Fall has come and gone and winter is here with a vengeance.
Neither means I have to put the FC away. Au Contraire, slippery conditions are sometimes the most fun in an FC.

I have a disagreement with a friend on the operation of the Limited Slip Differentials as found in the FC.

I believe this behavior to exhibit itself on FOUR different FCs, encompassing both turbos and NAs and both S4 and S5 models.


My friend owns both an S5 Turbo and an S5 N/A both have the original viscous LSDs.
I own an S4 GXL with the original clutch pack LSD.
And we have a mutual friend that has an S4 Turbo with its original clutch pack LSD.

My buddy's house is on a hill. Several times one of the four cars has had the wheel closes to the curb on a patch of ice or some wet leaves or something over the past few months. Each of those times, we are unable to move our cars without either first rolling backwards onto bare pavement or spinning one wheel on the slippery debris.

The disagreement we have is whether this is normal or not... I think all of the cars may have weak LSDs which are now acting as open diffs. He thinks this is normal behavior.


Now, it's my understanding that on a viscous, you should see some wheel spin until the fluid heats up and the wheel with traction gets some torque.
And that the clutch pack differentials are superior because they will have almost NO wheelspin because the clutches are always engaged by a spring. Any wheelspin is actuall the clutches slipping. The more worn out the diff is, the more wheel spin you will get.

He believes that in order for the LSD to operate properly, both wheels must have at least a little traction to provide some resistance which will activate the differential.

I thought this was only the case in Torsen type differentials found as stock equipment in the FD.

My citing is this:
http://www.howstuffworks.com/differential.htm

I know it's basic, but it's still relevant.


I'm just a bit confused because I was told that the diff in my car was rebuilt shortly before I bought it... I wasn't provided with papers, so one of three things is possible.

1.) It wasn't rebuilt. It's worn out and I'm right.
2.) It was rebuilt wrongly. It's still worn out or spring preload was set wrong.
3.) I am wrong and this is normal behavior for the clutch pack LSD.


Keep in mind we have not measured breakaway torque of either of the cars. I prefer a scientific method that gets me numbers...
Though they all pass the idiot test where dumping clutch results in a two black tire marks on the road. This could just indicate that both tires have equal traction and I don't accept it as positive proof that the LSD is indeed working. I've lit up both tires in an open diff vehicle.

Someone settle this argument and keep my user text in mind when answering. Show me some proof... A link, test reference, or something...

Also, does anyone know the breakaway torque of the factory diffs?

 

More info is here:
http://forum.miata.net/vb/showthread.php?t=315039

Where it shows the factory breakaway torque to be around 40 ft/lbs... Meaning that the wheel that has traction is always getting 40 ft/lbs of torque to move the car while the remainder of what the engine is putting out goes to the slipping wheel.

 

my understanding of clutch type LSDs is that they have 2 or perhaps 3 "modes" if we can call them that.

the first mode is the rest mode, where u are not applying any torque to the diff with the drive shaft. In this mode, the diff has some limited slip behaviour due to preload on the clutch pack inside. the amount of this preload should be based on the spring in the diff and how worn the clutch packs are.

the second mode is the one when we start applying torque to the diff through the driveshaft. This mode can only occur if the diff is resisting spinning. this resistance can come in the form of either
a) the wheels have traction and the torque applied to the diff is accelerating the car
b) the wheels or a wheel have/has no traction and the torque being applied accelerates the wheels
In this mode, as you apply torque to the diff and the diff resists this torque, the force created inside the diff applies pressure to the clutch plates inside. as this force grows the pressure applied rises and the limited sleep behaviour increases.

In your example, you have 1 wheel on ice (very little resistance). So in order to go into the second mode of the LSD (and create limited slip behaviour beyond that of the preload) you would have to punch the gas hard initially and then let off once the lsd starts to work. The hard punch of gas will work to accelerate the spinning wheel with torque applied by the engine. The wheel will resist this acceleration and the net force in the lsd will lock it up. once it locks up a bit, it will now be applying force to the other wheel, and the torque in the driveshaft will be accelerating the car up the hill (instead of accelerating the spinning wheel). Now you just have to keep the throttle applied enough to keep the car in place or moving forward, and the resistance force will be the car trying to roll back down the hill (as opposed to the resisting force being the spinning wheel resisting acceleration).

I dont have links to back this up, but this is the behaviour i saw in my 86 gxl and this is the understanding ive built over the years from bits of info here and there. the fact that your wheel spins so easily in the ice probably indicates that the preloaded force ont he LSD is below spec. which could be several things. bad fluid, worn disks, worn spring.

the possible third mode of the LSD would occur if you can distinguish between positive and negative force applied through the driveshaft (engine accelerating and engine braking at high rpm). im not sure if all clutch type LSDs distinguish between these, but i believe that some high-end race ones do. so u can have the lsd act differently under engine braking then it does under acceleration.

 

Let's put it this way.

Autocrossers with a lot of bar and a lot of spring tend to prefer clutch-type LSD's compared to torque-biasing LSD's.

Why?

In the tight corners, there is a tendency to lift the inside rear wheel. Torsen's will have no slip-limiting effect unless the e-brake drag is used. Clutch-types will still be able to put power to the ground, even though the lifted wheel has no traction.

The problem you pointed to comes from wear.

Mind you, I have no experience with LSD's in a static situation, i.e. car starting from a stop with something that affects the traction of the tires underneath the tires.

 

Viscous diffs work by shearing the fluid inside the differential and using the resistance to shear as a fluid coupling between the two drive axles. In other words, the two wheels are coupled together using fluid. When the fluid heats up, it becomes easier to shear, and the diff looks more like an open unit.

Viscous diffs don't really wear out - but as the fluid ages (or as I mentioned, as it gets hot) it shears more easily so the differential looks more "open".

Your clutch differential only has a maximum lockup it can provide. If it didn't allow any slip, it would be impossible to turn the car without sliding the rear end (think of a dirt track car running a spool). The idea is there is a maximum wheel speed differential that is "allowed" before more power is coupled over to the other side. As the clutches and springs wear, this speed gets higher and higher, and eventually it will become infinite - an open differential.

Incidentally, there are fancier diffs out there that have different lockup characteristics on acceleration and deceleration. This is good, because a diff that has a lot of lockup coming out of a corner may be appealing but a diff that looks open may be appealing on corner-entry for stability reasons.

Your clutch-pack differentials sound worn out to me. You can always have them rebuilt to a higher lockup or to change the ramp. Check out diffs-online and see if they do Mazda diffs.

JV