Rear Frame

Posted by Butch on 02/07/2013

Rear corner at top of trailer

T.I.G. weld at exterior of the rear corner. (Compare with M.I.G. weld on the interior)

 

The top corners of a trailer at the front and rear are important to consider when contemplating which designs have high durability. The corners at the rear of the trailer are critical to the torsional stability of the trailer. If the rear corners are not sufficiently rigid the rear frame of the trailer can sway back and forth in a motion known to the industry as 'racking'. When not controlled, racking can lead to metal fatigue and failure. The result of excessive racking is cracking in the corners of the rear frame and abnormal wear to the rear door hardware.

Rear Header

 Rear header contoured for a perfect fit with the trailer's top rail.

 

4-Star incorporates several design features to control racking. The rear header is contoured to exactly match the profile of the top rail. The header fits snugly into the radius of the top rail, similar to a jig saw puzzle piece, limiting its range of motion. The rear corner post is notched so that the joint formed where it meets the rear header lies on two different planes at right angles to one another. Notching it this way redirects the lines of force away from the joint minimizing the likelihood of the joint cracking. The three elements forming the rear corners (top rail, header, corner post) are buttressed with two large gussets, one from top rail to rear header and one from rear header to rear corner post.

 

Inside view of rear corner gussets

Inside view of the rear corner showing the large, 3/16" thick gussets used to stabilize the joint.

 

Racking starts down at floor level. A little flex down here can translate into a lot of movement up top. We reinforce our corner posts with large tail light gussets. On trailers with widths that do not allow for tail light gussets we extend the corner posts down through the floor so that the rear threshold and bottom rail are lapped over it to provide increased rigidity. The size of the rear corner posts and rear header beams have a tremendous influence on the potential of the rear frame to rack. Generally, the wider the members, the more resistance to racking the structure will be. Our rear headers and corner posts are ‘L’ shaped with dimensions of 4 inches wide in the rear and 6 inches wide on the sides.

 

Rear view of trailer showing tail light gussets.

Tail light gussets add to the stability of the rear frame.

 

Many manufacturers use a simple miter or butt joint to form their corners. Be sure to examine our corners and compare them to other manufacturer’s designs. We think you'll agree with us that ours is the superior design