1986 Ford F-250 HD Towing Axle

Towing and Traction Aids

A traction aid in the form of a locker or limited-slip is as useful to a tow rig as to any 'wheeler, even when towing. We chose to incorporate the new Detroit Truetrac for the Ford Sterling in our buildup. It proved to be virtually seamless, whether towing or solo. It got us off a field of soft dirt in two-wheel drive while towing a pair of loaded grain trailers weighing over 30,000 pounds. The open diff couldn't in the same situation, and just spun a tire. The ice and snow performance was outstanding, because the gear-drive Truetrac doesn't rely on traction to release the clutches in a turn like a plate-type limited-slip. The truck rolls around a corner as smooth as glass but has a higher bias ratio than most other limited-slips for improved traction on soft ground.

We got to try out the new Truetrac application for the Ford 10.25 and 10.50 axles. It's a pretty hefty unit, about 10 pounds heavier than the stock open carrier. If your Ford is quipped with the optional Trac-Lok limited-slip, don't cry to see it go away. It has such a low bias ratio as to be nearly ineffective. The Truetrac has a bias ratio of about 3.5:1. (On its best day, the OE unit was under 2:1.) The best part is that the Truetrac is seamless on the street, despite its good bias ratio, because it uses gears instead of clutches.

The End Result

To recap what we learned from this, any towing axle should start with a diff-temp gauge and go from there as needed. A high-quality gear oil with friction modifiers and/or a high-capacity cover are the next steps. Limited-slips and lockers don't seem to add much heat, but they will prevent a tow rig from being stuck in a rest area parking lot-which our test rig once experienced.

We learned that hard parts are seldom necessary for an axle used within the truck's Gross Combined Vehicle Weight Rating (GCWR), or even a few steps beyond. You simply don't get the sorts of torque spikes when towing that kill so many axle parts on the trail. Bigger tires may dictate some upgrades, or a machine that tows hard and 'wheels. In our case, the truck runs tires that are only about 15 percent taller than stock, and it has plenty of reserve strength.

Installing a Truetrac is home-wrench-friendly if you have some axle savvy. You do not need to change the ring-gear setup and, assuming it was correct before, it will stay that way after the installation. Reuse your old carrier-bearing preload settings by making sure you reinstall the same shim thickness (or the same shims) in exactly the same position. It pays to set up a dial indicator before to check backlash. Your double check, shown here, is when you check after and the backlash is the same or very close. In our case it was exactly the same at 0.012 inch, well within the 0.011- to 0.016-inch specification.

The Amsoil Series 2000 75W-90 synthetic has been a mainstay in high-performance gear lubes, but over the time we were in tests, it was replaced by the upgraded Severe Gear SAE 75W-90. The Series 2000 uses top-quality PAO (Poly-Alpha-Olefin) synthetic base stocks and has a pour point of -51 degrees, so it's great in any sort of cold weather above true Arctic conditions. It has a high Viscosity Index of 146, showing that it will retain its viscosity over a wide temperature. It does well in wear tests, having an 0.040mm scar diameter on the ASTM four-ball wear test.

This is a sleeper gear oil. LE607 mineral-based oil is an industrial gear oil with very high load capacity. It's a straight grade-90, but one with a relatively low pour point of -11 degrees. The trick part of this oil is the additive package. It contains something called Almasol, a solid film lubricant additive that attaches itself to metal surfaces in a microscopic layer. It won't build up beyond that microscopic level, but "repairs" itself as the layer wears away. According to LE, it dramatically increases the load capacity of the oil and reduces gearbox operating temps by up to 50 degrees. Various wear tests place it in the highest tier of gear oils, most notable by making it to the 13th stage of the FZG wear test. This oil also has an extremely high resistance to emulsifying in water and will separate rapidly.

Axle Temp Tests

We used an '86 Ford F-250HD 4x4 diesel as a test mule. It has 4.10:1 gears and uses the legendary Sterling 10.25 full-floating rear axle. We installed a diff-oil temp gauge and ran the truck under varying conditions with three different lubricants and in a couple of different configurations. The testing was done in northern Ohio, where steep hills are in short supply, so we substituted heavy loads over a measured course driven at the same speeds and with the same number of starts and stops each time.

Three courses were used: Course A, a 45-mile loop of mixed rural roads at 50 to 60 mph and freeway at 70 mph with a total weight of 7,345 pounds; Course B, a 25-mile loop at 40 mph over rural roads, with many stops and starts and a total weight of 18,625 pounds (7,345 truck, 10,180 gravel and 1,100 trailer); and Course C, a one-time event with two loaded grain trailers, with very soft tires-a 5-mile loop at 20 mph with a total weight of 34,345 (whew!).We also pulled the grain trailers across a plowed field to test the Truetrac.

Our tests did not generate the massively high temps a truck towing 10,000 pounds over the Rocky Mountains might generate, but it did show us how the axle reacted to various situations and products. In monitoring temps, we noted that speed plays a part. At low speeds, temps were always lower than at higher speeds. When we slowed down to 25 mph, even with our test load, temps dropped dramatically. Repeated accelerations from a dead stop dramatically added temperature.

TEMP TESTS*