Drop Axle vs Straight Axle Trailer: Deck Height Guide

Two choices on a leaf spring trailer axle change how high the trailer frame and deck sit: a straight or drop axle beam, and overslung or underslung springs. This guide covers what each one is, what it does to frame and floor height, and which Dexter beams Southwest Wheel offers in each style.

Once you have picked a style, measure for the beam with How to Measure for a Custom Dexter Trailer Axle.

Key takeaway

• Drop axle: The spindle center sits below the axle tube. Drop spindles lower the trailer floor.
• Underslung springs: Lower the frame. Overslung springs raise it.
• Availability: A 4" drop is offered on most capacities. A 6" drop is offered only on the 3,500 lb. custom beam. The 4,400 lb. beam is straight only.
• Check the overhang: A drop beam has its own overhang range, so check your numbers again before you switch.

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 axle tube.

The drop size is measured from the center of the spindle to the center of the beam. On a 4" drop axle, the spindle center is 4" below the center of the beam. On a 6" drop, it is 6" below. With the same springs, hangers and tires, the frame sits about 4" lower on a 4" drop axle than on a straight axle.

Drop spindles lower the trailer floor. Southwest Wheel's 4" drop beam pages describe the drop for trailers that need a lower ride height.

The sources do not print a single deck-height number for every trailer. The final height also depends on the springs, the hangers and the wheels and tires. If you need a specific deck height, call 1-800-866-3336 before you order.

Overslung or underslung

Overslung puts the spring seat on top of the axle tube. Underslung puts the spring seat under the axle tube.

Underslung springs lower the frame, and overslung springs raise it. Underslung springs on a straight spindle also lower the floor, so a drop spindle is not the only way to bring the deck down.

On a straight axle, it does not matter which side of the tube the spring seats are welded on. The axle can be rotated to put the seats on top (overslung) or underneath (underslung). Where the seats end up still changes the ride height. On a cambered beam, the order form asks whether the seats are welded over or under the axle.

Leaf springs need room to move. Hangers that are too short can let the axle hit the frame, and if the equalizer hangers are too short, the spring eye can hit the frame. Hangers that are too long need bump stops.

What each choice does to frame and floor height

Which beams come in a drop

Southwest Wheel has stock Dexter beams and Custom Built Dexter Trailer Axles. Not every capacity comes in every style:

Stock 4" drop beams:

• 3,500 lb. 4" drop beam
• 5,200 lb. 4" drop beam
• 7,000 lb. 4" drop beam
• 8,000 lb. 4" drop beam

Custom built drop beams:

• 2,000 lb.: 2,000 lb. 4" drop custom beam
• 3,500 lb.: 3,500 lb. 4" drop custom beam, 3,500 lb. 6" drop custom beam
• 5,200 lb.: 5,200 lb. 4" drop custom beam
• 6,000 lb.: 6,000 lb. 4" drop custom beam
• 7,000 lb.: 7,000 lb. 4" drop custom beam
• 7,200 lb.: 7,200 lb. 4" drop custom beam
• 8,000 lb.: 8,000 lb. 4" drop custom beam

Switching from straight to drop

A drop beam is not a drop-in swap for the straight beam of the same capacity. Each drop beam has its own overhang range, and its minimum is higher than the straight beam's. For example, the stock 3,500 lb. straight beam page prints 14" to 17.5", and the stock 3,500 lb. 4" drop beam page prints 15" to 18.5".

The hub face changes too. If you measured the hub face on a straight axle and are switching to a 4" drop of the same capacity, add 2" to the measured hub face. For example, an 89" hub face on a 5,200 lb. straight axle becomes 91" on a 5,200 lb. 4" drop.

Total overhang is the hub face minus the spring center. Before you switch to a drop, work out your total overhang and compare it to the range printed on the drop beam's page. If it falls outside that range, do not guess a new hub face. Measure again or call 1-800-866-3336 before you order.

Camber and spring seats on custom beams

On a custom built beam, camber and spring seat position are one choice on the order form:

• Straight, seats ship loose
• Straight, seats welded on
• Cambered, seats welded over the axle (overslung)
• Cambered, seats welded under the axle (underslung)
• Cambered, seats ship loose

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

Lead time, returns and shipping on custom beams

• Lead time: Custom beams are built to order. Delivery is 2-3 weeks after the order.
• Returns: Once the order is placed, a custom axle cannot be altered, changed, cancelled, or returned for any reason.
• Shipping: Shipping is one charge for the entire order.

Straight or drop checklist

1. Capacity, from the label or your trailer's needs
2. Straight or drop, and whether your capacity comes in a drop
3. Switching from straight to a 4" drop? Add 2" to the measured hub face
4. Overslung or underslung springs
5. Hanger length, so the springs have room to move
6. Total overhang (hub face minus spring center), inside the range for the beam you pick
7. Stock beam or custom built beam

Need a specific deck height? Verify fitment before ordering or call 1-800-866-3336.

Frequently asked questions

What is the difference between a straight and a drop axle?

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, which lowers the trailer floor. The drop size, such as 4", is measured from the center of the spindle to the center of the beam.

Which is lower, overslung or underslung?

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

Can I get a 6" drop axle?

Only on the 3,500 lb. custom built beam.

Does the 4,400 lb. beam come in a drop?

No. The 4,400 lb. custom beam is straight only.

Is there a 2,000 lb. drop axle?

Yes, as a 4" drop custom built beam. The stock 2,000 lb. beam is straight only.

Does switching to a drop change my measurements?

Yes. For the same capacity, add 2" to your measured straight hub face. Drop beams also have their own overhang range, so check your total overhang against the drop beam's page, and call if it does not fit.

Can I change or return a custom axle?

No. Once the order is placed it cannot be altered, changed, cancelled, or returned.

Picked your style and need the capacity next? See How to Measure for a Custom Dexter Trailer Axle.

Need a drop? Pre-configured axles are straight only, so compare your options in pre-configured vs custom built Dexter axles.

Straight or drop, the beam ships alone. Finish it with Build a Complete Dexter Trailer Axle.

Hanger length and overslung or underslung mounting also change ride height. See Hanger Spacing for Single and Tandem Leaf Spring Axles.

Southwest Wheel - Your Trailers Best Friend Since 1919.

References

• Trailer Axle Terminology, page 3 (Straight axle, Drop axle, Overslung and underslung), page 4 (Camber).
• LIT-008-00, Dexter Axle applications guide, page 9 (Axle Types), page 10 (Specifying Axles).