The trailer's roof can be flat or arched. Generally the roof is secured and braced to the top rail and struts. The trailer's undercarriage contains the moving parts of the running gear and suspension. Wheels turn on axles, and springs suspend the body and absorb vibration and shock. Most of today's trailers replace springs with rubber torsion suspension, so the axles themselves absorb the shock from the wheels.
Rubber cushions the axles.
On a tandem-axle trailer, the two axles are each rated at 3,500 pounds. A trailer using a 5,000-pound coupler is rated to hold only 5,000 pounds. So the lower coupler rating overrides the 7,000-pound rating of the axles. Wheels are 15 to 17.5 inches, with radial tires. The length of axles affects wheel placement.