What's the DIFF???

[Steve83]

This is an updated version of my old "Diffinitions" post on ORC (which is now virtually dead, despite a nice new face). I want to make sure this info survives, even if ORC doesn't.

This is for anyone wondering what all the different names/types/brands/models/variations of differential do, and why you might want one over another. Most of the links take you to a manufacturer's official tech page, including an exploded view of the particular diff, and a detailed explanation of how it works.

An automotive differential is a mechanism for dividing the engine's power between 2 shafts, most often between the 2 axleshafts within an axle. It typically involves a "carrier" which contains 4-6 bevel gears; 2-4 pointing to the center (usually 2 on a "pinion shaft" perpendicular to the carrier's axis) commonly called "spider gears", and 1 on each axleshaft concentric with the carrier's rotation. When the vehicle is rolling straight, the spider gears don't rotate on the pinion shaft, but merely convey power from the carrier (which is driven by the ring & pinion gears) to the axle gears which ultimately turn the tires. During a turn, the spider gears spin to allow one axle to turn faster than the other. The problem is that the spider gears can spin fast enough to allow ALL the power to go to ONE axleshaft, leaving the other NOT TURNING AT ALL. This is an "open" differential and is used on almost all production vehicles; front, rear, or all-wheel drive. Some 4WD vehicles are manufactured with open diffs.

The easiest solution to the wheel spin allowed by an open diff is to eliminate all differentiation by either replacing the carrier & its contents with a solid piece of metal called a "spool", or by making the carrier & its contents solid by welding everything together (a "Lincoln Locker" named for the brand of welding machine), or by replacing only the contents of the carrier with a "mini-spool". But this produces unpredictable behavior on-road (especially on ice), accelerated tire wear, & poor steering response.

A better solution is to limit how fast the spider gears can turn by braking the axle gears - "limited slip". This involves simple springs pushing on friction materials inside the carrier, but sometimes requires additives to the oil. They also wear constantly, so they require service to continue to function. Also, they can be easily overwhelmed by wear, engine power, or driveline torque (big tires or a heavy vehicle). Some brands of LSD's are Ford's Trac-Lok (made by Eaton), GM's Positraction (also now made by Eaton), Dana's Hydra-Lok, and Auburn LSD, and each uses a specific carrier different from the open diff's.

An even better solution is "torque biasing" in which a mechanism varies the braking effect to send the torque to the tire with traction. This type uses even more unusual carriers. GM's Gov-Lock uses an extremely complex & delicate arrangement of springs, counterweights, and clutches to do an effective job, but they’re famous for exploding when over-revved. The Tractech (now owned by Eaton) True-Trac uses a much simpler, more robust, & more elegant arrangement of helical gears, but still suffers from wear. Zexel's TorSen claims not to suffer from wear and is made in 3 configurations, but is only available for a few axles & transaxles.

A solution that virtually eliminates wear is the Tractech (now owned by Eaton) Detroit Locker, which uses toothed rings held in engagement by heavy springs to transmit power from the ring gear to the axleshafts. They replace the stock carrier, but require no modification to the axle housing. There is no differentiation (in the pure sense) since they act as a spool, but the differential effect is still achieved when the rings disengage under extreme driveline torque, momentarily allowing one axle to freewheel to relieve the torque. There are many copies of this design marketed under many names, and there are also "mini" versions which replace only the contents of the carrier but are considered far weaker. While VERY effective & dependable (except minis), these show the same unpredictability as a spool when driven on ice. Some also produce an audible popping during normal on-road operation which bothers some drivers.

The last & most functional type is the "selectable locker" which gives the driver direct & immediate control to change from an open diff to a spool, and a recently-introduced type switches from LSD to spool. A few older 4WD's were manufactured with this feature, and it has recently returned in a few high-end SUVs/SUTs. ARB sells the most popular which uses an air diaphragm in a proprietary carrier to apply force to the clutches that lock the diff. While effective & convenient, it is expensive to buy, expensive to install, and requires an on-board air source which is also expensive. It requires some drilling of the axle housing for the air line to be routed thru, and some owners have found air leaks to be a problem. The Eaton Elocker promises simplicity over the ARB since it uses an electromagnetic solenoid mechanism to lock the diff, but they have little history to judge their quality. Those offered in high-end SUVs/SUTs see little real use. Most early selectable-lockers were cable-operated.

This is an excellent illustrated & animated tutorial on the functioning of differentials, and the rest of the web site is a WEALTH of information & links.
This is another article comparing types of diffs.

BEFORE POSTING IN ANY TECH WRITEUP THREAD, read this post.

[Fireguy50]

should be in the FAQ section

[Steve83]

If a mod wants to move it there; fine by me.

[never lift]

so do stock broncos have trac-loc?

[Bronco4Life]

Originally Posted by never lift

so do stock broncos have trac-loc?

Some do, it was an option.

Look at your door code, and then compare it to the codes listed in the chart in the axles info sticky.