At our shop in Bend, Oregon we replacing trailer axles on a regular basis. Getting the right axle for your trailer requires understanding how to correctly determine the following details:
Axle Length or "Hub Face" is the measurement from the base of the wheel stud on face of one hub to the base of the wheel stud on face of the other hub. This is the same for Torsion and Spring Axles. The distance between the Spring Centers or Torsion Brackets to the Hub Face will vary depending on the wheels and tires that you intend to use.
Spring Centers: When hanging the axle on the spring, the spring brackets have a hole in them that will fit over the bolt that holds the springs together. Spring Centers are the CL (centerline) measurement of the spring mounting pads on the axle. The spring center, if it is mounted to the frame of the trailer built from 2" channel or tube, can be measured from outside of the frame on one side to the inside of the frame on the other (assuming that your spring mounting pads are welded to the center of the frame structure).
Spindle Type: If building a trailer with a low deck height is the goal then a drop spindle is the way to go. Motorcycle trailers and car haulers often use these spindles to keep the ramp approach angle at a minimum.
Hub Pattern: When messuring your hub pattern you need to measure the diameter of the circle of the bolt pattern. On a 5 bolt trailer this is located between the two bolts opposite of the first bolt from which you are starting. If there is a specific tire and wheel combination that you would like to use, select it from the available options.
Hub Stud Size: 8 bolt hubs and larger have various different stud sizes depending on the capacity. Smaller hubs use 1/2".
Idler or Brake: Idler means no brakes. The most common brake on 3.5k trailers is Electric Manual Adjust.
Tube Options: 3.5K and 7K trailers usually 2-3/8" round tube painted steel axles, however boat trailers often use square tube galvanized or rust resistant axles.
Greese Packed is the standard on many axles and the default when ordering. These are maintained by dissasembling the hub and hand packing the cleaned bearings with greese.
E-Z-Lube is common on many Dexter axles, if, after you remove the grease cap, you still have a greese zert that screws directly into the spindle you have an E-Z-Lube axle. (Accu-Lube is a similar and used by Al-Ko/Hayes Axles).
Nev-R-Lube greeseless bearing cartridge.
Spring Position: We need to know the spring position so that we can determine which side of the axle to weld the spring hangers. Since most axles have a camber (a bend), putting the axle on the wrong side of the spring reduces capacity and will wear tires quickly. When installing an axle with camber the camber needs to be up. When the trailer is under a load the axles will flex and "appear to straighten".
All prices are in USD