Problem with my rear LSD?

hellyhans

Forum Member
Joined
Feb 15, 2009
Messages
2
Location
Montreal,quebec,canada!
Hi,
a lot of good info on the site and happy to be part of the community.

So I have a 2002 forester S sport edition MT.
And tonight I wanted to just go into some 6 inch deep snow, and I go stuck!

I couldn't believe it.
I've driven off road before and i'm not the kind to go full on gas! lol

So anyways, I noticed that I had one rear wheel on the road and the others in snow.
But my rear tires were not spinning, only one of them was.
It was spinning with the front opposite side which is normal. But I thought my model has the rear LSD. so it should have locked the 2 rear wheels no?
I tried giving full gas thinking it should warm the fluid to lock it, but it didn't.

I also noticed that my car smells like transmission fluid, but its not leaking anywhere.. so maybe its a breather or something.

Can someone give me some info concerning this LSD issue? and maybe why my cars smells like that? should I replace the fluid?
 
:welcome: hellyhans!!!:) Wish I had solid answers to your questions, I don't think you really have a problem with your LSD, they just work like that. I've been in situations with my MT where only one wheel was spinning, and the MT is supposed to be a 50/50 split front to rear. A little bit of brake with your foot on the gas might have helped get your other wheels going.

Is your smell like ATF? Look into the power steering if it is. If it smells like gear oil there are breathers on the gear box and rear diff could be coming from there. Is it a constant smell, or did you just notice it while you were trying to get unstuck?

Where are you located? Might want to put that in your profile. Glad to have you around.
 
I dont like viscous LSD's.

Sometimes if you get the car stuck in deep enough snow, the body is wedged on top of snow, so there isnt enough weight on the tiores to get you to go.

Since you have a manual, here is a trick to use. What is happening is the wheel with least resistance is getting all the power. Here is an ancient trick.

Use the parking brake.

Applying the parking brake SLGHTLY (this takes a little practice) will put enough drag on the free spinning whell to transfer power to the non spinning wheel, and in theory, if that Tire has traction, the car should start to move.

The other trick is to use the foot brake for this, which may require getting the car in gear, allowing tires to spin, and slowly applying some brake pressure.

nipper
 
^ what he said! Spinning the diagonally opposite wheels is common - with or without LSD.
 
Hi guys, thx for the super welcome.
I updated my info in my profile a bit :P
So what I did today, i brought the car over to my friends place and he has a car lift, so I check what the smell was, and I have a leaking front drive shaft seal leaking on my exhaust.

I just got my transmission and center diff replaced under warranty by my dealer and I just got the car 1 week ago, and already a leaking seal.
Most likely the guy didn't seat it properly.
Will be taking the car back to the stealer.

And Im most likely going to replace my fluids in my tranny and diff myself with some new good stuff.
 
The fluid in the viscous LSD as it gets hot from operating in slippery conditions can get hot when used to limit slip (NOT STOP SLIP). As it gets hot, it loses its effectiveness and works more like an open diff. Of course, if the diffeence in grip between the left and right wheel is too much, the diff cannot apportion enough drive tot he wheel with grip. So the end result is what you discovered.

I have slowly climbed a hill with 2 diagonal wheels in the air, but the diff was working well enough to let me do that.
 
Rally said:
The fluid in the viscous LSD as it gets hot from operating in slippery conditions can get hot when used to limit slip (NOT STOP SLIP). As it gets hot, it loses its effectiveness and works more like an open diff. Of course, if the diffeence in grip between the left and right wheel is too much, the diff cannot apportion enough drive tot he wheel with grip. So the end result is what you discovered.

I have slowly climbed a hill with 2 diagonal wheels in the air, but the diff was working well enough to let me do that.
Click to expand...


You have that very backwards. The way a viscous coupling works is that has altenating plates attached to each side of the output. In these plates are holes (calibrated) that creat shear forces. These shear forces cause friction in the fluid and the fluid heats up. The fluid used is silicon, and it sealed. As the fluid heat ups, it gets thicker and expands. From this point there are two differnt kinds of VC. One uses this heat and expansion to engage a mechanical clutch to thransmit force. The other type uses the thickening of the fluid with those plates to transmit the force.

https://en.wikipedia.org/wiki/Viscous_coupling_unit

The old style that we so love was a mechanical clutch, wither a clutch pack or a cone clutch. This was engaged all the time. When the forces on the two wheels (I forget the exact numbers but I can look them up) had a specific variation, the clutchews would disengage and you caould go around a corner without any drama.

The reason they went to the VC arangement is that it is much lighter and cheaper to make. The down side to them is that they really are limited in what they can do. the new electronic control traction systems are a step above (a gaint step) above a VC, but still no where as good as an old fashiobed posi-rear.

Now if you let a VC overheat (mismatched tires) the fluid cooks (this is how you get torquebind too) and the plates get glued together.


nipper
 
so, out of interest, i have never had the centre diff distribute unevenly, so that diff must be better/different type than the rear right?
 
benwarn said:
so, out of interest, i have never had the centre diff distribute unevenly, so that diff must be better/different type than the rear right?
Click to expand...

Same basic idea except that in the center diff the pressurized silicon fluid forces the plates to contact each other. This means that there is a mechanical connetcion of drive forces. the rear diff relies on the fluid getting thick and hot to transmit the power.


nipper

 
benwarn said:
ah, so we really want a centre diff in the rear
Click to expand...

Its the other design of a VC. It uses the liquid to slide clutches togehter as opposed to the liquid transmitting force. What you really want is a good old fashioned posi rear. i think the center diff would be too harsh for a rear diff, as the action of the front is muted by splitting the power 2 more times from the output of the center.



Hell, we just need lockers :)


nipper
 
In my experience the centre viscous coupling is quite effective at distributing torque to the front or rear if one has more grip than the other. The rear viscous coupling on the other hand does not seem to be very effective at all for distributing torque side-to-side. Mine seems to have no 'slip-limiting' ability at all.

Contrary to what others have said, I don't believe you will have much success by 'drag braking' with the park brake or foot brake - this is only effective with torsen type limited slip diffs.

Some people on here have had good results with Cusco brand clutch-type limited slip diffs for preventing cross-axle wheel spin when offroading.
 
https://www.torsen.com/files/Torsen Traction Differential Technical Sheet.pdf

Torsen are limited slip as opposed to locked and unlocked. It converters the diff into a live axle when there is slip. They are great on the track or on the road, but they need some traction to work. When one wheel is spinning and the other has power, the Torsen wont react, as it thinks it is turning a corner.

Thats why you wont see torsen's in the rear diffs of any serious off road vehical (but they may be in the center diff).

https://www.ritchiewiki.com/wiki/index.php/Torsen_Differential

https://en.wikipedia.org/wiki/Quattro_(all_wheel_drive_system)
(read #2)

And here
https://home.wanadoo.nl/xavira/differential/dif.htm

Where Hummer uses them all around, even Hummer states its weakness when one wheel looses complete traction:
"The HMMVV, or Hummer, uses Torsendifferentials on the front and rear axles. The owner's manual for the Hummer proposes a novel solution to the problem of one wheel coming off the ground: apply the brakes. By applying the brakes, torque is applied to the wheel that is in the air, and then five times that torque can go to the wheel with good traction. "

Drag breaking has worked as long as there have been differnentials.

nipper
 
nipper said:
Drag breaking has worked as long as there have been differnentials.

nipper
Click to expand...



In cases of cross-axle wheelspin, drag braking via the park brake or foot brake:
  • will work if you have torsen-style cross-axle diffs;
  • will not work in the case of conventional open diffs or viscous-coupling LSDs on the cross-axles as the driver can only apply equal braking to the spinning and non-spinning wheels and non-torsen diffs have no torque-biasing capability to send more torque to the non-spinning wheel;
  • can have a detrimental effect if you have viscous-coupling LSDs on the cross-axles, since viscous-coupling LSDs become more effective as the difference in speed of the output shafts increases and by drag braking you are probably slowing the spinning wheel and thereby reducing the difference in speed of the output shafts.
The type of drag-braking that is automatically applied by electronic traction control such as that included in the VDC systems on some newer Subies differs fundamentally from that which can be applied manually by a driver in that it can brake wheels individually. That is, it can apply braking to a spinning wheel but leave the wheel with traction unbraked and free to drive the vehicle out of the situation.
 
Last edited:
simxs said:
In cases of cross-axle wheelspin, drag braking via the park brake or foot brake:
  • will work if you have torsen-style cross-axle diffs;
  • will not work in the case of conventional open diffs or viscous-coupling LSDs on the cross-axles as the driver can only apply equal braking to the spinning and non-spinning wheels and non-torsen diffs have no torque-biasing capability to send more torque to the non-spinning wheel;
  • can have a detrimental effect if you have viscous-coupling LSDs on the cross-axles, since viscous-coupling LSDs become more effective as the difference in speed of the output shafts increases and by drag braking you are probably slowing the spinning wheel and thereby reducing the difference in speed of the output shafts.
The type of drag-braking that is automatically applied by electronic traction control such as that included in the VDC systems on some newer Subies differs fundamentally from that which can be applied manually by a driver in that it can brake wheels individually. That is, it can apply braking to a spinning wheel but leave the wheel with traction unbraked and free to drive the vehicle out of the situation.
Click to expand...


But that is assuming everyone has electronic control, which a lot of people do not. Also if you have a real mechanical LSD or a tporsen, its not needed to do. My repply was addressed to the vehical at hand, and open diffs in general. I have used it myself on open diffs so it does work, it just takes practice.

There also is a method of jabbing the brake pedal while applying the brakes (I have not used this yet) that people reports works by using the pulsing of the abs to shift torque.

nipper
 
nipper said:
My repply was addressed to the vehical at hand, and open diffs in general.
Click to expand...

So was mine. The additional note about electronic traction control is just an addendum though very much related in that ETC is another method used to prevent cross-axle wheel-spin. Perhaps I should have said "On the other hand, if you have traction control...".

nipper said:
I have used it myself on open diffs so it does work, it just takes practice.
Click to expand...

I'll have to agree to disagree about this working on open diffs:). However, us Sydney-siders have a trip coming up (https://offroadsubarus.com/showthread.php?t=692) where there should be ample opportunity and several Subies around to further test this technique. I'll report back here if we get the chance to do so.
 
simxs said:
I'll have to agree to disagree about this working on open diffs:). However, us Sydney-siders have a trip coming up (https://offroadsubarus.com/showthread.php?t=692) where there should be ample opportunity and several Subies around to further test this technique. I'll report back here if we get the chance to do so.
Click to expand...

It would be sweet to see some real world testing on this one. Also might finally end the debate on weather it works or not.:iconwink: Just need to test both an LSD and an open diff car.
 
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