Originally Posted By: Jim Allen
Some of you seem to have missed the point here. The original poster is replacing the factory diff with an aftermarket Truetrac, which has different lube requirements than the factory diff. Eaton had some recommendations specific to their unit and they made them. It wasn't bashing or condemnation of a particular oil, just a recommendation for their unit. Simple.
One reason Eaton does not want synthetics or slippery additives used with the Trutrac is that it affects the bias ratio of the limited slip. The gear type limited slips rely on a certain amount of gear friction to operate. If you reduce friction with various base oils or additives, it reduces the effectiveness of the unit to a degree. I've not heard of any noise issues using synthetics, but the Eaton guy would probably know better than me.
Want to Know More?
The Truetrac has three sets of element gears (a.k.a. pinion gears) in mesh around side gears that are splined to the axle shafts. There may be more or fewer element gears depending on the torque capacity of the unit and the desired bias ratio. Both the side and element gears are helical, though the gears are specially cut to provide more resistance and friction. The element gears are also in mesh with the element gears on the other side.
They operate partly on the one-way worm gear principle, meaning that when they are "powered" from one direction (e.g. the outside wheel speeding up as the car turns) they rotate freely but they resist turning in the other direction (when driven by the ring gear thru the carrier). The other friction comes from the axial thrust of the element gears into the housing. Torque tends to wedge the gears into their pockets and create extra resistance. In both these situations, decreasing friction would tend to decrease the bias ratio of the unit. The friction is carefully calculated and the lubricant is part of that equation.
In use, the gear type limited slips are very smooth and have a generally constant bias ratio, meaning that they do not gain much extra braking force from increases in input torque, as do the plate or cone type limited slip types, but they react faster to changing traction conditions. The clutch type limited slips need a fraction of a second to “ramp up,” i.e. allow the gear separation forces a chance to increase clamping force on the clutch packs beyond the preload. The gear type limited slips also maintain their bias ratios over a longer period. As the clutches wear in plate or cone type limited slips, their bias ratio decreases and they revert back to open diffs. Most plate types are practically open diffs by the time they hit 80K miles. Some go sooner. The bias ratios of gear type limited slips do degrade over time but the drop is very small and very gradual... slow enough that you can call them "life-of-the-car" limited slips.
The side and element gears allow for speed differential in turns, according to the gear ratios of the tooth counts, but do so while applying torque to both tires. This makes them popular on road race courses because they offer a smooth and predictable action. In simple terms, a driver can power through a turn and power out of a turn more aggressively without the danger of losing control.
They are also better in ice than plate type limited slips, because they do not need tire grip in a turn to "break away" the clutch pack to let the unit differentiate in a turn. I have been running a Truetrac in my pickup for a few years now and it's almost seamless, even on ice. In four-wheeling, they can be overcome by wide variances in traction side to side, ore so than a preloaded clutch type LS, but with brake pedal modulation, the bias ratio can be controlled by the driver. Ditto when they are combined with electronic traction control systems that use the ABS brake system.
Good information to know, since you have the truetrac. What gear oil are you running? How do you feel about synthetics in this differentials? What viscosity should I run? What is a really good dino gear oil? What about semi-blend gear oil?
