How to Measure a Trailer Axle

You measure a trailer axle in two places: hub face to hub face, and spring center to spring center. Hub face to hub face is the total width across the outer faces of the hubs. Spring center to spring center is the distance between the middles of the two spring seats. Order a replacement with both figures and the capacity rating, because getting either wrong gives you an axle that physically will not fit.

The two measurements

Hub face to hub face. Measure across the trailer from the outer machined face of one hub to the outer machined face of the other. Do not measure to the tire or the wheel rim.

Spring center to spring center. Measure from the center of one spring seat to the center of the other. This determines where the spring perches must be welded and it is the figure most often mismeasured.

Capacity

Stamped on the axle tube or on a tag. Common ratings are 3,500, 5,200, 6,000 and 7,000 lb. Never fit an axle rated lower than the original, because the trailer's GVWR was certified against that rating and dropping it makes the plate on the tongue untrue.

Brake flange

If the axle carries brakes, note the backing plate bolt pattern and the drum diameter, commonly 10 inch or 12 inch. An axle that fits dimensionally but takes different brakes still means buying new brake assemblies.

Bolt pattern

Count the studs and measure the circle. 5 on 4.5 and 6 on 5.5 are the usual patterns on light trailers. This has to match your wheels or you are buying wheels too.