Traction Aid: Your Guide to Lockers, Limited Slip Differentials, and Spools

The first automakers had a lot of things to figure out, such as, how to get the engine’s power to the wheels. It was found early on that you could not match the speed of the engine to the speed of the wheels. Countless hours of innovation and engineering came up with brilliant inventions, like clutches and differentials, that would correctly transfer power in a usable manner. Drivetrains had to be invented that would multiply gear ratios and allow differentiation in speed of certain parts to allow for a smoothly working system.

Since then, entire books have been written on the engineering behind differentials and clutch systems, but we’re going to attempt to give you the basics of axle differentials in this article. While differentials can be found in other places in the vehicle (like all-wheel-drive transfer cases), for practical purposes we’re going to be referring solely to what’s inside your axles.

What Makes the Difference?

Differentials are found in the drive axles of vehicles. Energy comes through the powertrain from the engine to the transmission through the transfer case and down the driveshaft that spins the pinion gear of an axle (or independent suspension centersection in IFS or IRS). That axle’s pinion gear turns a ring gear that is mounted on a differential gear that turns both axle shafts perpendicularly to the rotational axis of the driveshaft, thanks to the angled cut of the ring-and-pinion gears. The ring gear and pinion gear are not a 1:1 ratio but instead divide the driveshaft’s rotational speed by whatever the gear ratio is (For example: a 4.10:1 gear ratio will turn the axle shafts around 1/4 of the speed of the driveshaft.) Not only is the differential a ring gear carrier that transfers power, it also allows for a differentiation in wheel speed.

The appropriately named “differential” in your axle does just that; it allows differentiation of wheel speed on either end of a drive axle. This is done so there is no binding in the drivetrain and so a vehicle doesn’t “hop” when turning, allowing for smooth operation. When a vehicle turns one way or the other, the inside tire in the turn is rotating at a slower rate than the outer tire. If both axle shafts were locked together, both tires would have to spin at the same speed no matter what. While this would be fine going straight down the road, even changing lanes on a highway can slightly differentiate wheel speed. This slight variation in wheel speed necessitates a differential in each drive axle in order to operate a vehicle smoothly.

Off-Road

But the same thing that makes a differential great can make it a detriment in the dirt. The allowance for a difference in wheel speed means that one tire can be completely stopped while the other tire spins (with an open differential). Therefore, if one tire loses traction while the other side has it (traction), only the tire without traction will spin. This is how you get stuck. And that’s why we have the need for traction-aiding differentials.

A traction-aiding differential is one that greatly inhibits or eliminates any difference in wheel speed. There are two basic types: Limited slip differentials or locking differentials, aka lockers. A third type of traction device for an axle—used for off-road or racing purposes only—is a spool; a ring gear carrier that is really not a differential but instead a solid piece of metal that totally locks both axle shafts together in your axle’s centersection. There are a great number of variations of limited slip diffs and lockers, but let this be clear: A locker 100 percent locks both axle shafts in an axle together, allowing for absolutely no differentiation in wheel movement (when locked) unless something breaks. Anything less than 100-percent locked would have to be considered an open diff, a limited slip diff, or just not working.

What’s Right for You?

This is a question for you to answer based on the information you’ve learned. If you’re daily driving your 4×4, a limited slip differential or selectable diff lockers are always going to be your best bet. The benefits of the limited slip versus the locker is that it’s “on” all the time, and that can even help on the street. The selectable locker is nice because it absolutely locks both tires (both sides of the axle) together when engaged, and allows completely normal operation on the street when disengaged. If you’re off-road 100 percent of the time, a slightly less-expensive automatic locker might work out very well for you. And if you’re not at all concerned about tire wear or turning radius, you might as well just run a spool.

Open Differentials

An open differential is one that applies the same torque to both wheels but can allow for a differentiation in rotational speed (of the axle shafts). Therefore, while the same force is being applied to both sides of the carrier at all times, an open differential will allow for one wheel to spin slower than the other, or even completely stop, while the other wheel spins faster. Open diffs accomplish this through the use of side gears and spider gears inside the carrier.

Limited Slip Differentials

A limited slip differential (LSD) inhibits a difference in wheel speed in an effort to keep a tire from slipping when traction lessens on one side of the axle. An LSD will still allow for wheel speed differentiation so you can drive normally on paved roads with one installed, making it a popular choice for street-driven trucks. For the purposes of this article, we are going to concentrate on the two main types of aftermarket LSDs available: A clutch type and a gear-type limited slip.

01. CLUTCH OR CONE LSD

A clutch-type limited slip diff uses either a clutch plate stack or cones. Clamping force and friction are used to keep uneven wheel spin to a minimum. The internals of a clutch-type LSD carrier are similar to an open differential, with the preventive force of the clutches always engaged ready to inhibit wheel spin. As more torque is applied, more force will be applied on the clutches (or the cones ramping up) and the more coupled the wheels will become. These types of differentials will occasionally need rebuilding and replacement of wearable items.

02. GEARED LSD

A geared limited slip differential is a torque-sensitive mechanical LSD that does not have cones or clutches. It uses helical worm (or spur) gears that press against the carrier and create friction to inhibit wheel spin. The torque-biasing is only applied to the more slowly spinning wheel when needed and otherwise remains in an “open” position. You could consider this type an automatic limited slip. Because gear-driven limited slips only bias torque when needed, they do no create any binding like a clutch-type LSD or a locker possibly could, so they can be used in front ends with no negative impact.

Automatic Locking Differentials

An automatic diff locker is one that goes into an unlocked “open” position when turning and then reengages for straight line driving, locking both wheels together. This sensing for disengagement and re-engagement is done through completely mechanical operation and has been found to be a reliable, heavy-duty, no-fuss option that many off-road enthusiasts prefer. But some users find that the automatic engagement and disengagement are not always spot-on, and sometimes an automatic locker will suddenly pop or slightly engage in a corner. This “pop” can sometimes sound like you’ve just broken an axle shaft and will certainly get your attention when it occurs.

Drop-In Locker

A less-expensive locking differential option is a “drop-in” locker. A drop-in locker is a differential retrofit that goes into your existing open carrier and turns it into an automatic locking differential. These are generally the cheapest type of traction aid you can add (aside from a spool) to your axle, but they are sometimes not as strong as a complete replacement locker.

Selectable Locking Differentials

A selectable diff locker is one that goes from an open differential to a 100 percent locked differential (which essentially acts like a spool). The engagement and disengagement actuation of a selectable locker is most often done using compressed air or electricity to actuate the locker. When a selectable locker is locked, there is no allowance for a difference in rotational speed on either side of the axle. No matter what, both tires will turn together. In a front application, it will be very difficult to turn the steering wheel with the locker engaged if both tires have traction.

LOCKER PACKAGES

Selectable locker packages are often sold complete with everything you need (minus the install kit) for swapping out your existing differential gears for a locking one. Electric-locking diffs usually have two wires that come out of the axle housing and go to a switch. Air-actuated lockers use air lines and solenoids, and sometimes the air compressors are sold separately. If you’re in a hurry to finish an install before a trip, make sure you got an air compressor included in your purchase if you’re running an air locker.

Spools

Sometimes, no differential is the best choice. In more extreme off-road situations (like racing) many drivers opt for a spool instead of a differential in their rear axles. A spool locks both axle shafts together and allows for no differentiation in wheel speed. This is more easily dealt with when there is tire slip due to varying traction in dirt/mud/sand. There is no lighter, no stronger, and no cooler-running centersection option you can use than a spool. Spools are a single chunk of machined metal that have no moving parts so they are not only inexpensive but also extremely strong, compact, and make no heat. This is not a viable option for a street-driven vehicle, nor is it an option for a front 4WD differential—your ability to turn would be greatly inhibited. Spools should only be used in rear axles.

One Axle at a Time?

Assuming you have a 4WD vehicle, you have two differentials: one front and one rear. When adding a diff locker, you do not need to match both ends with lockers. While hard-core off-roaders like to have lockers in both axles, you’ll still see a huge improvement in traction with just a rear locker. As long as you are not changing gear ratios, you can address one axle at a time with no consequences.

Dana 60 or Ford 9-Inch?

Dana 60s and Ford 9-inch axles are the two most popular axles for off-roaders. Both can be made semi-floating or full-floating; both can take 35-or 40-spline axle shafts if modified, and both have plenty of aftermarket housings and differentials available. The Dana 60 has a 9.75-inch ring gear, and the Ford 9-Inch has a 9-inch ring gear (outer diameter). However, many argue the Ford 9-inch is as strong as a Dana 60, due to the 9-inch’s ring gear being thicker thanks to the smaller inner diameter of the ring gear. But this difference in ring gear inner diameters makes a difference in differential size. The Dana 60’s differential (left) is larger than the Ford 9-inch (right). So, unless you’re running a spool, a Dana 60 is a stronger option based on differential sizes.

Your Ride Differentials

Most stock 4x4s are produced with an open differential. Limited slip differentials are an option as a build upgrade when the vehicle is made, but very few vehicles are assembled with diff lockers. There are a few exceptions: Toyota offers an electrically locking differential in both axles of their Land Cruisers and in the rear of Tundras and Tacomas. Jeep offers front-and-rear lockers in its Wrangler Rubicon. And Ford has a stock rear electric locker option for even their Super Duty trucks. It should be noted that traction-aiding differentials (like the ones described above) can be part of, but are not the same as the electronic traction-aiding systems found on VW Touaregs and many newer high-end 4x4s.

Gear Change

Speaking of gear ratio changes, a gear shop is going to charge you around the same amount of money to tear down your axle and add a locker whether you’re changing gear ratios or not. If you’re thinking of changing gears, now is the time. Of course, you’ll spend more money buying the gears, but you’ll be saving money in the long run by not having to pay for an install twice.

Auto vs. Selectable

An automatically locking differential uses side-to-side wheel rotation as an indication of whether to be locked or not. It can sense unnecessary or unwanted wheel spin and will lock both axles together in an instant. An auto locker will theoretically unlock when turning or cornering and lock back up in a straight line. In theory, this sounds wonderful. And there are some higher-tech OEM auto-locking systems that work beautifully. But an auto locker can sometimes slightly engage when you don’t want it to (like when turning). And they’re not always the easiest to drive with on icy roads for the same reasons. Also, tire wear can speed up with an auto locker. Therefore, many enthusiasts often choose a selectable locker if it’s for a daily driven 4×4. Being able to leave the differential gear completely open while driving on the road is often easiest and best for street/highway use. But a selectable locker is usually more expensive and does add exterior pieces to your locker that could possibly fail outside of the axle (wiring, air lines, solenoids, relays, switches, and so on). For that reason, many off-road dedicated rigs will be outfitted with an auto locker.

Editor’s Note: A version of this article first appeared in the Fall 2016 print issue of Tread Magazine.