One of the best scenes in “My Cousin Vinny” was when Mona Lisa Vito, played by the adorable Marisa Tomei, is on the witness stand as an automotive expert. The case hinged on a set of tire marks. When asked how the marks were made and what a limited slip differential is, she said “A limited slip differential distributes power equally to both the right and left tires,” and then aptly explained, in a heavy New York accent, what an open differential was with “Anyone who’s been stuck in the mud in Alabama knows you step on the gas; one tire spins and the other tire does nothing.” Funny and accurate.
We may not deal with Alabama mud too often, but rocky hills or a deep sandwash are all too familiar of terrain and having the ability to get out of that sand and back to terra firma requires traction. Most vehicles come stock equipped with an open style differential. They work fine on the street as they allow the two axles to work independently from one another, like what’s needed to go around corners.
But, they can actually make things worse in the dirt as they have a bad habit of sending all the power to the one tire that has the least traction, thus causing that tire to spin uncontrollably and possibly dig a hole that may be tough to get out of. To combat this and give you all the traction that you need, there are things such as lockers and limited slip differentials.
For the absolute in both tires rotating at the same rate, there are lockers. They are either air or electrically operated and are great for hard core 4 wheeling where putting the power equally to both wheels is needed for slow climbs over big rocks. But for us who drive trucks on trails and in the desert, they may not be the best call.
That would probably be an LSD, or limited slip differential. There are two main designs of LSD’s as one uses helical gears and the other uses springs and clutch style friction plates. Eaton makes both styles of LSD’s, and they have been around as long as autos themselves. Though now a huge international conglomerate with ties from aerospace to electrical relays, the first product they offered was a patented internal gear truck axle way back in 1911 as Torbensen Gear and Axle. Now, they offer everything from the iconic “Detroit Locker” to the E-Locker to the subject of this install, the Trutrac.
According to Eaton, “The Truetrac operates as a standard or open differential under normal driving conditions, allowing one wheel to spin faster or slower as necessary. When a wheel encounters a loss of traction or the terrain changes, the gear separation forces take effect and transfer torque to the high-traction wheel. The helical-shaped gears mesh with increasing force until wheel spin is slowed or completely stopped. When the vehicle exits the low traction situation, the differential resumes normal operation.”
The owner of this truck likes to drive off-road and the open differential that his Ford F150 had was ok to a point, but says that even with 4 wheel drive there were times when we was worried about losing traction when he needed it most. These were either sandy or muddy conditions or when going up a steep hill. He said he went with the Trutrac because of the towing he does and felt the helical gears were the stronger option over the spring and plate design.
He may have let it go if not for the fact that he really needed to change the gearing. Having swapped out stock tires for some 35-inch BFG KO2’s, the 3:73 gears were now not close to being right. He stated that the engine was barely turning 1800rpm at 75mph (not that we condone going over the posted speed limit…). Their wrongness was especially evident whenever the owner tried to pull his fully loaded toy hauler up any hills. He was afraid he was going to hurt the engine or trans. A change to 4:56 gears was required, so he figured he do it all and add an LSD while things were apart.
G2 Axle and Gear also offers a lot of products to their customers. They produce everything from gear sets to complete bolt in-ready rearends. They also had the appropriate 4:56 gear set to fit the 8.8 Ford rearend in this F150. Forged from 8620 steel, heat-treated and machined to OE specs, these gears will put the powerband back where it needs to be. Not known for great low-end torque, the 5.4L V8 will at least have a chance with the 4:56’s, and can only benefit from the addition of the Trutrac. Some think that a Trutrac in back and an on-demand locker up front is the ultimate off-road setup for serious chasing and overlanding with a 4x4 pick-up. We think that setup could put a hurt on winch builders everywhere.
This install happened at the Riverside CA. 4-Wheel Parts store. Both the front and rear differentials received the G2 4:56 gears though a Trutrac was only installed into the rear one. Speaking of installing things, know that each end is about a 4-hour job plus parts, so expect a bit of sticker shock. Also know that the gears must be broken in reasonably slowly. Short trips of less than 50-mile duration is recommended for the first 200-500 miles, and no heavy towing for about the first 1000-miles.
Follow along and see how easy it isn’t to get a truck ready to take on multiple roles. From tow rig to overland explorer, a 4x4 truck can do it all; if it has the right pieces, that is.