Do You Need a Locker?

A buddy recently bought a mountain bike with full suspension, a zillion gears, and tires knobby enough to make Ivan Stewart proud. Where does he ride it? In the neighborhood with his kids, and sometimes on beach bike paths. He would have been better off with a fixed-frame cruiser on street meats, but he needed the mountain bike. It was cool. What does this have to do with lockers? Well, some people buy 4x4 accessories the same way. They “need” brush guards on a Suburban, fender flares over stock-size rubber, or mud tires on a commuter truck. Obviously they just want a certain look, but justify it by convincing themselves they need the parts.

Adding these accessories usually does no harm to the vehicle or the driver except in the area of the wallet. Traction aids, especially locking differentials, are a different matter. You may think lockers are cool after watching a CJ crawl over a vertical boulder. But unless you’re planning on doing that same sort of driving, adding a locker could negatively affect your vehicle’s performance. There are some key questions you should ask yourself before you buy lockers for your rig. And maybe what you really need is one of the alternative devices that will still improve your traction without a locker’s downsides. To research this article, we spoke with traction-aid manufacturers and retailers to try to get an unbiased view of the products on the market. Our experts didn’t always agree with one another (some even offered opinions that were 180 degrees apart), so use this information as a starting point in your search for more traction. And be sure to talk to local experts about your particular 4x4 and where you drive it to determine which traction aid suits you best.

WHEN FOUR-WHEEL DRIVE ISN’T

It sounds good in theory until you lose traction under one of those wheels. As Powertrax engineer Andy Gerrick explained, a spinning wheel is a “wheel with zero traction, so it can sustain zero torque. But since an open differential always provides equal torque to both wheels, when one wheel loses traction, both wheels receive zero torque and there is no force to propel the vehicle.” If you have loose road or trail surfaces at both ends of your truck, suddenly your four-wheel drive is no-wheel drive. LOCK ’EM UP

The logical solution then is to connect the axleshafts so that all the engine torque isn’t lost to the spinning wheel. But the trick is to be able to disconnect the shafts when needed for cornering. Which brings us to the locker. Locking differentials typically replace the spider and side gears with a device that prevents differentiation except when the wheels must turn at unequal speeds. In Tractech’s Detroit Locker, for example, spring-loaded drivers with meshing teeth lock the axleshafts together when the wheels are turning at the same speed. In a corner, the teeth on the outside wheel’s driver disengage and allow it to turn faster than the rest of the assembly. The inside wheel’s driver remains locked to the power source—a central driver or spider assembly—and propels the vehicle. When wheel speeds again match, the teeth realign and the axleshafts are linked again.

Off-road, the advantages are huge. When the axleshafts are locked, both sides are receiving engine torque, even if one wheel loses traction or is hanging in the air. You simply won’t have a spinning tire. So when it’s time to crawl up that slickrock face, or heave yourself out of that deep mud hole, you’ve got both tires working for you full-time. Even in a situation where both wheels have bad traction, at least they’ll be working together to scrabble for footing.

However, there are disadvantages. The locker sends power constantly to axles, bearings, U-joints, and other components that may not have been designed for those kinds of torque loads. Breakage is possible. Also, locker disengagement is usually accompanied by a lot of bang and clatter. In a corner, tires may scuff until the drivers disengage, although this isn’t as noticeable in the dirt as it is on the pavement. You may find, too, that having both rear wheels pushing equally creates understeer, or an initial resistance to turning, at the front of the vehicle even if the locker is in the rear. On very slick surfaces—ice, for example—or in off-camber situations, a locked rearend is prone to side-slipping, which will require some countermeasures on the driver’s part.

The advantages/disadvantages question sparked a lot of disagreement among the experts we interviewed. One retailer said lockers are so harsh that they should be used only in a few applications: racing, deep mud, or rockcrawling—situations where tires come off the ground. Yet one manufacturer said he couldn’t imagine an off-road circumstance where you wouldn’t want a locker. Still unsure if a locker is right for you? Read on to see what other options are available.

LOCKER ALTERNATIVES 1: On-Demand Units

Lockers like the Detroit mentioned above are considered automatic lockers because no action from the driver is required to operate them. There are also on-demand lockers that are electrically or pneumatically engaged at the push of a button. They’re locked when you need ’em and disengaged—like they’re not even there—when you don’t.

Unfortunately, on-demand availability is somewhat limited. Only Toyota and Mitsubishi equip some of their 4x4s with on-demand lockers. We’ve used them in Land Cruisers, Tacomas, 4Runners, and Monteros and have found them invaluable on the trail. Most recently, we drove a 2001 Tacoma Double Cab that was equipped with the device. It engages electrically through a differential lock sleeve that is meshed to the diff case. When the system is actuated, the sleeve slides and couples with the side gear to restrict differentiation.

ARB Air Locker Inc. is the only manufacturer that offers an aftermarket on-demand system. As the name implies, an on-board air compressor actuates the ARB. When you engage the unit, air pushes a clutch gear that in turn locks a splined bevel gear to the differential case. This prevents the gearset from rotating, and both axle shafts are mechanically linked to the ring gear. Turn off the unit and the compressed air leaves through an exhaust port, releasing the clutch and bevel gears to their original positions.

LOCKER ALTERNATIVES 2: Insertable Lockers

Besides automatic versus on-demand, there’s another way to classify locking differentials: full-carrier-replacement or insertable. Full-carrier-replacement lockers—like some Detroit and ARB units—are the stronger of the two types, as they are installed inside a new, stouter carrier. As one retailer put it, “A locker is only as strong as the case it’s in.” This also means, however, that they are more complicated to install and require the removal and re-setup of the ring gear. If your mechanical skill level isn’t to engine-rebuild levels, it would be best to leave this installation to a pro.

Insertable lockers are simpler to install, as they fit into the existing case. Though insertable lockers are relatively new to the traction-aid scene, there are a number of them on the market, including Tractech’s Detroit E-Z Locker and Gearless Locker, and Powertrax’s Lock Right and No-Slip Traction System. Many of these units are designed with clutches and springs that allow them to disengage more smoothly than a traditional locker, giving them pavement manners more like a differential with a limited slip. (You can read in detail about the operation of the Powertrax No-Slip in the accompanying story, “Do-It-Yourself Traction?”) Insertable lockers provided one point of agreement among our retail experts: Since they are not as strong as replacement-carrier units, they should not be used on vehicles with radically modified engines, suspensions, or tire sizes. They are best suited for near-stock and lightweight vehicles. Even Tractech’s own sales literature designates its E-Z Locker as “for the occasional off-roader.”

LOCKER ALTERNATIVES 3: Limited-Slip Differentials

If you hear someone using the terms “locker” and “limited slip” as if they’re the same thing, they’re wrong. Both are traction aids but they work on completely different principles. A limited-slip differential is so named because, in a low-traction situation, it transfers some of the engine’s torque to the wheel that has bite.

Limited slips (or posis, which is short for Posi-traction, the GM name for its Eaton units) fall into two different types. Most units, such as the Auburn, Trac-Lok, and Eaton limited slips, use clutch plates, springs, cones or some other means to apply friction between the side gears and the case to resist differentiation. Two other limited slips, the Detroit Truetrac and Zexel Torsen, use gears instead of clutches or springs to transfer torque from the spinning to the non-spinning wheel. The limited slip’s advantage is that it performs like an open differential—smoothly differentiating through turns —until traction is lost at one wheel. The limited slip’s operation is virtually transparent to the user, which is why it’s so popular among OE truck makers. However, our experts disagreed wildly about which limited-slip system is best. One retailer spoke so highly of the Truetrac that he claimed he could sell far more of the units than Tractech could ever deliver to him. The gear-type unit responds more quickly to loss-of-traction situations, he said, and there’s no heat (energy) loss like there is through the clutch systems. Another maker claimed a gear-type limited slip is virtually useless if one wheel has no traction, since there’s no pre-load on the side gears (as is the case with clutch-pack systems) to offer any resistance to differentiation.

The basic disadvantage to any kind of limited slip is the same as its basic advantage: It operates most often like an open differential. No spring pack, clutch, cone, or gearset is going to offer the same kind of positive link between the axleshafts as you will find with a locker. Also, limited-slip differentials tend to not be as strong as lockers.

No matter what their stance was on lockers versus limited slips, all the retailers we spoke with had one notion in common: They spend a lot of time with prospective buyers discussing vehicle types and expected driving conditions before recommending any particular traction aid. Be sure you do the same before plunking down your hard-earned cash. Don’t be like my biking friend and buy something because it’s cool. Buy one because you need one.