In garages, on trails, and around campfires, debate will always rage over what single modification is most effective in creating a capable 4x4. Is there one magic mod that can dramatically improve your 4x4's trail performance, or is capability a sum of many components? Well, one school of thought for many experienced 'wheelers is that a differential locker, limited-slip, or spool is one of the most basic and effective mods you can make to your truck, because depending on which device you install, you can have either temporary or full-time torque divided to both wheels fed by a differential. Clearly, installing one of these devices creates a dramatic improvement over a stock "open" differential, thus allowing you to more easily negotiate difficult terrain. To understand why this is, you must first understand how a differential works.
Multi-Tasking
Whether you drive a Super Duty or a Sportage, the differential in your truck performs three basic tasks. First, the spider gears and side gears allow your wheels to turn at different speeds when you take a corner or make a steering correction. If this didn't happen, the tires would have to skip or slide during these maneuvers. That would be tough on driveline components and tires. Second, the diff converts the rotational torque of the engine and driveshaft into forward/backward tire movement. This is accomplished via the drive pinion and ring-gear, which alters the direction of the torque by 90 degrees. Third, the ring-gear and the drive pinion determine the amount of gear reduction that a diff produces: If your truck has 4.10:1 gears, the ring-gear makes one complete rotation for every 4.1 rotations of the drive pinion.
An "open" diff (one without a mechanism to limit or control the amount of engine torque directed toward a tractionless tire) will always transfer torque to the tire with the least amount of traction. Obviously, this action is worthless and frustrating when your vehicle is hung up on a rock, forcing its way through mud, or trying to push through snow. The result of this is that a four-wheel-drive vehicle without a locker or limited-slip diff in either axle really only has two-wheel-drive. That same vehicle with a locker in the rear, but an open diff in the front, only actually has three-wheel drive. The cure? Get torque to both wheels fed by each differential. How? Install a locker, limited-slip, or spool.
But which one? Well, that's the question you have to answer. Your answer depends on what type of vehicle you have, and most importantly, what you do with it. Answering that question will create yet another question. Do you install them in the front, in the rear, or in both differentials? Once again, the answer lies in what you do with your vehicle and what type of four-wheel-drive system it's equipped with. It also depends upon the number of dollars you wish to spend.
Spools
On the extreme end of things are the spools. Full spools are manufactured from a solid piece of aluminum or steel, and they effectively couple the ring-gear directly to the axleshafts. They are super-strong, lightweight, and send 100 percent of the torque to both wheels fed by the differential. These are not recommended for street use. In fact, they're generally used only in racing applications or on the rear of 4x4s that only see trail duty, due to their complete lack of differentiation. Mini-spools also are available, and they're less expensive than full spools, they're not as strong, and they too don't differentiate wheel speed during turns, and are best suited to trucks used only off-highway.
Who they are: Auburn Gear
What they make: Pro-Series limited-slip, High Performance Series limited-slip, mini spools, and spools.
The facts: The High Performance Series is designed to be a replacement for OE limited-slips, while the Pro Series (pictured) provides higher torque and preload in a fast-acting unit. The Pro Series is offered in more than 30 different models, while the High Performance Series is available in almost 25 models. When installing Auburn differentials it is important to use a high-quality non-synthetic 80w90 GL-5 oil treated with GM or Ford limited-slip additive. This will eliminate clutch chatter generated when the clutch cone engages and disengages in rapid succession, ensuring pleasing driveability that resembles that offered by an open diff. Auburn also offers mini-spools and full spools.
Get more info from: Auburn Gear, Dept. FW, 400 East Auburn Drive, Auburn, IN, 47606-3499, 219/925-3200, www.auburngear.com.
Who they are: ARB USA
What they make: ARB Air Locker
The facts: When activated by the driver via a switch, air pressure from an onboard air compressor locks this locker, thus sending 100 percent of the torque to both wheels on the differential. The rugged ARB Air Locker is more complicated to install than mechanical lockers because it requires the installation of an air compressor and air lines. But the system is transparent until you need it, resulting in a retention of normal differential operation. The ARB Air Locker is available for more than 100 different applications and there are, the company tells us, more than 1,000,000 of them in use around the world, which speaks volumes.
Get more info from: ARB USA, Dept. FW, 20 South Spokane Street, Seattle, WA 98134, 206/264-1669, www.arbusa.com.
Who they are: Randy's Ring & Pinion
What they make: Yukon Performance XYZ Pozitraction for GM 12-bolt 33-spline differentials, Yukon Schir Grip Positraction for Mopar 8 3/4 Chrysler differentials, Yukon spools, and mini spools
The facts: In addition to a number of steel mini-spools, Yukon offers full spools in both steel and aluminum for most popular differentials. The Yukon Pozitraction unit for GM 12-bolts is available with either solid-steel clutch discs or carbon-fiber clutch discs, and it can be ordered with either 400- or 800-pound preload. New to the lineup of limited-slips is a unit that fits the Mopar 8 3/4-inch differential (pictured).
Get more info from: Randy's Ring & Pinion, Dept. FW, 11630 Airport Road, #300, Everett, WA 98204, 425/347-1199, www.ring-pinion.com.
Who they are: Eaton Automotive
What they make: ELocker, Eaton Limited-slip
The facts: The "rebuildable" Eaton limited-slip unit is the same as is found in many OE applications (Eaton has sold more than 1.2 million limited-slips to GM last year for light trucks and SUVs), and it employs race-bred carbon friction discs to restrict wheel rotation (in excess of 100 wheel rpm) in order to provide more driving force to the wheel with the most traction. It is fully automatic, and boasts a unique design that prevents lock-up from occurring at speeds above 20 mph. The ELocker (pictured) is Eaton's newest product, and it's derived from the same unit found in the new Hummer H2. It allows the driver to manually select full locking capability, while otherwise allowing for standard open differential operation.
Get more info from: Eaton Corporation, Torque Control Products Division, Dept. FW, 26101 Northwestern Highway, Southfield, MI 48076, 248/354-2757, www.eaton.com.
Who they are: Big End Performance
What they make: Spools, mini-spools
The facts: Spools are manufactured for GM 12-bolt (30- and 33-spline) and Ford 9-inch (28-, 31-, 33-, and 35-spline) differentials from 4140 chrome-moly. The GM spool weighs in at only 9 pounds, while the Ford 9-inch weighs in at just 8 pounds. Mini spools are available for a variety of GM 10-bolt applications, as well as GM 12-bolt and Ford 8.8-inch and 9-inch applications, and they're constructed from 8620 non-leaded steel. An optional chrome-moly cross-pin is available for Ford 9-inch applications to replace the notoriously weak stock pin. All products are CNC lathed in the USA.
Get more info from: Vehicle Specialties Inc., Dept. FW, 7940 New Jersey Avenue, Hammond, IN 46323, 800/424-8741, www.vsihp.com.
Who they are: Drive Train Specialists
What they make: Spools, mini-spools
The facts: Spools are available for most popular applications and spline counts, and some are available in aluminum; most are constructed from steel. Mini-spools are available for most applications as well, and aluminum is available for some Ford 9-inch applications.
Get more info from: Drive Train Specialists, Dept. FW, 26400 Groesbeck Highway, Warren, MI 48089, 800/521-0628, www.drivetrainspecialists.com.