Trailer Axle Terms: A Plain-English Glossary

Trailer axle parts come with their own vocabulary. Order forms ask for a hub face and a spring center. Product pages list BT8 and BT9 spindles, E-Z Lube, inner wiring and a GAWR. If a term is new to you, it is easy to order the wrong part. This glossary explains the trailer axle terms you will see on Southwest Wheel product pages, order forms and Dexter manuals, in plain English.

Each term has a short definition, where it matters and a link to the guide or parts that go with it. The terms are in alphabetical order. On a Dexter axle, always follow Dexter's instructions. On another brand of axle, follow that maker's own manual.

Key takeaway

• Measure two numbers first: Hub face and spring center set the size of a leaf spring axle. Overhang is the hub face minus the spring center.
• Match the spindle: The spindle decides which bearings, seal and hub fit. A BT8 and a BT9 spindle use different bearings and seals.
• Every part has a rating: The axle system is limited by its lowest rated part, so keep the springs, wheels and tires rated for the same capacity as the beam.
• Follow Dexter's bulletins: Use zero offset wheels or wheels within 1/2" of zero, torque the wheel nuts in stages, and look for about three threads past the nut.

Trailer axle terms, A to W

Axle description number

A coded number on the Dexter axle label. It identifies the axle model, hub face, spring center, brake setup, lubrication and other features. Where it matters: it is the fastest way to order an exact replacement. See How to Identify Your Old Trailer Axle and Order a Dexter Replacement.

Axle label

The identification label Dexter puts on its Sprung and Torflex axles. It shows the axle model, the capacity, the hub face, the spring center and a serial number. Per Dexter Product Bulletin 422, newer labels are placed on the rear of the axle tube, toward the frame mount. Dexter also etches the axle lot number on the tube. Where it matters: read the label before you measure. If it is gone, measure the axle instead.

Axle spacing

The distance from one axle to the next on a tandem or triple. The Dexter catalog labels it C. Where it matters: it is set by the springs and hangers. See Hanger Spacing for Single and Tandem Leaf Spring Axles.

Bearing cup and cone

Dexter's standard wheel bearing setup uses opposed tapered roller bearing cones and cups inside the hub. The cone holds the rollers. The cup is pressed into the hub. Each hub has an inner and an outer bearing. Where it matters: replace the cup and cone as a set, and match them to the spindle. Shop Trailer Axle Bearing Kits.

Bolt circle

The number of wheel studs and the diameter of the circle they sit on, such as 5 on 4.5". Where it matters: the wheel has to match the hub bolt circle exactly. Dexter Product Bulletin 409 warns that some bolt circles differ by so little that a wrong wheel might still go on. Shop Trailer Wheels, Tires and Assemblies.

Brake flange

The flange welded to the axle behind the spindle. The brake assembly bolts to it. Where it matters: an axle cannot take brakes without one. Custom Dexter beams of 3,500 lb. and up always come with a brake flange. On the 2,000 lb. custom forms, the brake flange is an option. The stock 2,000 lb. beam does not have one. See Dexter Axle Options: Tube, E-Z Lube, Pre-Wire and Brake Flange, or shop Brake Flanges.

BT8 and BT9 spindles

Two common 2,000 lb. spindle types. A BT8 spindle uses an L44643 bearing at both the inner and outer positions and a seal for a 1.25" shaft. A BT9 spindle uses an L44649 bearing at both positions and a seal for a 1.50" shaft. The custom 2,000 lb. Dexter beam has a Type BT9 spindle with 1-1/16" inner and outer journals. Where it matters: identify the spindle before you buy bearings, seals or a hub. See the TruRyde Bearing Kit for BT8 Spindle and the TruRyde Bearing Kit for BT9 and BT16 Spindle.

Camber

A slight arch built into the axle tube. It makes up for the expected deflection under load, so the tires stand more upright and the tread meets the road evenly when the trailer is loaded. Where it matters: camber is chosen together with the spring seats on the custom form. See Overslung vs Underslung and Camber: Spring Seat Options for a Custom Dexter Axle.

Capacity

The load an axle is rated to carry, such as 3,500 lb. On a tandem, the axle ratings add up, so two 3,500 lb. axles give 7,000 lb. Where it matters: pick the capacity from the loaded trailer weight, then match the springs, wheels and tires to it. See Choosing Trailer Axle Capacity: Hitch Weight, GAWR, Wheels and Tires.

Cotter pin

A pin that locks the spindle nut on standard grease and oil hubs. Dexter's newer D-Flat spindles add a cotter pin hole and use a castellated nut retainer with a cotter pin. Where it matters: use a new cotter pin when you reassemble the hub. See Trailer Hub Bearing Adjustment and Grease, Step by Step.

Double eye spring

A leaf spring with an eye at each end. Dexter double eye springs are held to the axle tube with U-bolts and tie plates. Where it matters: a double eye spring does not use the same hanger as a slipper spring. Shop Trailer Leaf Springs.

Drop

On a drop axle, the spindle center sits below the axle tube, so the frame rides lower. On a 4" drop axle the spindle center is 4" below the center of the beam. A straight axle has the spindle center in line with the tube. Where it matters: a drop is a separate choice from camber. The 2,000 lb. straight beam uses a 1-3/4" tube, and the 2,000 lb. drop beam uses a 2-3/8" tube. See Drop Axle vs Straight Axle: Deck Height Guide.

End play

A small amount of in-and-out movement of the hub on the spindle. Dexter says tapered roller bearings need a minimal amount of it, and that it is essential to bearing life. Where it matters: it is set when you adjust the spindle nut. See Trailer Hub Bearing Adjustment and Grease, Step by Step.

Equalizer

The part at the center hanger of a tandem or triple that connects the springs of one axle to the next. It helps transfer load from one axle to the other. Where it matters: if the equalizer hangers are too short, the spring eye can hit the frame. Shop Trailer Equalizers.

E-Z Lube

A Dexter spindle that has been drilled and fitted with a grease fitting in the end. Grease pumped into the fitting goes to the inner bearing, flows back to the outer bearing and comes out the grease cap hole, so you can grease the bearings without removing the hub. Dexter designed E-Z Lube to allow immersion in water. Dexter strongly recommends against a pneumatic grease gun. Where it matters: E-Z Lube is a choice on most custom forms, and the 4,400 lb. custom beam comes with it as standard. For grease, see Dexter #2 Lithium Complex Trailer Grease (088-071-00) or Timken wheel bearing grease in a 14 oz. cartridge or a 1 lb. tub.

GAWR (Gross Axle Weight Rating)

The load carrying capacity of the axle system, measured where the tires meet the ground. It includes the wheels and tires and is limited by the lowest rated part. Where it matters: a 3,500 lb. beam with tires rated lower than the beam does not carry 3,500 lb. See Choosing Trailer Axle Capacity: Hitch Weight, GAWR, Wheels and Tires.

Grease

Dexter specifies a lithium complex, NLGI No. 2 grease with a dropping point of at least 215 C (419 F) for its grease-packed bearings. Do not mix grease types. Dexter says bearings should be lubricated every 12 months or 12,000 miles. Where it matters: use a grease that meets the full Dexter spec. See Dexter #2 Lithium Complex Trailer Grease (088-071-00), Timken grease in a 14 oz. cartridge or 1 lb. tub, or all Trailer Grease.

Grease cap

The cap on the front of the hub. An E-Z Lube cap has a rubber plug in the end. An oil cap is clear and shows the oil level. Where it matters: on an E-Z Lube hub, grease comes out of the cap hole when the hub is full.

Grease seal

The seal pressed into the back of the hub behind the inner bearing. It keeps grease in and dirt and water out. Where it matters: replace it if it is nicked or torn. On an E-Z Lube axle, Dexter says to replace the seals any time the hubs come off, before you grease the bearings. The seal size follows the spindle, such as a 1.25" seal for a BT8 spindle.

Hanger

The bracket welded to the trailer frame that holds the end of a leaf spring. A single axle uses a front and a rear hanger. A tandem adds a center hanger with an equalizer. Where it matters: hangers that are too short can let the axle hit the frame, and hangers that are too long need bump stops. See Hanger Spacing for Single and Tandem Leaf Spring Axles, or shop Hanger Shackle Kits.

Hanger spacing

The distance between hangers along the frame. The Dexter catalog labels it A on a single axle and B and E on tandem and triple axles. Where it matters: it is set by the length and type of leaf spring.

Hub face

The measurement from the wheel mount on one hub to the same point on the other hub. It is generally measured from the base of a wheel stud to the base of a wheel stud on the opposite side. Where it matters: it sets where the wheels sit, and it is the first measurement on every custom form. See How to Measure for a Custom Dexter Trailer Axle.

Idler or brake

An idler hub has no brake. A brake hub carries a brake drum or a disc brake rotor. Where it matters: a brake hub needs an axle with a brake flange. See Build a Complete Dexter Trailer Axle, or shop Trailer Hubs with Parts.

Inner wiring (pre-wire)

Dexter runs the electric brake wire through the axle tube. Dexter calls this inner wiring. The order form calls it pre-wire. Where it matters: every custom Dexter form asks whether you want the axle pre-wired. See Dexter Axle Options: Tube, E-Z Lube, Pre-Wire and Brake Flange.

Lug torque

How tight the wheel nuts are, measured in ft. lbs. Dexter Product Bulletin 409 says to start all nuts by hand, tighten them in stages in the torque sequence, and torque them before first road use and after each wheel removal. Check and re-torque after the first 10, 25 and 50 miles. In general, about three threads should be visible past the end of the nut. Where it matters: loose wheel nuts can let a wheel come off. The full table is in Trailer Hub Bearing Adjustment and Grease, Step by Step. Shop Lug Nuts for Trailer Hubs and Drums.

Nev-R-Lube

A Dexter bearing option. Its opposed tapered roller bearing cones are sealed inside a one-piece double cup. They are lubricated, assembled and sealed at the factory, and no further lubrication is needed. Where it matters: Nev-R-Lube hubs use a different service procedure and have their own wheel offset limits in Dexter Product Bulletin 404.

Nut retainer

The part that locks the spindle nut on an E-Z Lube axle built after February 2002. Where it matters: pry it off gently and set it aside when you remove the hub. See the Dexter Axle Spindle Nut Retainer for E-Z Lube Axles (006-190-00).

Overhang

The hub face minus the spring center. Where it matters: each custom form lists the overhang range allowed when the spring seats are welded, so check it before you order. See How to Measure for a Custom Dexter Trailer Axle.

Overslung and underslung

Overslung puts the spring seat on top of the axle tube, which raises the frame. Underslung puts the spring seat under the axle tube, which lowers the frame. Where it matters: it changes frame height as much as a drop does. See Overslung vs Underslung and Camber: Spring Seat Options for a Custom Dexter Axle.

Pre-configured axle

A stocked, exact Dexter part number built to a set hub face and spring center. It is straight, cambered and underslung, with spring seats pre-welded, E-Z Lube spindles and brake flanges. Where it matters: if your size is on the list, it is the simpler order. See Custom Built vs Pre-Configured Dexter Axles: Which to Order and Pre-Configured Dexter Trailer Axles.

Slipper spring

A leaf spring with an eye at the front and an open slipper end at the rear. Where it matters: it does not use the same hanger as a double eye spring, so match the hangers to the spring.

Spindle

The machined end of the axle that the bearings ride on. Where it matters: the spindle sets the bearings, seal and hub that fit. Shop Trailer Axle Spindles.

Spindle nut

The nut on the end of the spindle that holds the hub on and sets the bearing adjustment. Where it matters: follow Dexter's adjustment steps for your spindle. Shop Trailer Spindle Nuts.

Spring center

The distance from the center of one spring seat to the center of the other spring seat. Where it matters: it has to match the springs and hangers on the frame. On the custom form it is optional. Enter it only if you want the seats welded. See How to Measure for a Custom Dexter Trailer Axle.

Spring seat

The seat on the axle tube where the leaf spring mounts. Seats can be welded on at the factory or ship loose for you to weld. Where it matters: welded seats need a spring center on the order. See Overslung vs Underslung and Camber: Spring Seat Options for a Custom Dexter Axle.

Tie plate

A plate used with the U-bolts. Per the Dexter service manual, double eye leaf springs are held to the axle tube with a system of U-bolts and tie plates. Where it matters: tie plates come off with the U-bolts when you replace a spring. Shop U-Bolt Plates.

Torflex

Dexter's rubber torsion suspension axle. It does not use leaf springs. Where it matters: the custom Dexter beams are standard spring (leaf spring) axles. If your trailer rides on a torsion axle, see Dexter Torflex Rubber Torsion Suspension Axles.

Tube and wall

The tube is the round axle tube between the spindles. The wall is how thick the steel of the tube is. The order form calls this choice Wall Thickness. Where it matters: the U-bolts have to fit the tube. The 2,000 lb. straight beam uses a 1-3/4" tube, the 2,000 lb. drop beam uses a 2-3/8" tube, and the 3,500 lb. forms offer a 2-3/8" tube and heavier 3" tubes. See Dexter Axle Options: Tube, E-Z Lube, Pre-Wire and Brake Flange.

U-bolts

The bolts that clamp the leaf spring to the axle tube. They are sized to the tube, so a 1.75" tube and a 2-3/8" tube take different U-bolts. Where it matters: Dexter Product Bulletin 488, U-Bolt Thread Change on 3.50" Diameter Axle Tubing, page 1, says to always use new U-bolts and hardware when servicing or replacing a leaf spring on an existing axle. Dexter Product Bulletin 505 changed light duty U-bolt nuts to serrated flange nuts with higher torque values. See Build a Complete Dexter Trailer Axle for the torque table, or shop U-Bolt Kits.

Wheel offset

The relationship of the center line of the tire to the hub face. Dexter Product Bulletin 404 recommends wheels with zero offset, or within 1/2" positive or negative, on Sprung and Torflex axles up through 15,000 lbs. Where it matters: wheels outside that range can unevenly load the bearings, decrease bearing life and potentially cause a failure. Use trailer wheels, not automotive wheels. Nev-R-Lube hubs have their own limits.

Frequently asked questions

What is the difference between hub face and spring center?

Hub face is measured across the hubs, where the wheels mount. Spring center is measured between the centers of the two spring seats. The difference between them is the overhang.

Is a BT8 spindle the same as a BT9?

No. A BT8 spindle uses L44643 bearings and a 1.25" seal. A BT9 spindle uses L44649 bearings and a 1.50" seal. Check the spindle before you order bearings, seals or hubs.

What does pre-wire mean on a Dexter axle?

The electric brake wire runs through the axle tube. Dexter calls it inner wiring.

Does every Dexter axle have a brake flange?

No. Custom Dexter beams of 3,500 lb. and up always do. The stock 2,000 lb. beam does not, and the custom 2,000 lb. beam offers it as an option.

What wheel offset does Dexter recommend?

Zero offset, or within 1/2" positive or negative, per Dexter Product Bulletin 404. Nev-R-Lube hubs have their own limits.

How many threads should show past the wheel nut?

In general, about three threads, per Dexter Product Bulletin 409. The exact amount varies with wheel and nut thickness.

Where is the label on a Dexter axle?

Per Dexter Product Bulletin 422, newer labels are on the rear of the axle tube, toward the frame mount.

What grease does Dexter specify?

A lithium complex, NLGI No. 2 grease with a dropping point of at least 215 C (419 F). See Dexter #2 Lithium Complex Trailer Grease (088-071-00) and Timken grease in a 14 oz. cartridge or 1 lb. tub.

Ready to measure? See How to Measure for a Custom Dexter Trailer Axle.

Building the whole axle? See Build a Complete Dexter Trailer Axle.

Southwest Wheel - Your Trailers Best Friend Since 1919.

References

• Dexter 2,000 - 8,000 lbs. Axle Service Manual, page 49 (Hubs/Drums/Bearings: standard bearings, E-Z Lube, Nev-R-Lube), page 50 (Hub Removal: E-Z Lube retainer after February 2002), pages 52 to 54 (Bearing Lubrication; Recommended Wheel Bearing Lubrication Specifications), page 55 (Seal Inspection and Replacement), page 57 (E-Z Lube Lubrication), page 64 (Suspension Systems: double eye leaf springs, U-bolts, tie plates, equalizer).
• Dexter Light Duty Axle Catalog LIT-122-00, page 18 (D20 Leaf Spring: 1-3/4" tube for straight axles, 2-3/8" for drops), page 25 (3,500 lb. tube and overhang), page 64 (inner wiring system).
• Dexter Product Bulletin 404, Wheel Offset Recommendations, March 26, 2019, page 1.
• Dexter Product Bulletin 409, Wheel Attachment and Torque Requirements, October 14, 2019, pages 1 to 3 (Wheel Selection; Torque Requirements; thread engagement).
• Dexter Product Bulletin 422, Axle Label Improvement, May 29, 2020, page 1 (Location).
• Dexter Product Bulletin 474, Spindle Nut Attachment, February 2, 2023, page 1.
• Dexter Product Bulletin 488, U-Bolt Thread Change on 3.50" Diameter Axle Tubing, December 22, 2023, page 1.
• Dexter Product Bulletin 505, Serrated U-Bolt Flange Nuts, November 15, 2024, page 1.
• Trailer Axle Terminology, page 1 (Hub face, Spring center, Overhang), page 4 (Spindle), page 5 (Idler and brake).