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4x4 Truck Differential Parts To Increase Torque - Divide & Conquer

Posted in Product Reviews on May 1, 2002
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Photographers: The Manufacturers

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.

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.

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,

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,

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,

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,

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,

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,

They're called many things, including posis, and quite simply they're designed to automatically transfer some of the torque from a wheel with no traction to the other wheel-the one that does have traction. Depending on the manufacturer, limited-slips accomplish this by incorporating centrifugal fly-weights, special gear setups, or friction clutches that sense wheel slippage and transfer power to the other wheel. This action can be triggered by torque or wheel speed (depending on the manufacturer). Although they're sometimes generically referred to as lockers, limited-slips never actually lock the axleshafts together, thus they're technically not lockers, and don't perform as such. How fast a limited-slip activates varies by manufacturer, and some require a fair amount of throttle input to trigger the transfer of power, while some don't. Limited-slips generally exhibit good road manners, are reasonably inexpensive, and offer a vast improvement in traction over open differentials, though traction with limited-slips still is nowhere near as secure as it is with lockers.

Unlike limited slips, a true locker will never allow either axle to turn slower than ring-gear speed. This means that a locker will never allow a high-traction tire to sit still while the driver inputs torque and spins a no-traction tire.

There are two kinds of lockers: automatic and selectable. Automatic lockers transfer all of the available torque to both wheels as soon as side-to-side wheel speed differences cause heavy springs to activate the locker's clutch and engage the spider-gear teeth to those on the clutch members to make both axleshafts operate as if they were one. Automatic lockers are durable and generally easy to install, but tend to generate clunking sounds (caused by the locking and unlocking of the locker), and to cause increased tire wear. Selectable, or manual, lockers accomplish the same goal as an automatic locker, but instead of clutches, the driver controls when the locker is engaged via a switch or lever. This guarantees open-diff driveability and offers standard open-diff four-wheel-drive operation when wanted, with the option of full locker capability on demand. This is especially important at the front axle, where a full locker can cause turning difficulties, and can put extreme stress on breakable parts. So the ability to disengage a locker in the front axle can make tight turns in difficult terrain easier to negotiate, and can help preserve U-joints. Also, it's important to use selectable lockers in the rear if you tow. Automatic lockers are hard on parts when towing. Selectable lockers tend to be more expensive than automatic lockers and they tend to require a longer installation time because of the associated air lines, cables, or electrical wiring required to activate them.

Go Shopping
Now that you're familiar with the three basic types of traction-enhancing differential devices, how they operate, and what their characteristics are, you can peruse the units shown here for one that best suits your needs. The list includes a number of companies that manufacture limited-slips, automatic and selectable lockers, and spools. Clearly, we couldn't list application information for each and every truck, but you can easily acquire that info by visiting manufacturers' Web sites or by simply contacting the manufacturer directly.

Who they are: Spicer Dana
What they make: Trac-Lok
The facts: The Trac-Lok limited-slip has been around for a long time. It features a clutch-plate-disc arrangement as well as a one-piece case and two pinion-mate gears. It's available for Dana 20, 35, and 44s.
Get more info from: Spicer Dana, Traction Technologies Group, Dept. FW, 2100 West State Street, Fort Wayne, IN 46808, 219/483-7174,

Who they are: Powertrax
What they make: Lock-Right locker, No-Slip Traction System
The facts: The Lock-Right locker (pictured) is said to be manufactured from "high-strength, ultra-resilient, heat-tempered steel alloys for maximum toughness" and can be installed in just a couple of hours. The No-Slip Traction System is a limited-slip unit that is built to withstand more than 6,800 lb-ft of torque per axle for very small differential applications (representing 75 percent more torque capability than the axles themselves) to well over 35,000 lb-ft for large differentials (representing up to 250 percent more torque capability than the axles themselves). Lock-Right lockers are available for almost 60 automotive applications, while the No-Slip Traction System is available for almost 70 automotive applications.
Get more info from: Powertrax, Dept. FW, 245 Fischer Avenue, Building B4, Costa Mesa, CA 92626, 800/578-1020,

Who they are: Tractech Inc.
What they make: Detroit Locker, Detroit E-Z Locker, Detroit Gearless Locker, Detroit Truetrac, Detroit Suretrac, and the new Detroit Electrac.
The facts: Tractech's three-locker lineup includes the legendary Detroit Locker, which is designed for extreme conditions and is available for diffs with 3,000- to 70,000-pound capacities; the Detroit E-Z Locker, which is designed for the occasional four-wheeler and which features fewer parts than the Detroit Locker as well as a design that reuses many conventional diff parts; and the Detroit Gearless Locker, which is for Dana 30, 44, and AMC 20 differentials, and has no gear teeth, which means it's quieter and smoother. The two limited-slips in the lineup include the Truetrac, which uses helical gearsets as opposed to clutch packs, and the heavy-duty Suretrac, which is a heavy-duty clutch-pack type unit for severe applications in larger (over 10,000-pound capacity) differentials. New for 2002, Tractec is debuting its Electrac, which offers full locking capability.
Get more info from: Tractech Inc., Dept. FW, 11445 Stephens Drive, P.O. Box 882, Warren, MI 48090, 810/759-3850,

Who they are: Precision Gear
What they make: Track-Lok, Powr-Lok, spools, mini-spools
The facts: The Track-Lok limited-slip is available for GM 10-bolt differentials with 28-spline axles. It uses clutches on both sides of the side gears as well as an "S" spring between the side gears to provide for initial clutch preload. The Track-Lok has larger side gears and spider gears than those provided in OE applications, and this means improved durability and load-carrying capacity. The Powr-Lok limited-slip is available for the Dana 44 (19- and 30-spline) and the Dana 60 (30- and 35-spline) differentials. Spools and mini-spools are available in either 4140 steel or 7076 T-6 aluminum.
Get more info from: Precision Gear Inc., Dept. FW, 12351 Universal Drive, Taylor, MI, 48180, 734/946-0524,

Who they are: Zexel Torsen
What they make: T-1, T-2, T-2R
The facts: The T-1 is a crossed-axis, helical-higher torque-bias limited-slip unit that was originally introduced on military Humvees, while the T-2 limited-slip is a parallel-axis helical unit that offers low backlash and quiet operation. The T-2R (pictured) is much like the T-2, but stays locked with lower torque inputs. In normal driving conditions, the units deliver power equally to each rear wheel, but if a wheel slips, the unit sends up to 67 percent of the available torque to the wheel with traction. All three of these units are clutchless and torque-sensing, and are designed for the life of the vehicle.
Get more info from: Zexel Torsen, 2 Jetview Drive, Dept. FW, Rochester, NY 14624, 716/464-5000,

Who they are: Ox Trax
What they make: Ox Locker
The facts: The Ox Locker is cable-activated, and allows the driver to select either full differential lock or open differential. The locker and heavy-duty diff cover are manufactured from billet steel and aerospace alloys, and the control cable is heavy-duty aircraft quality material. When the driver selects "lock" by pulling the heavy-duty lever, a raised shifting fork on the inside of the diff cover engages the unique four-post differential. Currently, the locker is available for Dana 30, 35, and 44 differentials, with Dana 60, AMC 20, and Ford 9-inch availability in the works.
Get more info from: Ox Trax, c/o Superior Axle & Gear, Dept. FW, 3954 Whiteside Street, Los Angeles, CA 90063,

Who they are: Genuine Gear
What they make: Quick Lok
The facts: This locker is available for a number of light-truck and SUV applications, and installs in about an hour. It's said to be an economical version of the Detroit Locker, and as with many lockers, it replaces the side and spider gears, but will reuse the side gears in some applications.
Get more info from: 4-Wheel Parts Wholesalers, Dept. FW, 801 W. Artesia Blvd., Compton, CA, 90220, 800/421-1050,

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