Funny thing happened to me the other day.....

I was round at a mates house who lives in a bit of a rural area and had no choice but to park in his field/garden.

His field not being the flattest in the land.
When I reversed out of his field my rear left tyre lost contact with the ground as I was driving up a banking and span wildly leaving me stranded as no power seemed to be going to any of the wheels!!!????!!!

My only choice was to select first gear and do a full U-turn in his field and drive out forwards........ bizare no?

So my question is do we have 4WD in reverse......

or can somebody explain whats going on?

P.S. the field was dry as bone.

 

No transmision boffins out there?

 

it wont drive as it requires all wheels to be in contact because subarus dont have lockable diffs like some landrovers which would then allow it to climb out on three wheels or lsd diffs which i know is gonna cause some arguments but if they did have lsd surely it should have got out which i have said all along they only have viscous couplings.
ah well bettewr wait for the abuse!

 

So why could I drive forward if only 3 wheels were in contact....

In reverse it stopped me dead spining the wheel not in contact with the ground in forward motion no problem!

Good explanation Scoobyboy but if your right wouldn't I of been completely stuck in forward and reverse?

Plums....

 

Were you driving forward downhill?

I snapped a driveshaft which meant I had no drive to one of the front wheels, most of the time I could still move OK but if I applied too much power it span away on the broken driveshaft.

It would not go up really steep slopes and felt as though the clutch was slipping, except that it was worse in lower gears

Paul

 

It wasn't really driving forward down hill as the gradient can't of been more than 5 degrees and the car stayed stationary while I was stranded, gravity played no part in freeing me!

Plums.

 

Imprezas _are_ full time 4wd, they are conventional diffs with a viscous coupling which opposes the any difference in speed across the diff. (Limited slip differential) There is however, a limit to the locking ability of the diff/coupling arrangement. I don't know what that limit is.

Locking diffs is only for serious offroad. Not high performance cars.

I know no reason why the lsd would not work in reverse. Only thing is that reverse is obviously a lower gear, hence more inclined to wheelspin.

Further, UK cars have an open (non-LS) front diff, though that doesn't seem relevant here.

 

From dim and distant recollection of the owners manual on my Legacy, the car is fitted with 3 limited-slip diffs. If I understood correctly, the diffs do the normal diff-type stuff, until a wheel loses traction - then the diff locks, giving power to both wheels equally.

I'm guessing that the hardware that makes it possible wouldn't work in reverse - since the locking mechanism probably only works in one direction.

No idea if this can be backed up, but it seems to make sense to me.....

 

Still not satisfied, Plums, are you? I am still waiting for the ultimate explanation, too ;-)

Reason: I made a similar experience last winter when going into reverse on a snowy parking lot. The car behaved like 2WD! I had the impression my Scoob was loosing traction at only one rear wheel(!) while moving backwards, being terribly slow meanwhile.
When changing to first gear, it seemed to be okay and 4WD-like again!

What's going on? Seems like there is no diff-locking in reverse indeed. So this would be kinda fake 4WD for me :-(

patGT

 

Right - I'm going to do my best to see if I've got it right in my own mind ...

Subarus use viscous couplings to control the amount of slip between the wheels (not sure if it is 2 or 3 couplings, there is certainly one between rear wheels, and one front to rear, I think the front pair have a traditional L.S.D.).

Viscous coupling work by having a liquid stored in container, and some impellors (think of propellors) that ONLY spin when one set of wheels is moving faster than the other. SO when one of your rear wheels start to spin, the liquid starts to get spun by only one set of the the impellors - this makes the liquid become more viscous very quickly, the sticky liquid then starts to drag the other impellors around - thus transferring power from the free-spinning wheel to the wheel with more traction.

Now I ASSUME this whole thing assumes the wheel spin will always be forwards and not reverse. If you run an impellor in reverse, it can't make the liquid more viscous, so no power will be transferred.

I'm no expert, but found all this out on a useful site..
http://www.howstuffworks.com/differential7.htm

What do you think?
Plausible??

Ian.

 

sounds good to me fubar!

 

How long does an LSD take to lock-up when cold. I would expect an car on the track LSD's to work quicker than if I jumped in my car and started doing donuts, only a thought, no sciene involved, now where are my car keys.

Mark A

 

Hmmmmmm seems plausible.... why not?

Cheers Fubar!

PatGT welcome mate you'll find scoobyne very addictive!!!

Just ask my wife!!!

Plums.....

 

Thx Fubar!
Yes, seems plausible although I would not really like it..
Because one would always have to care about avoiding going backwards under slippery conditions - very annoying and embarrassing, too. I already hear people laugh "we thought you had AWD.."

@All
Today, I sent a mail with that topic to the Subaru import dealer here in Switzerland. Wondering what they're gonna say. I will post their reply as soon as I get it.

Regards,
patGT -> Scooby freak nevertheless ;-)

 

Mr. Sempers is spot on.

Viscous coupling is a set of plates in a casing imersed in silicon fluid. Half the plates splined to the outer and half the inner.
When both wheels are rotating at the same speed, this lump rotates as one unit with no spinning of the plates, but as soon as there is a speed diff. the plates spin inbetween each other, heat the fluid which then goes more solid/stiffer and resists the speed difference. Obviously the resistance is limited by the size of the unit. Scoobs have a vicous centre and rear, while the front is a std. free diff. as in not an LSD at all.

Don't know why she wouldn't rev out of the field. Must have gone over the limit of the rear viscous.

Cheers
Daver

 

Quick addendum:

I believe the standard centre is 10kg/m (I presume that's the maximum torque transfer, can't convert to lb/ft)

Plates in a vc are flat plates perforated with holes and/or slots, running quite close together. Alternate plates are connected to each side. There's no reason I'm aware of that the vc unit should be directional...

Not that that helps explain.

http://www.fortunecity.com/silversto...action_4wd.htm

Has a good description of some of the diff principles (watch out for the popups ads), there's also an article on autospeed.com about (upgrading) impreza centre diffs.

- Mark.