This article was copied out of Petersens 4Wheel & Offroad Magazine, Alan Huber, August 2001, pgs 116-7. I feel this is a very nicely written article and is should be reproduced to others to read and learn from.

Whats the Diff?

We wanted to see for ourselves how each of these differentials stack up. We bolted'em in, we drove'em daily to work, we thrashed'em on the trails in Moab, on the Rubicon, and throughout Southern California. Here's what we found. Note that the test axle is a Ford 9-inch.


Limited Slip

Selectable Locker

Automatic Locker


Tried and True:


In our case (no pun intended). Yours is probably the same make as your axle.

Auburn Gear

A cone-and friction-material type diff. Other types of limited-slips use clutch packs or grease.

ARB Air Locker

Driver selectable by flipping on a supply of compressed air activating a solenoid to engage the locker.

Tractech Detroit Locker

A locker that senses torque and automatically engages gear teeth to spin axles at the same time.

Drivetrain Direct

A solid, machined unit that is internally splined to keep axle always turning together.

Budget Hit:

Price is from Driveline Direct at press time and is for differential unit only.

Free. Your truck almost certainly came equipped this way from the factory; they're also a-dime-a-dozen at a salvage yard.

$389. Prices vary depending on which type of limited-slip you desire – clutch pack units are less, gear driven ones are more.

$639. Ouch! This price is for the diff only; you must also install an on-board air compressor in order for the locker to operate.

$457. Detroits seem a little pricey at first, but they’re only 68 bucks more than a limited slip.

$169. Wow! What a swingin’ deal! The spool gets our unofficial Best bang for the Buck award.

Modus Operandi:

As a vehicle turns, an open diff allows the inside tire to rotate at a slower rate than the outside tire. This is needed because the inside tire make a smaller arc.

A limited-slip (or posi) allows different wheel speed same as an open diff, but it will try to turn both wheels together up to a point. Requires a friction additive.

Functions like an open diff when unlocked, butt when compressed air is applied, an annular piston connects, turning the axles at the same speed like a spool.

Application of torque causes the locker to turn axles together like a spool. Unit will, however, unlock when coasting through a turn, allowing the axles to differentiate.

Not much modus operandi here. A spool permanently locks both axles together so they are always turning at the same speed – always.

Pavement Manners:

The most street-friendly of all, open differentials are completely pavement benign which is why they are used most often by factories – no owner complaints.

You won’t notice it at all on the pavement (good!) except maybe in straight line full-throttle acceleration where the diff will probably drive both tires at equal speed

Just as invisible on the street as an open diff because, when unlocked, it is an open diff. ARB carrier cases are considered stronger than factory open cases.

Can be street driven (some were optional on factory muscle cars), but will chirp the inside tire on sharp corners when under power. Will unlock if coasting a turn.

Behaves the same way as a Detroit locker does when under power – chirping the inside tire on corners, but it won’t unlock when coasting.

Trail Tendencies:

Pretty much useless for all but the most tame trails because if suspension flex only slightly unloads a tire, all power will be sent to the wheel with the least traction.

You won’t notice it much at all on the dirt either (bad10 – especially if the suspension is twisted. Still, it is probably slightly better off-road than running an open diff.

The chameleon of the bunch! What was invisible on the street becomes a trail terror once you’ve flipped the switch and engaged the locker for off-road use.

Working like a spool, or an activated ARB, the Detroit will positively gear lock those tires together to claw the terrain in unison whenever you’re on the gas.

Again, two tires, forever turning together…at the same speed…always…like planets in the orbits, provide max forward trail momentum.

Coolest Feature:

Did we mention that they’re free? You could replace the spider gears with a lunchbox locker or weld them to create what’s known as a Lincoln Locker.

You can cause the unit to turn both axles in unison by applying brake pressure with the throttle. This holds the spinning tire while transferring torque to the other.

The coolest feature of selectable lockers is their, well, selectability! Completely off on the street, you won’t know it’s there until you need it – and then it works great.

A simple, strong design that has well proven itself over the years; the Detroit has seen (and survived) action in all sots of motorsports.

Maximum strength and utter simplicity make the spool a dynamite choice. It’s the least expensive diff in the test, so you can’t go too wrong.

Annoyance Factor:

These things are so Villa they just won’t rock your world. The slightest bit of opposite-corner suspension twist will leave you spinning your wheels.

If you’ve owned a muscle car you know limited-slips work great on pavement, but off-roading is a different animal, and many times we were left spinning tires – again.

The annoying trait with this unit stems from the design complexity. Although ARB produces a high-quality piece, O-rings and air fittings (in general) can fail.

Besides the aforementioned tire chipping, the Detroit gets tiring on sweeping pavement curves because the locking/unlocking action causes the truck to weave.

Noisy tire chirping will net you dirty stares from pedestrians as well as some increased tire wear. Of course the same is true of the Detroit.

Living With It:

You can live with it, but why would you want to? You might as well buy a Pontiac Aztek and go read Car and Driver.

Limited-slips are easy to live with; your 4x4 is nice to drive on the street with no bad manners, but off-road you might get left behind. Can you live with that?

Air lockers are great dual purpose diffs, disappearing on the street yet still awesome in the out back, just be sure to change your gear oil religiously.

If you minimize jerky throttle application and try to coast through corners (or at least keep steady pedal pressure) many drivability issues won’t be issues any more.

Like the Detroit, be prepared for steering that is a little heavy, and keep tire pressure high to reduce chirping, but at least there’s no lock/unlock weave.

Unabashed Opinion:

Get rid of it or modify it as we specified above in the Coolest Feature(s) Category – unless, of course, you enjoy being stuck.

Our four wheeling adventures now take us beyond the capabilities of a limited –slip. If you don’t want to be limited off-road, go for one of the diffs to the right.

The ARB in out Ford’s 9inch worked primo until a seal let go while we were halfway up a steep, soft, dirt hill. It was probably from a lack of maintenance on out part.

Yes, the Detroit can cause some (minor) handling problems, Yes it causes tire chirp, And yes, it will take you places off-road you thought were physically impossible.

Less drivetrain slop than the Detroit and less handling problems. Does it chirp tires? Yes. Do you care? No. Put 30 lbs. In your tires and drive.