

 

NOW: When I romp on it in the snow, ALL tires spin. Prior posts seem to suggest that this is not how Subaru AWD works ie one, two (maybe three) tires may spin, but not ALL at the same time. Am I mistaken as to ALL four wheels spinning at the same time? I turn around and look at the tire footprints after a "burnout" in the snow, and it sure looks like a true AW burnout.

 

Juan

 

If that were the case then both front tires would have to break free at the same time with equal amounts of friction (or lack of) between the two. But as Setright said if the traction is different between the two front wheels then the one with less traction will get the power. Since you have the rear LSD that would help out some, but depending on how stuck you are the transfer may not be enough to move the car. As for the 4EAT's there is a 90/10 split between front and back so the transfer of power to the rear is much faster then say a rear wheel assist drive car like the Honda. Also remember that the torque split is 50/50 in 1st and 2nd. I may be wrong, but the viscous coupling action of the manual is more of a dragging effect then a locking affect. If the front shaft spins faster then the rear shaft then the differential heat will cause the fluid to thicken. This in turn causes the rear shaft to try to match the speed of the front shaft. I have read that the manual has the ability to send 100% of the power front to rear. I'm not to sure that is correct. I think it has the ability to send the available power that the spinning tires aren't soaking up. BTW The VC is sealed with fluid mixed with air. It is believed that this style of differential can be ovecome in certain situations. Psst... BTW people get agitated around here if one compares the Honda with the Subie.

[zyewdall]

Moderators

 

Would be it be possible to write up a sticky or FAQ that goes through all the various 4wd/awd systems, and which cars and which years they were in? Written by someone who actually understands them all (i.e. not me). It seems like every few months, a new thread comes on and discusses this for awhile, with lots of opinions, and some facts. Might be nicer if people could just look it up.

 

A comparison to the VW/Audi/Mercedes/Honda/Landrover/4wd-truck etc might be nice too -- show how the subaru are better (if they are. I don't mean to be arrogant about subarus, but my GL wagon does seem to perform better than my 4wd truck till ground clearance becomes an issue, dispite both of them mechanically being identical, 4wd/2wd, with open differentials on each axle. Must be weight distribution?)

 

Zeke

[Setright]

The fluid inside the viscous coupling is oil. As it heats up it actually decreases in viscosity, but it does expand and that pushes the plates together ;-)

[Martinjmpr]

As I understand it, the automatic transmission subarus are front wheel drive with rear wheel assist, just like the honda someone mentioned. The rear wheels don't engage unless the computer senses the front wheels turning faster than the rear ones (or unless you hotwire around the duty C solenoid to lock the center clutch on it).

 

My understanding, according to Subaru, is that the AT has a 90 front/10 rear split under normal driving conditions which is different from the "automatic 4wd" systems which are 100/0 under normal driving conditions. So on a Subaru, all 4 wheels are always receiving power, which is the difference between the Subaru and the "auto 4wd" systems. If the front wheels start to slip the viscous coupling can send more power to the rear, up to 50%.

 

The MT versions have a center differential and are 50/50 at all times, which is apparently the reason that there is no significant difference in MPG between automatic and MT equipped modern Subarus.

[cannonball]

The fluid inside the viscous coupling is oil. As it heats up it actually decreases in viscosity, but it does expand and that pushes the plates together ;-)

 

 

First time attaching something so I hope it works.

 

Here is some info on the VC. It says silicone oil and air. It explains how it works, but it should be open to discussion on how it works in layman terms.

Center Diff.pdf

[mikkl]

Nice description. Thanks for posting it.

 

mikkl

[cannonball]

Some more interesting reading.

 

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

[Juan]

Some more interesting reading.

 

http://www.autozine.org/technical_school/traction/tech_traction_4wd_2.htm

 

Wow. Read both links posted in this thread. Once you get past the viscous part, it's really pretty straight forward.

 

Having been a cook, I can only imagine that the viscous reaction is much the same as that found in a bowl of a thick cornstarch/water mixture. Stir the mixture slowly with a fork and there is no resistance. Stir the mixture quickly and the mixture becomes so thick (viscous?)that power is transferred from the fork to the bowl as the fork gets "stuck" in it and begins to spin the bowl. Slow down, and the mixture returns to normal until you speed up again. It's really quite amazing and the change in thickness/resistance/ is instantaneous. The reaction is also variable depending upon how thick or thin you make the mixture at the outset.

 

Anyway, thanks for the posts.

 

Juan

[cannonball]

Wow. Read both links posted in this thread. Once you get past the viscous part, it's really pretty straight forward.

 

Having been a cook, I can only imagine that the viscous reaction is much the same as that found in a bowl of a thick cornstarch/water mixture. Stir the mixture slowly with a fork and there is no resistance. Stir the mixture quickly and the mixture becomes so thick (viscous?)that power is transferred from the fork to the bowl as the fork gets "stuck" in it and begins to spin the bowl. Slow down, and the mixture returns to normal until you speed up again. It's really quite amazing and the change in thickness/resistance/ is instantaneous. The reaction is also variable depending upon how thick or thin you make the mixture at the outset.

 

Anyway, thanks for the posts.

 

Juan

 

That's why in my earlier post I stated that I think it's more of a dragging effect than a locking effect. I don't see how the plate surfaces could actually join together since they are directly connected to the drive shafts. As you can see from my avatar I do a lot of cycling. This time of year I am on the trainer. My trainer has a fluid resistance unit, and the faster I turn the rear wheel the harder it is to pedal.

So it appears on a manual that there in no mechanical connection between the transmission and the drive shafts except the oil that uses centifugal force to spin the plates thus turning the shafts. If one spins faster than the other then the oil thickens or spreads (however you want to look at it) and it creates a dragging effect to the opposite set of plates causing torque transfer. If this is the case then Subaru has a lot of faith in that silicone oil. Also this why they don't want uneven tire sizes because it will wear out the oil prematurely rendering it useless. Even the the oil may work under normal conditions it won't have the ability to thicken anymore and you or stuck with an open center diff.

[rugbyben]

can anyone provide a digram of this center unit? I understand the concepts and physics but i would like to see how it is actually assembled. (without taking mine apart of course)