Overslung vs Underslung and Camber: Spring Seat Options for a Custom Dexter Axle

Every custom built Dexter axle beam from Southwest Wheel has one required choice on the order form: Cambered and Spring Seat Options. That one choice sets two things. It decides whether the beam is cambered, and whether the spring seats are welded on, over or under the axle, or ship loose for you to weld.

This guide explains what overslung, underslung and camber mean, what each of the five form choices gets you, and how to pick one. If you have not taken your measurements yet, start with How to Measure for a Custom Dexter Trailer Axle.

Key takeaway

• Overslung: The spring seats are welded over the axle, so the springs sit on top of the tube. Overslung springs raise the frame.
• Underslung: The spring seats are welded under the axle, so the springs sit under the tube. Underslung springs lower the frame.
• Camber: A slight arch in the axle tube so the tires stand more upright when the trailer is loaded.
• Loose or welded seats: Enter a spring center and the seats are welded. Leave it blank and the seats ship loose.
• Built to order: A custom axle cannot be altered, changed, cancelled or returned once it is ordered, so decide before you order.

Terms you need

• Spring seat: The seat on the axle tube where the leaf spring mounts. Leaf springs are attached to the axle with U-bolts.
• Spring center: The distance from the center of one spring seat to the center of the other spring seat.
• Overslung: The spring seat is on top of the axle tube.
• Underslung: The spring seat is under the axle tube.
• Camber: A slight arch in the axle tube so the tires stand more upright when the trailer is loaded.
• Straight or drop: On a straight axle the spindle center is in line with the axle tube. On a drop axle the spindle center sits below the tube. This is a separate choice from camber.

What "seats welded over or under the axle" means

Leaf springs can be mounted either over or under the axle tube. Where the spring seats are welded decides which one you get:

Spring mounting is not the only thing that sets height. Spring hangers come in different heights for more control of the trailer height. Hangers that are too short can let the axle hit the frame as the suspension moves, and hangers that are too long need bump stops.

If you need a lower floor, you have two ways to get it: a drop spindle axle, or underslung springs on a straight spindle axle. See Drop Axle vs Straight Axle: Deck Height Guide to compare them.

What camber does

Camber is the angle of the wheel to the road. A cambered axle is built with a set bend in the tube that makes up for the expected bend under load. When the trailer is loaded, the tires stand more upright, and ideally the tire footprint meets the road evenly across the width of the tread.

On a custom beam, camber is your choice. Other Dexter axles at Southwest Wheel come one way:

• Stock axles by capacity, such as 3,500 lbs. Straight Dexter Trailer Axles, are straight (not cambered).
• Pre-Configured Dexter Trailer Axles are cambered and underslung.

The five camber and spring seat choices on the form

Every beam in Custom Built Dexter Trailer Axles, straight or drop, offers the same five choices:

The spring center goes in the Spring Seat Measurement field under the camber choice. Fill it in only if you chose welded seats, and keep it inside the range shown on that beam's form. If you leave it blank, the seats ship loose.

The no camber, seats welded on choice does not ask for over or under.

Loose vs welded spring seats

Welded seats are set at the spring center you enter, so the beam arrives ready for the springs. Pick this when you have measured the spring center and know whether you want overslung or underslung.

Loose seats come with the axle and need to be welded at your spring center measurement. Pick this when you want to set the spring center and the seat position yourself.

On a straight axle that is not cambered, it does not matter which side of the tube you weld loose seats on. The axle can be turned to put the seats on top (overslung) or underneath (underslung). Once the seats are welded, install the axle the way Dexter specifies: every Dexter axle is directional, so the front faces forward and the top faces up. Turning the axle applies only to a straight axle with loose seats. Cambered, drop and pre-configured axles go in top up.

Southwest Wheel does not weld the spring seats on stock beams. Here is how each type of Dexter axle ships:

How to choose

1. Set the height first. Want the frame lower? Choose underslung, or a drop beam. Want it higher? Choose overslung. Then pick a hanger height that leaves the axle room to move.
2. Decide on camber. Want the tires to stand more upright when the trailer is loaded? Choose a cambered beam.
3. Decide who welds the seats. Have a spring center that fits the range on your beam's form? Choose welded seats. Want to weld them yourself? Choose loose seats and leave the spring center blank.
4. Check for a stocked axle. If you want a cambered, underslung beam with welded seats and your hub face and spring center match a listed part, a pre-configured axle is a stock item with E-Z Lube spindles. Compare them in Custom Built vs Pre-Configured Dexter Axles: Which to Order.
5. Build it on the form. If you need camber, welded seats or E-Z Lube on a size that is not stocked, build it on the custom form.

Camber and spring seat checklist

1. Decide on frame height: overslung, underslung, straight or drop.
2. Choose cambered or not cambered.
3. Choose welded seats or seats that ship loose.
4. For welded seats, measure the spring center and check it against the range on your beam's form.
5. For loose seats, leave the spring center blank and plan to weld the seats at your spring center. On a straight axle that is not cambered, weld them on either side and turn the axle for overslung or underslung.
6. Once the seats are welded, install the axle with the front facing forward and the top facing up. Every Dexter axle is directional.
7. Keep the spring eyes and equalizers free to rotate.
8. Double-check every choice. A custom axle cannot be changed or returned after you order.

Verify fitment before ordering or contact Southwest Wheel for assistance. Call 1-800-866-3336.

Frequently asked questions

What does overslung mean on a trailer axle?

Overslung means the spring seats are on top of the axle tube, so the springs sit over the axle. On the custom form it is listed as seats welded over the axle.

Which is lower, overslung or underslung?

Underslung. Underslung springs lower the frame, and overslung springs raise it.

What does a cambered axle do?

A cambered axle has a slight arch in the tube so the tires stand more upright when the trailer is loaded.

Do I need a spring center if my seats ship loose?

No. Leave the spring center blank and the seats ship loose. Weld them at your spring center measurement.

Does Southwest Wheel weld the spring seats on stock axles?

No. On stock beams the seats ship loose and you weld them. Pre-configured axles have the seats welded, and a custom beam has welded or loose seats, chosen on the order form.

Does it matter which side I weld loose seats on a straight axle?

No. A straight axle that is not cambered can be turned to put the seats on top (overslung) or underneath (underslung). Once the seats are welded, install it front forward and top up. This does not apply to cambered, drop or pre-configured axles.

Can I order a beam that is not cambered with the seats welded?

Yes. Choose no camber with the spring seats welded on, and enter your spring center.

Are pre-configured Dexter axles overslung or underslung?

Underslung. They are cambered, with the spring seats welded under the axle.

Can I return a custom axle if I picked the wrong option?

No. A custom axle is built to order and cannot be altered, changed, cancelled or returned. Stock axles can be returned within 30 days.

Need the hub face and spring center first? See How to Measure for a Custom Dexter Trailer Axle.

Placing the hangers for your springs? See Hanger Spacing for Single and Tandem Leaf Spring Axles.

Picked your seats? Add the hubs, brakes and suspension with Build a Complete Dexter Trailer Axle.

Replacing an old axle? Record its label, hub face and spring center first with How to Identify Your Old Trailer Axle and Order a Dexter Replacement.

Southwest Wheel - Your Trailers Best Friend Since 1919.

References

• Trailer Axle Terminology, page 1 (Spring center), page 3 (Overslung and underslung, Straight axle, Drop axle), page 4 (Camber).
• LIT-008-00, Dexter Axle applications guide, page 3 (Definition of Terms: Camber), page 9 (Running Gear Systems: Leaf spring), page 10 (Specifying Axles), page 12 (Installation Suggestions).