Overland Tech: Differential basics

This is a followup to an earlier post talking about the basics of 4WD systems.  This time it’s all about the differentials.

Anatomy of an axle

Quick rundown: Engine torque is generated in the engine, applied to the transmission by the clutch or torque converter, magnified by the transmission gears, then distributed front and/or rear via the transfer case.  This engine torque enters the axle via the pinion gear and then force is applied to the ring gear applying the torque to the axle shafts.  The differential is what determines how much and were that force is applied between the two wheels of that axle.

Before we get started, let’s talk about the term “differential” to start with.  The differential gets its name because it regulates the relative wheel spin between two wheels on the same axle.  When you’re going straight the wheels are turning at the same speed, no big deal.  As soon as you turn the wheels begin to turn at different speeds.  The inside wheel slows down and the outside wheel speeds up.  This is compounded by different wheel speeds front and rear.  This is why it’s a very very bad idea to put your 4×4 in 4wd while on a dry solid surface and make a tight turn.  The wheels are trying to turn at the same speed and will bind the driveline causing the 4×4 to buck.  If it bucks hard enough this could case catastrophic failure of driveline components like u-joints or possibly even damage the ring and pinion gear sets.  The various types of differentials help regulate the “difference” in wheel speeds side to side, and in some cases, front to rear.

OEM style “open” differential

The Open Differential. The open differential is just as the name applies. It does little to regulate the relative wheel speeds of an axle.  The advantage is there is little chance an open differential will bind unless someone jams the gears inside the differential.  The disadvantage is there is nothing to transfer power from one wheel to the other if one breaks traction.  This is where the “one wheel wonder”  and “one wheel peel” effect comes in.  Once the tire breaks traction and starts spinning it will continue to spin.

Limited Slip with helical gears
Limited Slip with clutch packs

The Limited-Slip Differential.  As the name implies, a limited-slip allows for a small degree of slippage between wheels.  The advantage over an open differential is that is still provides positive traction to both wheels – hence the slang “posi” used to describe limited-slips.  Once one wheel slips the differential, through springs and clutches or gears, will transfer power from one wheel to the other wheel that has traction.  If that wheel slips, it will switch power back.  If both have lost traction then both wheels will spin.  Limited slips are great for street-driven vehicles that see wet and snowy roads.  They are also great for vehicles that tow allowing both wheels of the rear axle to retain power when trying to move a heavy load.  Off-road limited slips are great at getting 4x4s through slick mud and tricky terrain where the axle articulates and might “unload” (a term used to describe a free-floating tire with little to no vehicle weight) one tire allowing it to break traction.  The major disadvantage to a limited slip is that their clutch packs can overheat much like a clutch does in a manual transmission.  If the wrong friction modifier is used, or water/oil/mud gets into the differential the limited-slip can “cook” and ruin the clutches then relegating the limited-slip to an expensive open differential.

One piece spool.  Very strong for race applications, not very streetable.

The Spool.  The reason I am talking about a spool before lockers is for one simple reason.  A spool provides absolutely no differential action between the tires.  It “locks” them together 100% of the time and does so permanently.  So they only true locker that is 100% locked 100% of the time is a spool.  Their advantage, 100% power distribution.  Their disadvantage, no differential action between wheels which will cause binding on dry hard surfaces.  They are only recommended for vehicles that see off road terrain exclusively.  Some people can get creative with their welder creating a “Lincoln Locker” or “Miller Locker” (named after the brand of welder they use) which is basically welding the spider gears of an open differential so they no longer rotate.  This is find for rednecks who want to be cheap and “go muddin'” but has no place in the overland community.

“Lunchbox” automatic locker

The Automatic Locker. The automatic locker, sometimes called a “lunch box locker,” is a spring actuated locker that is basically a more aggressive limited slip.  Rather than using clutches like a limited slip, an automatic locker has toothed gears that mesh providing a mechanical lock between the two wheels of an axle.  The advantage is in loose terrain the locker will act like a spool but when a certain differential torque threshold between the two wheels is reached (i.e. one is trying to spin faster than the other around a turn)  it will release, or as some say “unload.”  This releasing action can lead to what is known as “kicking out” when an automatic locker in the rear end locks and unlocks going around tight turn on dry pavement causing the rear axle to jerk one way or the other.  This can also happen in the front when the automatic locker locks or unlocks causing the steering wheel to jerk side-to-side really quickly.  Experienced drivers can roll through it, but if it catches you off guard it’s a real wake up call.

In-dash switches for selectable lockers. Could be air or electric.

The Selectable Locker.  When most people talk about lockers they most likely referring to selectable lockers.  They are, in many ways, the best of both worlds.  When on they provide 100% power with no differential action just like a spool.  When off they act like an open differential, or in some cases like a limited slip.  There are three main kinds of selectable lockers.
  • Manual: These are cable actuated via a lever inside the 4×4.  The driver shifts the lever just like they would their transmission or transfer-case to engage or disengage the locker.
  • Air: Probably the most popular and widely known selectable locker is the air-locker.  When you flip a switch a burst of air either from a compressor or tank engages an air-actuated solenoid that locks or unlocks the differential.
  • Electric:  Just like an air-locker, an electric-locker has a solenoid that locks and unlocks the differential, it just does so with a direct 12 volt power source rather than air.  Electric lockers are becoming a increasingly popular piece of OEM equipment.
The three types of selectable lockers have their advantages and disadvantages.  Cable actuated ones require no complicated air systems or the routing of vulnerable air lines or wires.  However the cables are prone to stretching and don’t always fully engage the locker.  (Remember when that 12-speed bike of yours would never shift right when you were a kid?  Same idea.)  As mentioned before, air-lockers require an air source.  This can be a small dedicated compressor, a complicated on-board-air system, or a simple compressed gas cylinder (usually CO2, like from a paintball gun).  Electric-lockers are less complicated since they don’t require air in addition to power, and the wires are easier to splice than patching a ruptured air line.  However, some electric-lockers on the market are just variable limited-slips and do not ever fully lock 100% the way an air or manual locker does.
So, all that said, does the overlander need to worry about this kinda stuff?

For some they might be lucky enough to have a factory equipped set of lockers.  Other, like myself with the LJ, might have a combination of an open front differential and a factory rear limited slip.  However it’s more common to see 4×4’s with open differentials front and rear.  Looking at all the options out there, it’s very tempting to consider a locker as an upgrade when dealing with an open differential.

Personally, I’m of the opinion lockers aren’t needed when overlanding.  I’ve driven Jeeps with mostly open differentials and a few limited slips (’97 ZJ was open/open; ’96 ZJ was limited slips front and rear; ’87 XJ was open/limited).  I’ll admit there have been a few times I’ve gotten stuck when I wouldn’t have if I had a locker.  There’s also a lot of times I’ve looked at a trail or an obstacle and took a pass because I knew it meant trouble with or without lockers.  That brings up an interesting point.
I’m currently looking at keeping the limited slip in the rear of the LJ.  I’m looking at getting a locker for the front.  Being budget minded, and focusing more on overlanding rather than rockcrawling, I’ll probably settle on an automatic locker.  I don’t want to have to deal with or worry about complicated systems requiring a mix of cables, air, and/or electric wires.  The more elements in a system the more it is prone to failure.  I believe a mechanical locker in the front of my LJ provides me with the increase in traction I want, without compromising the reliability of my overlander.  The trick is going to be knowing when to say, “no.”  Before I used to take a pass on an obstacle because I knew once I got unloaded the open-diffs would leave me spinning.  With a front locker and a rear limited slip it will be more temping to try some stuff I used to pass on, but knowing when to say no will mean keeping myself out of trouble.  The last thing any overlander wants to be is stranded.
The double-edged sword of a locker means a more capable rig decreasing the chance of getting stuck.  It also means increase capability with an ability to try more difficult terrain and obstacles.  So, whether wheeling open-open, with a limited slip, or lockers, know how your rig operates, how it handles, and how it behaves.  Also, know when to say “no.”  That doesn’t mean always playing it safe, it just means playing it responsible.  After all, we’re not driving trailer queens that can get hauled home at the end of the day.  Our overlanders are an extension of who we are and our temporary home on wheels.  We need them to be capable, but most importantly, reliable.