The argument for/against FM in a rear diff + LSD

[MolaKule]

Originally Posted By: turtlevette
You're saying all FM by definition decrease the coefficient of friction. If so I've been misunderstanding it.

In any case, the term FM in my opinion is used too loosely.


There is the strictly technical definition of friction modifier and then there is the colloquial definition of friction modifier.

The strictly technical definition of friction modifier is that of a chemistry that can either increase friction, decrease friction, or create a dynamic coefficient of friction.

A friction modifier that creates a dynamic coefficient of friction
alters the friction coefficient with respect to the relative speed of adjacent surfaces.

The FM chemistry in engines reduces friction between sliding surfaces regardless of speed.

The FM chemistry in manual transmissions modifies the friction coefficient with respect to the relative sliding speeds of the synchros'.

The FM chemistry in automatic transmissions modifies the friction coefficient with respect to the relative rotational speeds of the wet clutches.

The FM chemistry in LS differentials modifies the friction coefficient with respect to the relative sliding speeds of the clutches in order to prevent stick-slip and chatter.

Quote:
Because I thought for example F type transmission fluid had a modifier that increased COF. So you're implying it lacks a FM?


A Type "F" fluid lacks any of the FM chemistry found in the Dexrons, ATF+'s, and the Mercon ATFs.

 

[zeng]

Originally Posted By: turtlevette
You're saying all FM by definition decrease the coefficient of friction. If so I've been misunderstanding it.Because I thought for example F type transmission fluid had a modifier that increased COF. So you're implying it lacks a FM?In any case, the term FM in my opinion is used too loosely.


I'm not sure all FM decreases COF.

My understanding is varying components (like differentials/plain bearings/wet brakes/clutches

etc) have varying desired COF (or sweet spots) derived from different types of FM's.

For example, auto transmission wet clutches requires higher COF than hypoid gear sliding , CMIIMW.

 

[69GTX]

I wasn't too familiar with the Auburn LSD back in 2012 when I got the fluid changed for the first time. Per Auburn it was supposed to get 4 oz of FM. I was surprised that the shop put in 7 oz of CRC FM contrary to my written instructions. My concern was that it would make things too slippery and lessen the effectiveness. They insisted it was a proper amount. Can't say with my easy driving style that I've noticed any difference on factory fill or the most recent change.

 

[dailydriver]

Originally Posted By: dparm
Originally Posted By: asand1
With a name like "GKN Visco Lok" I would think it was a viscous (VLSD) limited slip differential, and not need any FM at all?



Well, BMW puts it in from the factory.

The viscous term is derived from this key point (in the PDF up top):

Operating Principle
> A self-contained silicone-fluid filled reservoir
and shear pump generates a hydraulic
pressure proportional to a speed difference
across the axle
> The speed difference between the feed disc
and the fluid filled channel of the pump disc
and the resulting fluid shear forces moves the
fluid from the reservoir to the apply piston
> The hydraulic pressure creates an axial force
via the apply piston, compressing the clutch
pack


That's kind of cool, a combo of a viscous and clutch pack LS diff!

 

[dailydriver]

Originally Posted By: 69GTX
I wasn't too familiar with the Auburn LSD back in 2012 when I got the fluid changed for the first time. Per Auburn it was supposed to get 4 oz of FM. I was surprised that the shop put in 7 oz of CRC FM contrary to my written instructions. My concern was that it would make things too slippery and lessen the effectiveness. They insisted it was a proper amount. Can't say with my easy driving style that I've noticed any difference on factory fill or the most recent change.


7 ounces of additive is not too much, as long as there was not a lot of it (or any) already in the mineral gear oil they used in that 10 bolt's Auburn.

 

[gamefoo21]

Originally Posted By: dailydriver
Originally Posted By: dparm
Originally Posted By: asand1
With a name like "GKN Visco Lok" I would think it was a viscous (VLSD) limited slip differential, and not need any FM at all?



Well, BMW puts it in from the factory.

The viscous term is derived from this key point (in the PDF up top):

Operating Principle
> A self-contained silicone-fluid filled reservoir
and shear pump generates a hydraulic
pressure proportional to a speed difference
across the axle
> The speed difference between the feed disc
and the fluid filled channel of the pump disc
and the resulting fluid shear forces moves the
fluid from the reservoir to the apply piston
> The hydraulic pressure creates an axial force
via the apply piston, compressing the clutch
pack


That's kind of cool, a combo of a viscous and clutch pack LS diff!

It's like a Torsion diffy and a GM Gov-Lock diffy git combined, then because German, the Torsion part was replaced with a viscous version because more complex and German.

LOL

 

[dparm]

Originally Posted By: gamefoo21

It's like a Torsion diffy and a GM Gov-Lock diffy git combined, then because German, the Torsion part was replaced with a viscous version because more complex and German.

LOL



Except that GKN is a British company...

 

[dailydriver]

Originally Posted By: dparm
Originally Posted By: gamefoo21

It's like a Torsion diffy and a GM Gov-Lock diffy git combined, then because German, the Torsion part was replaced with a viscous version because more complex and German.

LOL



Except that GKN is a British company...


Yup, just like; FFD, X-Trac, and Quaife.

 

[MolaKule]

Originally Posted By: zeng


I'm not sure all FM decreases COF.



Did I not just say that in my explanation post #4001452

????