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Axle Terms Explained

Every Term You Need To Know About Axles Defined
They say if you talk the talk, you gotta walk the walk. But sometimes the talking is the tricky part, especially when it comes to axles. Even the term “axle” itself can be ambiguous. Do you mean an axle shaft or the entire axle assembly? Add to it slang like “Lincoln Lockers” and “tons,” and things get even more confusing. Don’t worry, as we are here to help. We have come with up definitions for most common axle terms along with colloquialisms. Whether you are looking to swap entirely new axle assemblies under your 4×4 or just want to regear to account for larger tires, we can help you talk the talk. Read on to learn what is what so you no longer have to look puzzled when someone asks you about a kingpin.

Axle Terms

Axle Shaft: An axle shaft is the part of an axle assembly that is turned by the differential. It transfers power either directly to the wheel or to the wheel hub that the wheel bolts to.

Ball Joint: Ball joints connect the top and bottom of the steering knuckle to the end forging and hold the knuckle in position while allowing steering motion.

Bearing: A bearing provides a low friction surface that allows two parts to move on a single axis. Examples include tapered roller bearings for carriers and pinions, needle bearings, and thrust bearings.

Bearing Cap: The bearing cap captures the carrier bearings on an axle assembly. Note that they are position sensitive and should be marked before removal to ensure that they are replaced in the same location and orientation.

Bolt Pattern: The number of lug studs bolting the wheel to the axle and the diameter of the circle the lug studs make, expressed in inches or mm. Common patterns include 5×5 (Jeep Wrangler JK and JL) and 6×5.5 (Toyota Tacoma and 4Runner). For a complete guide to bolt patterns, check out our story here.

C-Clip: Small clips commonly used inside the carrier on semi-floating axles to retain the axle shaft. On the axle shaft’s spline, a receiver groove cuts into the end for the C-clip to grab the axle. There is a pocket machined into the carrier’s side gear that the outer diameter of the C-clip fits into.

Chromoly: Chromoly is an abbreviation for “chromium-molybdenum steel.” Often expressed as 4130 or 4340, which refer to the amount of chromium and molybdenum present. This material is known for its malleability or flexible memory and tensile strength.

CORE 44: Aftermarket G2 axle housings that use the latest Dana 44 ring and pinion with forged bearing caps and 3-inch diameter DOM axle tubes. American-made CORE 44 housings are available as bolt-in options for all Jeep Wranglers built since 1987.

Constant Velocity (CV) Joint: A type of axle shaft in use on independent suspensions that offers a smooth range of motion throughout the suspension travel as well as the ability to steer. Also available for solid axles from aftermarket manufacturer RCV.

Bearing caps aren’t sexy, but they are important. Make sure that they are installed in the same position that they were removed. 

Dana: Axle manufacturer producing everything from the light-duty Dana 27 and Dana 35 front and rear axles to one-ton Dana 60, Dana 70, and Dana 80 axles (and even larger medium and heavy-duty axles).

Drive Slug/Drive Flange: An alternative to a locking hub that foregoes the ability to lock or unlock the front end in favor of greater strength as a result of less moving parts.

Drop Out : A drop out is an axle design that uses a removable third member where the complete differential is removable from the entire axle assembly. Commonly examples include Ford 9-inch and Toyota axles.

End Forging: Often called an Inner C, the end forging is the component on the end of a front axle housing that the knuckle bolts to with either ball joints or kingpins.

FAD: A front axle disconnect found on Jeep Wrangler JL front axles. A two-piece long side inner axle shaft is used to allow the front axle to be decoupled on the pavement to increase fuel economy without the need for manual locking hubs.

Flange: In general terms, a flat mounting surface to couple two pieces together. Often used instead of a yoke at the end of the pinion to connect a driveline.

Full Floater: A full floater axle is a non-load carrying axle that has a hub, bearings, and spindle at the wheel end instead of a flange. Common in one-ton axle housings, the axle requires no support and is “floating” between the wheel hub and the side gear.

High Pinion: High pinion is another name for reverse rotation gear sets. This term refers to the pinion of the axle being above the centerline of the axle housing.

Housing: This term refers to the main body of the axle assembly, sometimes also simply called the axle.

Hub: A hub is a wheel-mounting device that transfers power to the wheel. This unit usually is supported by two bearings and is driven by the axle shaft. Locking hubs are a subset of hubs but often referred to simply as hubs.

IFS: Independent Front Suspension allows each wheel to move independently of each other over an obstacle. Commonly found on half-ton pickups and Toyota Tacomas and 4Runners.

Inner Axle: An inner axle refers to the axle shaft that mates to the differential, typically in a front-end application where it mates to a stub shaft with a U-joint or CV.

Axle knuckles come in lots of variations. Some are stronger than others and require different steering setups as well.
King Pin: An alternative to a ball joint used to mate a knuckle to an end forging. Found on older Dana 60 and Toyota solid front axles. As the name implies, a tapered pin on the end of the forging mates to the knuckle with roller bearings.

Knuckle: The steering knuckle is the main component that allows a solid axle to perform its steering function. The knuckle is usually retained to the housing by the ball joints or kingpins.

Locking Hub: The locking hub is a device that allows the wheel bearing hub to disengage from the axle shaft. This function is used in part-time 4-wheel drive vehicles to reduce the additional drag of the drivetrain when the 4-wheel drive is not needed.

Placer Gold: G2 chromoly axle shafts named for their distinctive color. Forged from high grade 4340 chromoly, and heat-treated with computer-controlled methods, which results in 25% deeper surface penetration over stock. The shafts taper to maximize torsional strength and reduce stress risers.

Pinion Gear: The pinion gear is the drive gear that connects to the driveline and turns the ring gear. They are only available as a matched set with the corresponding ring gear.

Pinion Angle: Pinion angle refers to the angle that the axle yoke is relative to the ground. When using a double-cardan driveline, the pinion should point at the transfer case output flange. When using a single U-joint at each end of the driveline, the pinion angle should be parallel to the transfer case output flange.

Race: A tapered cup that is a hardened surface for a bearing to ride on.

Ratio: A ratio is a quantity that denotes the proportional amount of one quantity relative to another. A lower gear ratio is actually numerically higher (4.88 gears are lower than 4.10 gears).

Reverse Rotation: The reverse rotation gear set was developed for use in front axle applications. The standard gear, when used in a front application, is running on the coast side of the gear. The convex (drive) side of the gear is capable of carrying more load than the concave (coast) side.

Ring and Pinion: A matched set of gears with teeth designed to mesh together. The ratio of the ring gear to the pinion gear multiplies vehicle torque before transferring it through the differential to the axle shafts.

Ring Gear: The ring gear is the gear that is bolted to the carrier and driven by the pinion gear (also called a crown gear if you prefer tea to coffee).

Rzeppa Joint: A Rzeppa joint consists of a spherical inner shell with six grooves in it and a similar enveloping outer shell. Each groove guides one ball. The input shaft fits in the center of a large, steel, star-shaped “gear” that rests inside a circular cage. Found on JK and JL drivelines and in closed knuckle front axles.

Rzeppa joints (on the right) are commonly found on Jeep JL and JK driveshafts. They can’t handle high angles or bigger tires so make sure to replace them with U-joint driveshafts if you lift your Jeep.

Semi Float: A semi-floating axle carries the vehicle weight and is supported at the inner spline end and by a bearing before the outer end.

Spindles: Spindles are machined stubs that mates to a hub with tapered roller bearings. They are found on a front axle assembly (where they bolt to the steering knuckle) or full floating rear axle (where it mates to the axle housing).

Spline: Splines are the raised portions of an axle shaft or differential that look like teeth. Splines provide a strong and effective way of easily joining two parts together that can still be separated. The more splines a given axle has, the stronger it will be due to improved tooth contact.

Stub Shaft: The outer axle, or stub shaft, is the outer most axle shaft of a multiple shaft assembly. Typically found in a front axle assembly where they mate to the inner axle with a U-joint.

Third Member: A third member is an axle design that uses a removable third member where the differential and ring and pinion can be removed from the entire axle assembly. Common examples include the Ford 9-inch and Toyota axles.

Truss: A structure found on the top or back of axle assemblies to increase strength and rigidity. They are often used to tie together suspension mounting points when used with link suspensions.

U-Joint: U-Joint is short for a universal joint and allows a rigid shaft to bend in any direction. They consist of a pair of hinge-like crosses located close together but oriented at 90 degrees relative to each other. They are found both in drivelines and in front axle applications.

Wheel Flange: A wheel flange is the flat mounting surface that the wheel mounts to. This flange is either part of the wheel hub or made into the axle shaft depending on which type of axle is in use (semi-float and full-float, respectively).

Wheel Stud: Wheel studs allow the wheel to bolt to the axle. They are available in a variety of lengths and diameters to accommodate different thicknesses of wheels.

Yoke: The pinion yoke links the pinion shaft to a driveshaft with a U-joint. This allows the axle to move up and down as the suspension cycles. Yokes are often referenced relative to the U-joint they accommodate, such as 1310 or 1350.

ARB’s Air Locker is a great example of a selectable locker. It operates via compressed air and can be engaged with the flick of a switch. 

Differential Specific Terms

Air Locker: Differential developed by ARB, it operates as an open differential until air pressure is applied, at which time the side gears lock together to act as a full spool. It is one of the strongest and most common aftermarket carriers.

Automatic Locker: Also called a mechanical locker, these include Detroit Lockers and Lockrights that do not require any input from the driver. They will allow for a tire to spin faster than the carrier (such as when turning a corner) but not slower than the carrier.

Carrier: Sometimes referred to as the differential case, this unit transfers the motion from the ring gear to the axle shafts. The ring gear bolts to the carrier, which can contain gears, clutches, and springs.

Carrier Break: As the number of teeth on the pinion gear decrease, it gets smaller, increasing the distance from the ring gear. Different carriers are available to account for this distance. Carrier breaks for Dana axles are between 3.73 and 3.92 ratios for Dana 44s and between 4.10 and 4.56 ratios for Dana 60s.

CORE Locker: The Core Locker is a pneumatically-controlled selectable locker with a built-in manual override for complete reliability. This heavy-duty locking differential is designed to handle the added stress of a fully locked differential by incorporating a one-piece body, coupled with a forged cap for maximum strength.

Cross Shaft: The cross shaft is a hardened pin that transfers a load from the carrier case to the spider gear assembly.

Detroit: Detroit Lockers are automatic lockers that completely replace the factory carrier (except in the case of Corporate 14 Bolt axles) and transfer power evenly to both axle shafts. Unlike a spool, which locks the two axle shafts together entirely, a Detroit Locker allows a tire to turn faster than the carrier, but never slower than the carrier.

Differential: The component that the ring gear bolts to that transfers power to the axle shafts. It allows the tires to rotate at different speeds, such as when going around a corner. Most vehicles come with open differentials, where the power routes to the tire with the least amount of traction or resistance.

Limited Slip: A limited-slip differential (LSD) is basically a preloaded differential. These units either use a set of clutches or gears to preload the spider gears up to a specific breakaway setting. This breakaway is the force required to allow the axles to differentiate. Note that clutch-type LSDs require a special additive to the gear oil.

Locker: Mechanical lockers use a type of ratcheting device to lock the axles together when power is applied to the unit and allow one wheel to disengage while turning. Selectable lockers enable the user to turn them on or off with air or electricity. These units will rotate both tires even if one is off the ground.

Mini Spool: A mini spool is a device that goes inside a factory differential carrier, replacing the spider gear assembly and converting it into a spool. A full spool is a one-piece unit that replaces the carrier. Neither provides a differential action between the shafts.

Selectable Locker: A selectable locker, as the name implies, allows the user to select when it is engaged. When locked, these differentials function like a spool. When unlocked, they can either function like an open differential (such as an ARB Air Locker or Eaton E-Locker) or a limited-slip differential (such as in the case of an Auburn Ected).

Side Gear: The side gear is an internally splined gear that receives the axle shaft. The spider gears then drive this gear.

Spider Gear: A spider gear is also called spider pinion gear. These are the small gears that make up the spider gear assembly. This assembly is usually made up of two side gears and either two or four spider gears. The cross pin of the carrier drives the spider pinion gears.

Spool: A spool permanently locks axles together. Spools are usually used in competition vehicles that require no differential action between the axles. They are not recommended for street use.

Setting up ring and pinion gears properly is a bit of an art form. It takes the right tools, knowledge, and experience. 

Set Up Terms

Backlash: Backlash is how tightly the ring and pinion gears mesh together or play between the gears. Measured with a dial indicator in thousandths of an inch.

Case Spreader: A tool used to expand the axle housing enough to allow the carrier to install with the required shims and bearings.

Coast Side: The coast side of the gear tooth is the concave side. The coast side is typically loaded when the vehicle is decelerating or in reverse (except with a low pinion gear in a front application).

Crush Sleeve: A crush sleeve is a single-use, collapsible spacer placed between the inner and outer pinion bearings to keep preload on them.

Dial indicator: An instrument used for tracking precision measurements down to one-thousandth of an inch.

Drive Side: The drive side of the gear refers to the convex or crowned side of the ring gear. This side of the gear is the side of the tooth that the pinion gear pushes off to turn the ring gear when accelerating forward.

Heal: The heal is the part of the ring gear tooth that the pinion contacts first. It is the very outside of the gear tooth.

Pattern: The pattern refers to how the ring gear is mating with the pinion. By reading this print, you can determine if shims are required to set up the proper gear mesh and where they must be placed.

Preload: The amount of drag or resistance that keeps something from turning freely.

Ring Gear Bolts: Graded bolts that attach the ring gear to the differential. They are torque to yield bolts and should not be reused. They should also install using a thread locking compound.

Run Out: Run out is an undesired movement of a part through its motion or wobble.

Root: The root of a gear tooth is the very bottom or base of the tooth that spans the gap between the teeth.

Shim: A shim is a thin piece of material used to fill small gaps or spaces between objects. In a gear set, shims adjust the pattern of gear mesh.

Solid Spacer: A solid spacer fits between the inner and outer pinion bearings and works with shims to preload the bearings. Unlike a crush sleeve, a solid spacer is reusable.

Spanner: A spanner is also known as a side bearing adjuster. This adjuster is used to move the carrier assembly side-to-side to set backlash instead of requiring shims.

Toe: The toe of the gear tooth is the inner half closest to the carrier. This is the part that sees pinion contact last.


Birfield: A Toyota term for CV joint, found in the closed knuckle front end in solid axle Toyota Land Cruisers, pickups, and 4RUnners.

Deep Gears: Deep gears refer to lower (numerically higher) gear ratios. Deeper gears are often needed to account for larger tires.

Fast Gears: Fast gears refer to higher (numerically lower) gear ratios. Faster gears will generally provide better mileage and higher top speed.

Hogs Head: A southern term referring to the center section of the axle.

Inner C: An Inner C is a commonly used term to describe an end forging, where the front steering knuckle attaches to the axle housing assembly.

Lincoln Locker: The process of welding the spider gears together to remove the differentiation from the carrier and essentially create a spool. The name is a reference to Lincoln welders.

Lunchbox Locker: An automatic locking differential that replaces the spider gears with plates inside of the factory carrier, such as a Lockright. They don’t require changing shims or bearings, so they are thought to be so easy to install that you can do it during your lunch break.

Posi: Posi-trac refers to a limited-slip differential (LSD) designed by General Motors, but it has become a generic term for an LSD.

Salisbury: A British term for a Dana-style axle with a cast-iron center section as opposed to a removable third member.

Sterling: Ford 10.25 or 10.5 axles found in the back of Super Duty trucks. The name Sterling comes from the town in Michigan where they are made.

Tall Gears: Tall gears refer to higher (numerically lower) gear ratios. Taller gears will generally provide better mileage and higher top speed.

Tons: Slang for one-ton axles typically consisting of a Dana 60 front axle and a Corporate 14 Bolt, Sterling 10.5, or Dana 70 rear axle.

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