Buyers Products Underbody Boxes: How to Choose The Right Set Up

An underbody box is the only truck storage that costs you nothing. No bed space, no sight lines, no added roof height, no weight sitting up high where it hurts handling. It hangs off the frame rail, below the bed, out of the way.

Which is exactly why people get the order wrong. A bed box is easy to shop — measure the bed, buy the box. An underbody box depends on your frame, your fuel tank, your exhaust routing, your spare tire, and your rear axle, and none of that is printed on a product page.

We carry Buyers Products underbody boxes in steel and aluminum, from 15 inches up to 72, in every common door and latch configuration. Here's the order we'd walk through choosing one.

Step 1: Measure the frame rail, not the truck

This is the step that causes most returns, and it takes ten minutes.

Get under the truck with a tape and find the continuous clear length of frame rail on the side you want the box, between the cab and the rear wheel — or behind the rear wheel, if you're mounting aft. Note everything in the way: fuel tank, DEF tank, exhaust, spare tire winch, air tanks, battery box.

Write down three numbers:

1. Available length along the rail, obstruction to obstruction
2. Available drop from the bottom of the rail to where you stop being comfortable with ground clearance
3. Available width outboard of the rail before the box sticks out past the body line

Then subtract. Your box has to fit that envelope plus bracket clearance.

On ground clearance: a box hanging lower than the frame's lowest existing point will find every driveway apron, curb, and job-site rut you own. If you work off pavement, go longer and shallower rather than deep.

Not sure what you're looking at down there? Send us a photo and a measurement and we'll tell you what fits.

Step 2: Steel or aluminum

Buyers Products underbody boxes come in two materials, and the right one depends on where you work rather than which is objectively better.

Steel — heavier, and that weight hangs low, which is fine for stability. Takes a hit and dents instead of tearing. The powder coat is the weak point: once it chips, road salt goes to work. Lower cost per cubic inch.

Diamond tread aluminum — meaningfully lighter, which is real payload back on a service truck already running heavy. Doesn't rust, which matters a lot on a box mounted right in the salt spray. Softer, so it dents more easily. Costs more per cubic inch.

For most northern-climate service trucks, we'd point you at aluminum. An underbody box lives in about the worst environment there is for coated steel — constant road spray, constant moisture, and you never see the underside to catch a chip early.

Buyers builds its diamond tread boxes from .100-inch aluminum, and the steel boxes from 14-gauge steel with a powder-coated finish.

Step 3: Pick your size

Buyers Products underbody boxes are organized by depth and height. Here are the options in each family.

Compact boxes — 12" to 16" tall

For tight frames, behind-the-axle mounting, and trailers. Straps, chains, recovery gear, hand tools.

The 18 x 18 workhorse series

This is the size most full-size trucks end up with. Good capacity without hanging low, and the most popular family of Buyers Products underbody boxes we ship.

The 48-inch is the one to start with if you're not sure. It fits most full-size long beds with clear rail, and it's the size we move most.

Deep boxes — 24 x 24

For flatbeds, service bodies, and anyone who needs real volume rather than just length.

Check your ground clearance carefully on these. A 24-inch-deep box hangs noticeably lower than an 18.

Diamond tread aluminum

Lighter and rust-free. The right call in salt states and on trucks already running heavy.

Compare the 24 x 14 x 12: steel at $329.95, aluminum at $399.95. Seventy dollars for rust immunity and a lighter box is usually money well spent if you're running winter roads.

Step 4: Door and latch style

Single drop-down door with a T-handle covers the large majority of the range above. Simple, fast, and the door doubles as a small work shelf. Good up to about 48 inches.

Double barn doors appear on the Pro Series smooth aluminum boxes with stainless steel doors. On anything 60 inches or longer, a drop-down door means bending over a five-foot horizontal panel to reach the back — barn doors take that away and let you reach straight in. They're also better in tight parking, where a drop-down door swings into the next vehicle.

Worth knowing: the Pro Series barn-door boxes are a significant step up in price, and availability moves around. Check current stock on the Pro Series or call us and we'll tell you what's actually on the shelf.

On latches generally: a single-point T-handle is adequate for most uses, but on a 48-inch door the far ends flex under a pry bar. If you're storing anything worth taking, a multi-point latch is the better buy — same logic as a deadbolt versus a doorknob lock.

Step 5: Don't forget the brackets

Buyers Products underbody boxes are almost never sold with mounting hardware. This is the most common surprise on an order, and we'd rather flag it now than have you waiting on a bracket while the box sits in your shop.

Buyers sells welded structural steel mounting brackets separately, sized to match box depth. You'll usually need a pair per box, and depending on the chassis you may need to drill the frame or use existing holes.

Shop Truck Box Mounting Hardware

Frequently asked questions

Do Buyers Products underbody boxes come with mounting brackets?
Generally no — brackets are sold separately and sized to box depth. Check the product page and plan for the hardware before you order.

Steel or aluminum for an underbody box?
Aluminum if you're in a salt state or watching payload, since these boxes live in constant road spray. Steel if impact resistance or budget matters more. On matched sizes the aluminum premium is usually modest.

What size Buyers Products underbody box should I get?
Measure your clear frame rail first — that number decides it. The 48 x 18 x 18 is the most common choice for full-size long beds. Go to a 24-inch-deep box only if you have the ground clearance for it.

Are underbody boxes weatherproof?
Weather-resistant, not submersible. Gasket condition and latch type decide how dry they stay — multi-point latches seal much better than a single-point handle because they pull the door evenly against the gasket.

How much ground clearance do I need?
Keep the bottom of the box at or above the lowest existing point on your frame if you ever leave pavement. Measure the drop before ordering.

Can I mount one behind the rear axle?
Often yes, and it's common on flatbeds and service bodies. Watch your departure angle and make sure you're not blocking the spare tire winch or hitch access.

Got your frame measurement? Browse every size, or send us the number and we'll narrow it down for you.

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