What is a VLSD?

[SATAN]

I am writing this because there are people that do not understand what a viscous limited slip differential (VLSD) is. There is nothing wrong with that, but let me clarify.

 

A VLSD is different from a CLSD (Clutch Limited Slip Differential). The CLSD has actual clutches that engage when the rear wheels turn at different speeds from each other. The VLSD while performing the same task, does not have clutches. I am not going to go into the EXACT differences of the two diff's. You can research that elsewhere if curiosity gets the best of you. Instead I will give you a somewhat simple idea.

 

The VLSD has a viscous fluid inside of a coupler. This coupler is part of the internal working of the LSD much like the clutches of a CLSD are. Obviously they are not the same though. Instead of clutches engaging when the wheels spin at different speeds, the inside and outside of the viscous coupler spin at different speeds from each other (this will make sense when you look at the picture).There are two sets of plates inside the viscous coupling. When they spin at different speed, the thick fluid inside is forced to try and catch each set of plates up with the other. When driving in a straight line, they are both spinning at the same speed, when turning a corner, one wheel spins faster than the other, in turn one side of the viscous coupling is spinning faster than the other. Below is a picture of the viscous coupling out of my Q45 VLSD. This is essentially what all viscous couplings will look like. Sadly you cannot see the inside of it because it is a sealed unit.

This is one side of the coupler...

And this is the other side.

 

In the second picture you can see where the splines of the input shafts are. Then you can see the track where the planetary gears ride on the outside. You will notice that the spline section and the track for the gears are two separate pieces. These are the pieces that are allowed to slip differently from each other. When they do, the fluid inside tries to catch both sides up again, creating your LSD effect.

 

Now some things to know about viscous LDS's...

 

It has been noted that you CAN burn the fluid inside the viscous coupling by over heating it. If this happens you can damage the fluid and change its viscosity. When this happens, it is no longer able to catch the two pieces of the coupling up with each other like it should. Essentially, your coupling is burned up and needs to be replaced with another working unit. I do not actually KNOW of anyone this has happened to, but have heard of it happening.

 

Changing how the viscous unit locks up by changing the fluid inside... Sorry but, you can't. You cannot change the fluid inside of the viscous coupling. It is sealed, you cant get in there without destroying it. Keep in mind this is NOT the same as changing the differential fluid, (which will still have zero effect on how the coupling operates). What some people do is actually shim the viscous coupling tighter. This doesn't really change how fast the unit locks up, but it does sort up wedge the coupling in place and causes it not to spin in the first place. Some people do this wit burned out units to try and help get their LSD effects back. Kind of a kludge fix.

 

How do you know if you have a VLSD or a CLSD?

 

Again if you refer to the pictures above, you will notice a washer in the center of the viscous coupling. This is to keep the longer input shaft from stabbing all the way through the viscous coupling from the wrong side. If this happened you would actually lock both sides together making a solid diff. Now before you get any brilliant ideas about doing this to lock your diff by knocking that center washer out... there is more to it than that due to the differences in the two input shafts. But I will not go into that here.

 

So, looking through the holes where the input shafts plug into is one way to tell. You will see that washer in the center.

Another way to tell, is to look at the input shafts them selves. One of the splined sections on one axle of VLSD input shafts, will be substantially longer than on the other axle. This is so it can plug all the way into the viscous coupling. On a regular CLSD they are close to if not the same length splines.

The third and hardest way, is to take the diff apart and visually inspect it.

 

I hope this clears up some questions and common misconceptions about the VLSD. Hopefully someone will sticky this so the information is easily found.

[johnc]

Below is a typical inner view of a VLSD. The units rely 100% on the shear strength of the fluid sealed inside the housing.

 

 

 

VLSDs tend to work very well on street cars and as the center diff in a 4wd setup. Except for a couple very expensive exceptions they are not as effective in racing. As mentioned above, they tend to heat up under sustained load and then fail to an open diff. Most VLSDs that are street driven are pretty much open diffs by the time the unit hits 75,000 miles.

Edited October 6, 2009 by johnc

[HowlerMonkey]

Are those drain and fill plugs?

 

If so, I'll venture a bet that it's the fluid that ages........or seeps out.

 

Crosses fingers hoping that drawing came from a nissan service manual.......because that unit looks servicable with simple snap ring pliers but I doubt seals would be easy to come by.

 

It might be possible that toyota fan clutch fluid could be used.

 

If it is the correct formulation, then toyota offers 3 viscosity indexes.

[rvannly]

Are those drain and fill plugs?

 

If so, I'll venture a bet that it's the fluid that ages........or seeps out.

 

Crosses fingers hoping that drawing came from a nissan service manual.......because that unit looks servicable with simple snap ring pliers but I doubt seals would be easy to come by.

 

It might be possible that toyota fan clutch fluid could be used.

 

If it is the correct formulation, then toyota offers 3 viscosity indexes.

 

 

Hey, I was thinking the same thing about the fan clutch fluid... same concept sort of, in a fan clutch system right?

[SATAN]

If you look at the viscous coupling in my post from a 1990 Q45, you can see there are NO access ports of any kind. It is NOT a serviceable unit. The same goes for all 300zx VLSD's if I'm not mistaken.