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Locking Differentials 101

Posted in How To: Transmission Drivetrain on July 1, 2011 Comment (0)
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Photographers: Fred Williams

In the world of off-road performance products, the differential locker is near the top. A differential locker is mechanism that sits inside of or replaces the differential carrier. The job of a locker is to “lock” both axleshafts in unison so they can rotate at the same speed. Unlike an open differential carrier, which allows for different speeds of the tires as they move through different arcs of a turn and only sends power to the wheel with the least resistance, a locker splits the power between both wheels evenly.

On-road an open differential is rarely a problem, as both tires on the axle are firmly gripping the road. Even in light off-road situations an open differential works just fine—until one tire is in the air, on mud, or on another loose traction surface. That’s when the power goes to the tire that can spin, the one with the least amount of traction. Once that happens you are known as a “one-wheeler peeler” and you lose all progress.

But with a solid locker, no matter how twisted your rig gets and puts tires in the air or how slick a mud pit you are in, each tire continues to drive forward.

Though a set of differential lockers will greatly transform the off-road performance of your vehicle, not all are alike. And while some may increase your rig’s abilities off-road, that same locker set can create negative handling quirks on the street. Gathered here is a list of some of the most popular locking differential options and the pros and cons often associated with each.

Automatic (AKA Full-Time) Lockers
For a locker to be considered automatic, it simply means that the locker automatically engages when power (throttle) is applied, while operating like a conventional differential for going around corners. This is unlike a selectable locker, which requires an external actuating source. Probably the best known automatic locker is the Detroit Locker (www.eaton.com), a full carrier replacement locker that allows your wheels to change speed when making turns but that locks the shafts together once power is applied. Traditionally, automatic lockers such as the Detroit are some of the strongest and most preferred in vehicles that spend the majority of their time wheeling hardcore terrain.

Pros
• Strong and simple units with no wires, lines, or cables to tend with

• Once you become accustomed to driving with automatic lockers they can be barely detectable on the street

• Since the auto lockers allow differentiation (change in speed between the shafts) they often are a little easier to steer off-road

Cons
• Some have a reputation for loud bangs and pops, since the auto locker is constantly locking and unlocking as you drive

• Because of this, they can create strange handling characteristics that are especially noticeable in short-wheelbase vehicles.

• Generally not recommended for the front of vehicles equipped with fulltime hubs.

Drop-In (aka LunchBox) Lockers
Drop-in, or lunchbox, lockers are automatic lockers that sit in place of the spider gears found in an open differential carrier. No gear ratio or carrier change is usually needed to install drop-in lockers, provided the vehicle already has an open carrier and not a limited slip. These are usually very inexpensive lockers and are installer-friendly. One of the most common is the Lock-Right (www.richmondgear.com).

Pros
• Low cost

• Ease of installation

• Lightweight

Cons
• Can create noticeable handling quirks similar to full-carrier automatic lockers

• Can be noisy

The Gov-Loc
Found in the back of thousands of GM pickups, the Gov-Loc is a somewhat unconventional limited-slip/locker combo. The Gov-Loc received its name from how the mechanism works. As the differential begins to accelerate, a governor is thrust outward and up a slanted side gear. As the rpm increases, the governor forces pressure onto a thrust block, which allows a small cam gear to force the limited-slip clutches in place. The idea is that the faster the carrier rotates, the more the governor applies pressure, thus engaging (or locking) both shafts together. Yet while the design and theory behind it are solid, we rarely see them function consistently, nor would we place them in the category of an automatic locker.

Limited Slips Are Not Lockers
For all of the advancements in limited slips, they still cannot replace (or equal) a locker. Most limited slips, such as the Eaton TrueTrac (www.eaton.com) and the Auburn Gear Positraction (www.auburngear.com), use a gear or clutch engagement system to keep the wheels rotating at the same speed when power is applied. Though some are more aggressive than others and may seem to work similar to a locker in loose terrain, as soon as you enter extreme wheeling situations you find that the limited slip ultimately leaves you a one-wheel wonder like an open differential. This is mostly due to internal thresholds that keep the limited slips from completely “locking” up.

Manual (aka Selectable) Lockers
Selectable lockers are designed to give you the best of both worlds. When the locker is set to open, most act like a standard open differential, though some work similar to a limited slip. When engaged the units lock 100 percent and allow zero differentiation in axle speed between the shafts.

Unlike automatic lockers, selectable lockers require an activation source. For the ARB Air Locker (www.arbusa.com) and the Yukon Zip Locker (www.yukongear.com), an air line and small compressor are required, while the OX Locker (www.ox-usa.com) uses a cable and Auburn Gear’s Ected (www.auburngear.com) is electronically actuated.

Pros
• Great for daily drivers

• Strong

• Excellent on-road characteristics

• Can increase maneuverability on- and off-road

Cons
• Higher initial cost

• More external and moving parts to contend with compared to an automatic locker

• When engaged the lockers function as spools, which can make steering difficult.

Welded Spider Gears (aka Lincoln/Miller/Hobart Lockers)
For years they’ve been referred to as Lincoln Lockers, but not because the welding corporation produces an off-road line of traction aids. Lincoln Locker is just slang for welded spider gears functioning as a spool. Welding an open differential’s spider gears together or to the case unifies the gears to make the axleshafts rotate at the same speed. This is great for off-road-only rigs, as you can have your rig locked up in less than an hour using only a drain pan, a can of brake clean, and a welder.

Pros
• Extremely cheap

• Effective

Cons
• Not good for driving on the street

• Welding risks differential damage

• Spider gears can be difficult to weld

• Weaker than full replacement carrier

• Not recommended for front end

Spools
Spools have a great history in drag racing and have become a staple in the off-road world, especially in mud racing. These lightweight fulltime lockers are extremely aggressive when used on the street, as they allow no difference in speed between the shafts. Spools are complete carrier replacement lockers and can range in material types.

Another type of spool is the drop-in style mini spool (Randy’s Ring & Pinion, www.ringpinion.com). The mini-spool is an equally aggressive traction aid that simply takes the place of your open differential’s spider gears.

Pros
• Strong

• Lightweight

• Simple

• Cheap

Cons
• Very aggressive

• Not meant for highway use

• Not C-clip compatible

• Not recommended for front end

Brake Bias Traction Control
Many modern 4x4 vehicles have what’s known as brake-bias traction control. These systems work by manipulating the vehicle’s antilock brake system (ABS) to lock up each wheel individually when it senses that one tire is spinning much faster than the others. The idea is that by applying brake pressure to the wheel that is spinning freely, the differential will engage the other wheel(s) and will hopefully grab and move you forward. Though we’re not huge fans of this type of traction control, some implementations of it are getting better, like Toyota’s A-Trac system.

Open Differential
An open differential is a differential carrier fitted with a set of spider gears that allows for different axleshaft speeds. When throttle is applied, the open differential sends power to the wheel with the least resistance, which usually is the tire with the least traction. This design is sufficient on-road because it allows the tires to spin at different speeds, so when you corner and turn the vehicle there is no bind. Off-road, open diffs can usually be compensated for by driving finesse and momentum, but they ultimately limit the potential of your 4x4.

Pros
• Great for street driving

• Cheap, as most vehicles are already equipped with them

Cons
• Weaker

• Usually leaves one wheel per axle doing all the work

• No mechanism to lock shafts together

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