Project Grey Mustang 5.0: Part 5 – Putting the Power Down with Eaton


The Eaton TrueTrac operates on a very similar principle: helical worm gears are utilized to transmit the torque from one axle to the other when their speeds vary.  This official Eaton video shows the differential operating in a variety of conditions:


Source: Eaton


These two diagrams show the different components that make the TrueTrac operate.  The side and pinion gears are the key.  Source: Eaton

Those left-right pairs of pinion gears are the secret to the operation of the TrueTrac.  As seen in the below cutaway view, the pairs of pinion gears mesh with each other in addition to the side gears.


Source: Eaton

When both axles are moving at the same speed, the pairs of worm gears do not move relative to the side gears or each other.  This allows “normal” operation of the differential.  However, when going around a corner or on a slippery surface, the difference in relative speed of the two axles causes the worm gears to rotate into each other, transmitting torque from one side to the other.  A force is created on these pinion gears, progressively “wedging” them into the pockets they are installed into and thus progressively “locking up” the differential.


As you can see from the above diagram, three forces act on the pinion gears, each in a different axis.  The resulting vector sum pushes the worm gear into the pocket and begins to “lock” the two wheels together.  Source: Eaton

The most important part of the limited-slip differential in terms of corner carving is the bias ratio.  The Torsen units found on the Track Pack and BOSS Mustangs have a bias ratio that runs from 2:1 on the “street” units to 4:1 on the race-only T-2R units found on the BOSS 302R.  The below table summarizes the bias ratios of these various LSD units.


Differential Type

Bias Ratio

Ford Trac-Lok  (standard)

< 2.0 : 1

Torsen T-2 (street)

2.0 : 1

Torsen T-2 (BOSS)

2.7 : 1

Eaton TrueTrac

3.5 : 1

Torsen T-2R (Race)

4.0 : 1

As you can see from this table, the Eaton TrueTrac gives about 88% of the bias ratio of the race-only T-2R for half the cost and with much improved durability. Sources: Ford Racing, Eaton

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