Building a Trailer Axle: Building a Straight Beam Axle - Shim Method

NOTE - Building your own axles should only be undertaken by a skilled and proficient welder equipped with a heavy duty, industrial strength welding machine. Good weld penetration with no porosity or undercut is required to ensure the strength and integrity of the axle is not compromised. If you have any doubts about your skills, leave axle building to the professionals, poorly built and poorly welded axles can cause accidents and kill people.

Straight Beam Axle Setup - Shim Method

There are many ways of setting up spindles within axle tubes correctly, the following is a tried and tested method that almost anyone with minimal equipment can accomplish.

Getting the spindles perfectly parallel and true to each other is the difficult part of building your own axle. Getting it wrong will cause a multitude of problems, from poor trailer tracking behind the tow vehicle, premature bearing failure, excessive tire wear, blowouts and even broken axles.


If like most home workshops, you will be lacking a lathe with a 4 jaw chuck and head bore to handle a 2" x 2" piece of axle tube or have a bed length to cope with a full length trailer axle, so you will have to improvise to compensate.

If you are not using machined line pipe for your axle tubes, you will need the following equipment –

• Vernier calipers with internal jaws

• A new tape measure (no wear/sloppiness and easier to read)

• Selection of shim steel including some various sizes of panel steel 0.5 to 1.0mm thick

• Tin snips/shears and scissors (don’t use your wifes ones!)

• Soft face hammer (a shot filled hammer is ideal)

• Set of flat files

• Square

• Flat straight work surface

Ideally you want to get a cheap set of bearings to match the good ones in you new hubs. Using a brass or mild steel drift, carefully remove the good bearing cups from the hubs (put them away somewhere nice and clean) and fit the cheapies. You will need the hubs to be dry fitted (no grease) to the spindles later on to double check measurements and it is preferable not to do any welding or dusty work in and around your good bearings.

A note about welding axles - to prevent damage to the hub, bearings and spindle, earthing the axle must be done through the axle tube only. If necessary, tack weld a tab or bolt to the axle tube to allow easy attachment of the earth clamp.

rhs-seam-weld.pngAn issue with using ERW/welded pipe as axle tubing is the seam weld internally running the length of the pipe. You can work around this if the seam is of a consistent depth or offset from the centerline of the section (SHS only), but the best thing is to get rid of it. If you are able to access one, beg, borrow or buy an electric file (like a belt sander in miniature). An electric file will considerably reduce the time, frustration and damaged fingers from doing the job manually.

The second best option is to tack weld an 8" long , 2nd cut or bastard file onto the end of a length of 1" x 1" hollow section or similar. This will give the file extra leverage and cutting power and assists in getting deep inside the axle tube.

Clamp the axle tube to whatever will hold it steady and with the seam at the bottom, pass the modified file over the seam and file it flat. You will only need to file around 6 - 8" of the seam at each end of the axle tube, but take care that you do not file past the seam especially at the outer edge of the tube. Here there is a tendency to taper off at the opening.

Axle tube length for straight beam axles is pretty straight forward, if the spindle has a hydraulic caliper yoke or drum brake backing plate mount, the axle tube can butt up against the flat surface of these and welded around. This doesn’t work in all situations, and you may need to set the axle tube 3/4" or more back from the yoke/plate to get good spindle alignment and weld penetration.

If a non braking spindle is being fitted, the axle tube should not be closer than 1" from the seal shoulder (the last bit of machined surface).

With the hub dry fitted (with the cheap bearings), measure from the hub face back to the 1" mark on the spindle. Double this measurement and subtract it from the original hub to hub face measurement. This is the axle tube length.


Once the axle tube has been cut to length, remove any burrs inside and outside and if required, file the internal seam flat.

plug-hole-position-us.pngDrill a couple of plug holes in the axle tube at least 1/2"diameter, preferably larger so that the spindle can be secured at the back end with some good welds.

With the Vernier calipers, measure across the inside flats of the axle tube and the diameter of the spindle. Find the difference between the two (the total gap) and divide by 2.

Cut up 4 sets of shims, for each spindle, to the thickness of the ½ gap measurement. If using a mix of thick and thin shims, try to slip the thin sections inside a sandwich of thicker shims and use a thin layer of grease/oil to help keep the shims together. The shims should ideally be as long as the internal section of spindle.

Slide the shims and spindle into the axle tube to assist the stub axle to centralize. With round axle tube, set the shims at either quarters or thirds around the spindle. On the square axle tube, center the shims on the flats where the stub axle will contact.


The shims and spindle should slide nicely into the axle tube and may require a couple of shim adjustments to get right.

Once the spindle is at the correct position in the axle tube, do a couple of checks to ensure that the hub face to hub face measurement is correct and that the axle tube is central to the hubs.

Tack weld the spindles in position and start taking 3 or 4 point measurements from hub face to hub face around the hubs. This measurement is critical to getting the hubs exactly parallel to each other. If adjustment is needed, use the soft face hammer to tap the hubs until all measurements are exact.


Lay another tack weld on each spindle, check your 3 or 4 point measurement and repeat until the spindle has at least 3 or 4 good tack welds.

Tack weld through the plug weld holes to secure the back end of the spindle.

Set your welder so that you can lay a good hot penetrating weld (practice on some scrap steel) and fully weld around the spindle and axle tube. Don’t worry too much about the shims, if you wish to remove them before welding do so, but you can leave them to become part of the axle.


Fill the plug weld holes up until the weld is flush with the top of the axle tube.

Let the axle cool down slowly – do not quench or spray with water! Doing so can cause hardening of the steel around the weld and creating a weak, fatigue prone, stress area.

Once cool, double check the mark at the center of the axle tube. Remark if necessary and use this to mark out and drill your spring center holes. Spring center holes need to be around 15mm diameter. As always, double check your measurements before drilling. In some cases, you may find yourself drilling into the back end of the spindle. Drill deep enough so that there is clearance for the spring bolt head to fit.

If fitting spring retaining plates, now is the time to weld these on. Welds on spring retaining plates should only be done axially with the axle tube and not across the face of the axle.

Tidy up any slag or welding spatter and file or grind any sharp edges.

Remove the hubs, knock out the cheap bearing cups, give the hubs a good clean and pack and fit the good bearings and seals to your new axles.

If you are painting the axle, use a good zinc rich etch primer coat and a couple of two pot enamel top coats to finish off.

If you are planning on galvanizing your axle, drill a 3/8" drain hole both ends of the axle tube, at the position where the back end of the stub axle sits. If your spring center holes line up with the end of the stub axle, this would be a bonus.

Click here to see another method of setting up and fitting stub axles - The Weld & Machine Method