Truck body extrusions and the EN 12642 load path

Short answer. A truck body is a load-bearing structure, not a skin. EN 12642 asks the side wall to hold a fraction of the payload — 0.4 × P on a Code XL body, 0.3 × P on Code L — and that force has to travel from the panel through the posts, into the floor cross members and down to the chassis. Aluminium extrusions decide whether that path stays continuous. We extrude side rails, posts, floor planks, roof bows, corner sections and trim in 6063, 6061, 6463 and 6082 as part of our transportation aluminium profiles range, with 6005A and other special alloys run on request, and machining, anodizing and powder coating in the same plant in Foshan.

What EN 12642 asks the body to do

EN 12642:2016 defines two body classes. Code L is the ordinary body. Code XL is the reinforced body that can restrain part of the load without lashings, provided the load is placed positive-fit.

P is the permissible payload, and the force is calculated as the fraction times P times 9.81.

Two points are worth stating plainly, because they are where enquiries go wrong.

First, a curtain is weather protection. On a Code L curtainsider the side wall contributes almost nothing, so the cargo still has to be lashed to EN 12195-1: 0.8 g forward, 0.5 g sideways and rearward. A body only counts towards restraint when it is certified to Code XL and the load is loaded gap-free.

Second, the numbers in the table and the numbers used in enforcement are not identical. The standard sets the Code XL side wall at 0.4 × P; enforcement guidance in several European markets credits an XL body with restraining 50% of the rated payload to the side when the load fills the bay. If your customer is specifying to the standard, design to 0.4 × P. If they are specifying to what a driver will be stopped for, ask which document they are working from.

Positive fit itself has tolerances: within 30 cm of the front, within 30 cm of the rear, within 8 cm of either side, and no single gap over 30 cm along the bed. That is a packaging and loading rule, but it is also a geometry rule for the body builder — the internal width and the post pitch have to let the load sit inside those gaps.

Where the force goes

A side wall test is passed by a load path, not by a panel. Follow the force and the failure modes sort themselves out.

The joint at the bottom of the post is the one to look at first. A weld that passes a static proof load can still carry a stress concentration that grows a crack at 200,000 km. If the design puts the post load into a thin rail wall, no alloy choice saves it — the section has to be thick enough at the joint, or the joint has to be a mechanical connection with a gusset.

Alloy: what the section is made of, and what the weld does to it

The structural members on a welded aluminium body are normally 6005A-T6. It extrudes well in the shapes a body needs, and it holds more strength after welding than 6063 does, which is why it is the usual choice for posts, rails and frame members that are welded rather than bolted.

6063-T5 or T6 belongs on the parts that are not in the load path: trim, capping, edging, cable duct, door track, and the sections that exist to close a gap or hold a seal. It anodizes evenly, which matters on visible parts, and it is cheaper per metre.

6082-T6 is for machined brackets, hinge blocks and parts where the load goes through a bolt hole rather than a weld.

One thing to be clear about: welding does not restore the parent temper. In the 6xxx family the heat-affected zone beside a weld is softer than the parent metal, and no filler wire changes that. A body designed as if the weld were T6 will crack in the heat-affected zone, usually after the warranty period. The practical answer is to keep welds out of the highest-stress region, thicken the section locally, and use a 5xxx filler wire matched to the parent — then let the joint carry what it can actually carry.

If your drawing calls for 6005A or another special alloy, we run it on request. A special alloy carries a 20-tonne minimum, because the billet has to be made for the order.

Floor: the part that gets abused

Floor planks fail from point load, not from uniform load. A forklift wheel puts a few square centimetres of contact under a tonne or more, and that load lands between two cross members.

Three things decide whether a floor survives:

• Span between cross members. The shorter the span, the more load the plank takes before the top face yields.
• Chamber depth and wall thickness. A hollow plank with deep chambers resists local crush; a thin-walled decorative plank does not, whatever the alloy.
• Wear surface. Trailer floors get dragged, not just loaded. Hard-alloy wear strips at the top face, or a plank extruded with a thicker top wall, extend life far more than a thicker bottom wall.

Interlocking tongues between planks are worth specifying: they stop plank-to-plank movement and spread a wheel load across two planks instead of one. The cost is a more complex die and a slower assembly — the benefit is a floor that does not open up at the seams.

Aluminium on a steel chassis: the corrosion you cannot see

Every aluminium body sits on a steel frame, and the joint between them is where corrosion starts. Two different metals plus moisture plus a trapped crevice is a galvanic cell, and the aluminium becomes the anode.

What actually works, in order of effect:

1. Break the electrical contact. Isolation tape or a coated shim between the aluminium and the steel, and sleeves or washers on the fasteners.
2. Do not trap water. Drain holes at the low point of every closed joint, and no horizontal ledge where dirt and water sit.
3. Keep the joint dry by design. A joint that can be washed and drained lasts; a lap joint sealed on three sides and open on one collects salt.
4. Use stainless fasteners with isolation. A plain steel bolt through an aluminium rail is a corrosion cell with a fastener thread in it.

Anodizing and powder coating help on the faces. They do not protect a cut edge, a drilled hole or a weld, so the design has to assume the coating stops at the joint.

Length: extrusion is not the constraint

This is the part of a profile enquiry that gets answered wrongly most often.

The press can push a long section. What limits the length we can supply as one piece is what happens after extrusion: a profile has to be aged, and a profile that does not fit the ageing oven cannot be fully aged in one piece. Cutting it in half to fit the oven and welding it back is not the same product.

So the honest statement is: the length we quote depends on the die and on whether the finished length can be processed as one piece. Bring the finished length into the enquiry rather than assuming a maximum — a body builder who needs a 7 m rail and one who needs a 5 m rail are asking two different questions, and only one of them may be a single-piece answer.

Two other length numbers do matter commercially, and they are different from the process limit:

• Container module. A 40 ft container takes two 6.0 m lengths; a 20 ft container takes 5.85 m. Above that, the profile ships in sections or not at all.
• Cut length tolerance. Saw accuracy and bundle packing decide whether the profile arrives straight enough to assemble without re-cutting on site.

What we can and cannot do

Sections can leave the plant cut, CNC machined and finished, and the shape of a quotation — what is charged per kilo and what is charged per operation — is set out in why aluminium extrusion quotes differ.

That last line is worth repeating to your own customer. EN 12642 applies to the complete body, tested as a body. An extruder supplies sections, tolerances and material certificates; the body builder or the converter holds the certification. A supplier who tells you otherwise is either confused or selling you something.

### What to put in the enquiry

A profile enquiry that can be quoted in one pass carries:

• the section drawing, with the alloy and temper
• the finished length, and whether it may be supplied in more than one piece
• the quantity in metres and in kilograms, so the mill can compare against the die
• which faces are visible, and which finish those faces need
• whether the part is welded, bolted or both, and where
• the load case, if the part is structural — the number the section has to carry, not a competitor's section number
• the destination port, so the packing and the shipping module are quoted together

Questions buyers ask about truck body extrusions

### Is 6063 strong enough for a truck body side rail?

For trim and capping, yes. For a welded structural rail in a Code XL body, no — 6005A-T6 is the usual choice because it keeps more strength next to the weld. If the rail is bolted rather than welded, 6063-T6 can work in lower-stress positions, but the section has to be sized for the load rather than copied from a welded design.

### Can you supply a body that is certified to EN 12642 XL?

No, and no extruder can. The standard certifies a complete body after dynamic testing. What we supply is the sections, the alloy and temper, the tolerances and the mill certificates, so that your body can be built and tested to the standard.

### Why does my aluminium floor crack between the cross members?

Because the span is too long for the wheel load, or the plank's top wall is too thin for the wear it takes. Point load from a forklift is concentrated and repeated; a plank sized for uniform load will pass a static check and still fail in service.

### Do you anodize truck body profiles?

We anodize to AA10 or AA15 in-house, and powder coat in the same plant. Anodizing is usually specified for visible trim and door track; structural rails are often mill finish, because the coating does not survive the weld and the cut ends anyway.

### What is the minimum order?

500 kg per profile on standard alloys. A special alloy such as 6005A carries a 20-tonne minimum, because the billet is made for the order. Profiles ship FOB Shenzhen or Guangzhou, cut to your list.