Although you may be tempted to think of trailers as simple, dumb sleds that follow along behind your truck, the fact is that trailers are vehicles in and of themselves. Trailers bend, flex and handle in at least as dynamic a sense as the tow vehicle pulling them, and they must do so while staying stable and predictable at all times. Axle specifications play an important roll in determining a trailer's load potential and handling characteristics.
Gross Axle Weight Rating (GAWR)
This is perhaps the single most relevant specification when shopping for trailer axles, and is defined by the lowest rated component in the system. In short, if your axle center beam will hold 8,000 pounds but the hubs are only good for 5,000 pounds, your gross axle weight rating cannot be higher than 5,000 pounds. Other factors included when determining the Gross Axle Weight Rating (GAWR) are the suspension system, rim and tire weight ratings.
No matter how strong your axle is, it will always bend a little bit under load. If the axle were completely straight when unloaded, it would push downward in the center when loaded. This would cause the tires to tilt inward at the top, gaining what is referred to as "negative camber." A little bit of negative camber will help your trailer to handle better, but it will wear your tires out very quickly. Most trailer axles bow upward in the middle so that, under load, the axle straightens out instead of bowing down. Camber is defined by the vertical surface of the tire's degree of variation from completely perpendicular to the road surface when not under load.
Overslung or Underslung
Overslung axles use a leaf spring mounted on top of the axle and underslung axels use a leaf spring mounted beneath. Mounting the leaf spring on top of the axle will give you several extra inches of ground clearance, but it will do so at the expense of handling.
Straight or Drop Spindle
A drop axle looks like a very wide letter "U." The wheel hubs attach to the upward tips of the "U" and the leaf springs attach to the bottom horizontal portion. A drop will get the trailer lower to the ground for better handling, but comes with even more of a ground clearance penalty than just using overslung springs.
This crucial measurement is the distance from the center line of one tire to the center line of the other on a single axle. In the case of dual wheels, the track width is taken from the exact center of each pair of wheels. In general, a wider track-width will yield more predictable handling and less trailer sway, but the increased width of the center beam will increase bowing under load and reduce ultimate load capacity.