The simplest and quickest way to build a dropped axle is to use pre fabricated dropper plates.
Dropper plates come in various heights and can be custom made to suit your specific requirements. They simply slide over both the spindle and axle tube, and it is just a matter of getting things straight and aligned and welding up. Solid round or square stock is recommended for the axle tube if you are able to access it. If not, 2-2 1/2" x 1/4" or heavier SHS is suitable for an axle rated to 3300 lb.
Most trailer part suppliers will have dropper plates in stock with standard drop center heights of 2 1/2", 4" & 6".
Dropper plates are commonly cut from 1" to 2" steel plate, but if you have access to a gas cutting plant or plasma cutter and have time on your side, you can make your own from 1/4" plate/flat bar and laminate multiple plates together for super strong dropper plates.
Cut the axle tube and spindle so that there is at least 3/8" sticking out the front and rear of the dropper plate to allow a good sized weld to be applied. Just make sure you have adequate back clearance between the dropper plate/spindle and the trailer/chassis.
Slide the dropper plates over the axle tube to the correct position and lay the axle on its side so that both the dropper plates longest edge is laying on a flat and level surface.
Double check the dropper plate positions, square the plates to the axle tube and tack weld together.
Dry fit the hubs to the stub axles using your cheap set of bearings and bolt up to the wheel rims.
Using a straight section of “U” or “C” channel longer than the overall width of the trailer, position the wheels opposing each other in roughly the correct position.
The wider the channel, the more accurate the contact will be (as long as the wheel doesn’t touch the inner face of the channel).
Slide the dropper plates and axle tube over the spindle and adjust the position of the rims to centralize the axle unit.
Working from one end, get the rim perfectly setup on the channel with 4 full, even points of contact.
Clamp if possible or sandbag the rim to prevent any movement. Adjust the dropper arm on the spindle to the exact position and tack weld together.
After the spindles have been tacked up, slowly spin the axle 360 degrees. The axle should spin freely without any movement or wobble from the rims. If any rim movement is noticed or there is any tightness when turning the spindle, there may be some misalignment. Double check your measurements, break the tack welds and adjust if required.
When everything is secured with good tack welds all around, it is ready for welding and you can unbolt the wheels and remove the hubs from the axle.
Turn up the welder and fully weld the spindle and axle tube to the dropper plate.
Never run the welder earth through the hubs/bearings or on the spindle. Always clamp the earth lead directly onto the axle tube - weld on a temporary lug if required.
Once welded, let the axle cool down slowly, do not quench or spray with water. The crystalline structure of the steel can be adversely affected by rapid cooling leading to potential weakening of the axle.
Once cool, double check position of the spring center holes, drill or weld spring plates as required.
Cut a square of flat bar and blank off the end of the axle tube before painting. If galvanizing the axle, predrill drain holes in the flat bar.
Remove any weld spatter and rough edges and thoroughly clean the spindle and hub before fitting the new seals and bearings.
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