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Auto vs. Manual Differential Lockers

Posted in How To: Transmission Drivetrain on May 1, 2011
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As far as off-road performance goes, there's nothing you can do to your Jeep that will make as big a difference as adding a rear locker. And once you add a front locker, you'll be amazed at the tractor-like clawing abilities your ride will possess. Lockers help offset the drawbacks of a stiff suspension because forward momentum is no longer impeded if you lift a tire. Lockers add finesse to your driving because you'll be able to idle over obstacles rather than take a running start and making a hail-Mary pass through difficult lines. Lockers have drawbacks. There, we said it.

Pneumatic Selectable Locker

When a vehicle makes a turn, the outside tire needs to spin faster than the inside tire. An open differential allows the axleshafts to spin at different speeds, but always sends power to the tire with the least traction. The drawback is if you lift a tire, that's the side that's going to spin. An automatic locker locks the axleshafts together when under drivetrain power. The axleshafts can only disengage and spin independently when the vehicle is coasting. The downside is when turning under power the inside tire will chirp and bark and the outside tire will scrub. Also, an auto locker will instantly engage or disengage when power is applied or removed, which can cause noticeable jerking, banging, and clanging. A manual locker allows you to engage or disengage the locking function of the differential at will. This allows quiet and seamless on-road drivability without giving up any off-road performance. The downside is high costs and sometimes complicated methods of engagement/disengagement, such as bulky cables or air lines. Here's the staff's take on the subject.

Hazel's Hammerings
I've run almost every locker ever made in a variety of vehicles ranging from big and leaf-sprung to small and coil sprung. When it comes to lockers, I've got some very definite opinions-but no set rule. In other words, my choice very much depends on my application.

For starters, let's talk front lockers. If my vehicle has locking hubs I'm gonna go straight for an auto locker. I like their simplicity and the fact I don't need to remember to turn it on or off when on the trail. The only downside is if you bust a shaft under power there's a higher-than-average chance you'll damage an automatic locker, so, I'll factor axle strength and tire size into the equation. I normally go for a Detroit Locker and, although they can increase turning radius, I've found I can turn sharply by modulating the throttle during slow, slinky trail work. On the other hand, if I'm running a vehicle with unit bearings or no locking hubs, I'll go straight for a selectable locker. I've never had a metallurgical failure with a pneumatic locker, but I have had my fair share of leaky seals, crimped air lines, and other associated troubles. It's been enough to steer me over towards the electric locker camp. I much prefer simply running a couple wires through the diff than routing a bulky cable or plumbing an air system.

In the rear, I let my wheelbase and suspension determine my locker selection. For a long-wheelbase vehicle with leaf springs like a J-truck pickup or even a Cherokee I'm more than happy to run an auto locker. I never even noticed the Detroit Locker in the rear of my M-715. On the other hand, running a Detroit Locker or automatic lunchbox locker in the rear of a short-wheelbase YJ or TJ Wrangler was enough to make my hair white. It was so bad I wouldn't let my wife or friends drive those vehicles, so I wound up switching over to selectable lockers. For the sake of stability and predictability, I'd really recommend a selectable locker in any short-wheelbase Wrangler, CJ, or vehicle of that ilk. That is, if you're not manly enough to run a spool. But that's a different story.

Trasborg's Tourettes
I don't know what Christian is talking about with automatic lockers making his hair go white. We all know he's got no hair on his dome. Heck, if he had money he could be a stand-in for Daddy Warbucks. But seriously, I totally agree with keeping automatic lockers out of short wheelbase Jeeps. I've been in and out of more Jeeps of all shapes and sizes than many of my friends have owned underwear. The quick answer on wheelbase and auto lockers is that I draw the line at 100 inches. If the wheelbase is in the two-digit category (less than 100 inches), an auto locker can be scary on-road. I hate hearing people talk about "getting used to it." While life is a series of compromises, if you do something to your Jeep that can make it change half a lane on the highway or spin you clear around when it is wet, that isn't a good change. I'd rather a spool or a good old Lincoln locker in the rear of a short-wheelbase Jeep.

Electric Selectable Locker

That said, weight comes into play too. The heavier the Jeep, the less likely it is to exhibit handling quirks with an automatic locker. I'm not sure what the numbers are exactly and some of it has to do with the type of suspension you are running. In a sub-4,000 pound short-wheelbase Jeep I'd stay away from an automatic locker, but I've also run an automatic locker in a 6,000 pound short-wheelbase Jeep without the big handling quirks. I won't even point out that you need to leave the kitchen sink home if your Jeep is that heavy, but I've seen it more than I care to admit.

I'm a big fan of selectable lockers. I like the idea of having the Jeep handle like stock when full traction isn't needed. Like Christian, if I have a short-wheelbase Jeep with unit bearings up front, selectable lockers front and rear are going to be my default. That said, I have really bad luck with pneumatic lines in an automotive environment and given my electrical bent, I find myself once again agreeing with Christian (I hate it when that happens). My ideal selectable locker would run on 12-volt power, not have any friction clutches to wear out, and would be able to be locked or unlocked with a bolt through the housing if (when) the solenoid ever died.

Money can sometimes be an issue, and we all know that a selectable locker costs more than an automatic locker. In those cases, I'd suggest a selectable up front and a spool out back. It would work out to about the same cost as a pair of automatics with way better on-road handling. Sure, you'll get some scrub of the tires in a short wheelbase rig on-road around tight corners. The other thing to bear in mind is that a rear locker alone can do wonders towards getting a longer way down the trail. Maybe you don't need to go full-gonzo with a pair of lockers right out of the gate. Save some coin by putting one in the rear and running it. You might find you don't need a front locker for the kind of wheeling you do. Or you can go the lunchbox locker route to save some cash. You won't get the same kind of strength, since a lunchbox locker is hindered by the strength of the factory carrier. But it is a relatively cheap way to figure out what is right for you before dumping all your money on high-end stuff.

Cable-Actuated Selectable Locker

Cappa's Locker Box
When I choose a locker for a specific Jeep I take into consideration how much I plan to drive it on the street, the wheelbase and weight of the Jeep in question, power output, tranny type, and the overall strength of the axles. Like Hazel, I prefer simplicity, and if I'm working on a dirt-mostly, short-wheelbase Jeep, I'm steering into a pair of automatic lockers. But I generally wouldn't want this combination on a high-horse, manual-transmission, short-wheelbase, street-driven Jeep-especially if I expected to toss the keys off to locker-inexperienced drivers.

Lunchbox Locker

The on-road handling quirks of a lightweight short Jeep with an automatic locker can be exponentially worse when you add a manual tranny and or V-8 power. Going with an automatic transmission absorbs a lot of the harshness, but not all of it. Most owners of daily-driven CJs and Wranglers are better off with a selectable locker out back. Heavier and longer wheelbase Jeeps like FSJs can get by with an automatic rear locker with far fewer on-road handling drawbacks.

Off-road, I'm all about simplicity. With fewer moving components, there are fewer things to go wrong. That means less spare parts for me to carry and fewer things for me to worry about. It doesn't matter if your selectable locker is engaged by air, cable, or electricity. They can all become problematic and each has its own advantages and disadvantages.

Automatic Locker

The truth is that most trails don't require a front locker all the time, if at all. But it's nice to have in reserve if things get nasty. In most cases the front axle is significantly weaker than the rear axle and you may not always want the added front axleshaft and steering U-joint stress caused by a front locker. So if I'm trying to get the most out of a budget front axle, I'll spend a little more to install a selectable locker. That's not to say I'll waste money on a crappy front axle, but not every application requires a Dana 60 1-ton frontend either. In fact, only one of my Jeeps has ever even had a Dana 60 front axle. I've successfully run 38-inch tires with Dana 44 axles on heavy fullsize Jeeps with up to 350 hp thanks in part to selectable lockers. Most of the time I leave it disengaged until I really need it, and then when it's locked I generally go easy on the throttle.

Yes, it's true that if you break an axleshaft with an automatic locker like a Detroit, the locker can fail too. And in this case the selectable lockers are generally stronger than their automatic counterparts. However, I've personally had more selectable locker problems than auto-locker failures. Ultimately, choosing between an auto or selectable locker for your Jeep is always a compromise between handling, simplicity, durability, and cost.

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