It is the number of axles that determine whether the trailer is a single or a double axle variant. In a single axle trailer, there is a sole set of wheels. Whereas in double, the set of wheels are twice the set. The count of wheels makes it easy to determine a trailer type - https://goo.gl/n39pgR
Good quality trailer axles provide a smooth trailering experience at the full load condition. They maintain the load offer long-lasting performance.
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