Pigs, Chunks, and Lunchboxes are amazing. Eat em’ up!
“Lockers before Light bars.” That’s what they say. The saying is true for some, but many don’t know what lockers are, what they do or how they work. So, let’s have an appetizing discussion on pigs and chunks.
What is a differential?
The differential, also known as “diff”, “pumpkin” “third member”, “chunk” or “pig” serves a lot of purposes. It is the part of the axle the converts the rotation from the engine from left to right to front to rear.
While it’s doing that it also is increasing torque by lowering the gear ratio.
For most off-road rigs, factory ratios range from 3.54-4.10 to 1. In 3.54 gears, for example, the driveline turns 3.54 times for every 1 rotation of the axle shaft (tires). But the other thing the diff does is make the drive on the road extremely predictable. When you turn a corner, the tires on the outside of the turn must travel farther than the tires on the inside. The best way to deal with this is to allow the tires on the outside to spin faster… Simple! The differential does just that. It allows one axle to spin at a different speed than the other through four gears called spider and side gears. It allows the tires to differentiate… hence the name, Differential. We call this type of setup and “open” differential.
The trouble is that this also allows the power/torque from the engine to be transferred to one side more than the other… and that is exactly what it does. When one tire has less traction than the other on the same axle an open differential allows the torque to go to that tire. On an open differential 4x4 you will often see one front and one rear tire spinning
A locked differential provides a significant traction advantage over an open differential when the traction under each wheel differs because it “Locks” the left axle to the right axle so the rotational force is effectively transferred to both tires equally.
“I’ve heard lockers will cause axles to break.” It is true that the entire load required to move a vehicle can be placed on one axle which is where this debate comes from. But it is also very true that shock loading any component is the easiest way to break it. When you push on something heavy, and it doesn’t move your next response might be to bump it (think breaking down a door). Why do we do that? Because it works. It works because the force spikes on impact. This is true with tires too. Get them spinning and then suddenly stop them and you create a load spike called a shock load. This travels up the drivetrain in an instant. Anything that can’t handle the spike simply succumbs, spits out its guts, and leaves you stranded on the trail. A controlled load on one axle is far less likely to break than a shock load from a free spinning tire that suddenly gains traction.
Automatic VS Selectable
Automatic Lockers either replace the differential carrier (the carrier “carries” the ring gear and the side and spider gears) or replace only the side and spider gears. They engage when there is power applied to the differential. For the most part it means the axle is locked all the time. On road that translates to some understeer because the rear axle wants to push the vehicle straight. Understeer is an undesirable behavior
that occurs when a vehicle turns less than is desired when a corner is being taken – IE you are turning the steering wheel to the left relatively sharply, but the car only turns left slightly. Additionally, when turning with automatic lockers, the inside tire will often break free slightly and “bark”. This is because the inside tire develops the least amount of traction, and it must keep up with the
faster moving outside tire because the axles are “locked” together.
The simplicity of an automatic locker makes them extremely reliable.
Selectable lockers, when disengaged, have no adverse effect on the way your rig drives. Cruising around it feels perfectly normal. Many OEM’s use selectable lockers because of this. You get the best of both worlds with respect to drivability. Smooth driving on-highway and when needed, crazy traction. They are more expensive and have more systems than an automatic locker. Air lockers have and electrical system, an air system, and a locking system. The three systems leave the units
with more exposure to failure. Most selectable lockers have been around long enough that with a professional installation, failures are very rare.