And How to Build It
Axle buildups are part and parcel on the tech side of Four Wheeler but we've never covered the particular needs of a truck that tows a lot. We decided to subject an old buddy to some tests and improvements in order to determine the needs of a towing axle and the cures to any problems.
Our test mule, an '86 F-250HD 4x4 with its Banks-turbocharged 6.9L diesel engine and C6 automatic, has never been anything other than a work truck. Its first 15 years were spent towing a variety of 25- to 30-foot fifth-wheel and tow behind camper trailers, as well as carrying a couple of different overhead campers. These days it's a farm truck, still hauling and towing a variety of equipment.
We started by talking with Carl Montoya at Randy's Ring & Pinion, to find out what special needs a towing axle has versus a 'wheeling axle. Carl runs Randy's retail repair/rebuild shop, where they do a huge amount of axle work as well as R&D for Randy's new products.
"It's all about heat," he said. "The most wasted axles we see here are in motorhomes, box vans, and pickups that tow."
An axle under load generates heat-sometimes well over 300 degrees oil temperature if you let it get that far. More load equals more heat, and that's almost always where failure starts. A trail axle may endure higher peak torque loads, but a towing axle endures a heavy load over a long period of time-from hours to thousands of miles at a time. A towing axle failure is typically related to lubricant failure from heat. When the oil fails, the hard parts follow rather quickly. Without boundary lubrication from the gear oil, massive amounts of friction across the surface of the gears can create temperatures high enough to melt the metal surfaces.
In order to find out what was going on in our test axle, we installed a diff temp gauge. You don't see these on many trucks, but they aren't hard to find. Auto Meter makes fine diff-temp gauge sets in a variety of styles and the means to install them in most any diff. For steel covers, they make a weld-in bung. We used both on our test rig to run the oil temperature tests. We designed an easily duplicated test course on the rural roads in the northwest Ohio wilderness. Some of it was solo, carrying only a 1,000-pound test load of tractor weights, and some was towing a grain wagon with five tons of gravel in it. We also had a chance to haul a pair of large grain wagons full of wheat.
We started by giving the bone-stock Sterling 10.25 full-floater a fresh fill of 85W-90 Valvoline mineral (petroleum)-based gear oil to get our baselines with an "average" off-the-shelf oil. We opted to stay within the 90-weight realm rather than moving up to a 140w. The experts we consulted told us that a sustained 200 degrees (or more) was the best indicator to use a 140-weight oil. Oils lose viscosity with heat and at higher temperatures, and a 140-weight will retain the boundary lubrication needed to protect the gears where a 90-weight might not. The downside is that the heavier oil creates more parasitical drag (costing mpg) and creates more fluid friction at lower temps.
Figuring we couldn't test every type of gear oil on the face of the globe, we looked for two benchmarks from both the synthetic and the mineral realms to use as upgrades and found Amsoil and Lubrication Engineers (LE).
Amsoil is a well known supplier of quality synthetic lubricants. We obtained enough of their Series 2000 75W-90 for a couple of oil changes. We were partly through the tests when we discovered that the Series 2000 oil has been replaced by a new formulation called "Severe Gear." The Severe Gear may be better, but the Series 2000 did plenty good, reducing the solo temps by 15 degrees and the towing temps by 25. The synthetic base oil, combined with Amsoil's additives, were responsible for the drops in temp.
LE is lesser known to the 'wheeling public but a force in the commercial and specialty formulations market. The LE607 Almasol straight 90-weight mineral-based oil has been making a name for itself in the racing and high-performance markets along with the company's "bread and butter" commercial realm. This is a sleeper oil that four-wheelers should be looking at if they don't run in temps below about zero degrees Fahrenheit. It dropped our solo temps by 20 degrees and towing by 30. This oil relies heavily on its additive package, the foundation of which is Almasol, a proprietary wear- and friction-reducing element.
This is probably the most important element in building a towing axle. Even an average oil can survive long towing work if it stays cool. The universal magic number is 225 degrees. At that temp, oxidation is occurring in a major way. Some sources show that the rate of oxidation increases dramatically at oil temps above 140, and increases by 100 percent for every 50 degrees above that. The byproducts of oxidation are acids that will corrode and pit metal parts. The most common type of EP (Extreme Pressure) additive package, sulfur/phosphorus (S/P), begins to break down at temperatures above 200 degrees as well, leaving your hard parts less protected from wear. They also convert into acidic components that can eat into parts.
One of the best ways to keep an axle cool is by increasing oil capacity. The Mag-Hytec cover chosen for our Ford 10.25 test mule increased the capacity of our axle by 3 quarts. Not only does this cover have more surface area with which to radiate heat (plus cooling fins), the increased capacity gives the oil more "dwell" time in which to cool off. In our case, the Mag-Hytec knocked an honest 25 degrees off the oil temp with no other changes. When combined with the LE607, a total of 55 degrees was taken off the oil temperature, and 50 degrees with the Amsoil in the same conditions.
What it boils down to is that any increase in capacity or any improved means to radiate heat is useful. By the way, overfilling the housing doesn't count. Oil levels that are too high actually make the axle run hotter due to fluid friction and aeration. The higher-capacity covers increase capacity while keeping the actual level the same, or close to it.