Things You'll Need
If you have a trailer and you are not sure about the axle size, you can measure this yourself with just a few supplies and a helper. Knowing the axle size of your trailer is very important, in that you must know the maximum capacity of each axle so that you don't overload the trailer.
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Jack the trailer and take both the tires off of the axle with a lug wrench.
Measure the "track width." Measure from the flat side of the wheel hub all the way to the flat of the wheel hub on the other side. You will need a partner to help you measure. This distance is the track width.
Measure the "spring distance." Hold the tape measure in the center of the spring mounting pad and ask your assistant to do the same. Measure the distance across the two points. This is your spring distance.
Measure the "axle drop." Take a measurement that runs from the spindle center line and goes to the center line of the axle tube. This is what's known as the axle drop.
Purchase a replacement axle. Using the three axle dimensions, coupled with the maximum weight capacity for the vehicle that's towing the trailer, you can find a replacement axle that fits your trailer and the capacity of your vehicle.