Posted by Butch on 02/07/2013

In 2001 4-Star Trailers began bonding door skins using the same structural adhesive used in over 10 million cars and trucks built by DaimlerChrysler, General Motors, Ford, Volvo, Nissan, Mazda and Mitsubishi over the last 15 years. We chose adhesives formulated by the Lord Corporation after reviewing the motives of these large automotive corporations for choosing them for bonding roof and door panels. Attributes such as impact resistance, vibration and noise dampening as well as superior strength were among the reasons sited by these manufacturers for the use of adhesives. Attributes we realized are important to trailer owners and their cargo.

Testing a bonded door using strain gage to measure force.

We take a lot of pride in what we build.  After years of our own testing and developing fixtures, we found that an adhesive bonded door was 4 times stronger than a welded door.  We believed, as did many of our competitors, that 4-Star's welded doors were the class of the industry.  However, compared to a welded door, 4-Star's bonded doors provide added security and peace of mind.       

You might imagine that a company that has been welding all aluminum trailers for nearly 20 years would be skeptical about ‘gluing’ something together. You’d be right.  But after rigorous testing, we discovered bonding was stronger, cleaner, and more durable than welding.  In our tests we took examples of each door type and froze them, baked them, dropped them, pulled them and hammered them. In every test the bonded doors outperformed the welded, taped, and riveted door.

The bonded doors turned out to be solid compared to a welded door. It does not flex as easily. This is a good thing when the security of your cargo is dependent upon the strength of your door. It’s a bad thing when the frame around the door opening is the slightest bit out of square or out of plumb. (Plumb means to be flat, straight, or to have all 4 corners of the opening lying on the same plane.)

There are several causes for a door opening in the side of a trailer to become out of plumb. One of them is heat distortion caused by welding. Welding generates a lot of heat. The side posts of a trailer act like bacon in a frying pan when heat is applied to one side. They tend to bend or curl around the weld. There are several techniques that are employed to minimize heat distortion in welded assemblies, but it cannot be completely eliminated.

If the door is plumb, which all bonded doors are, and the opening into which it is hung is out of plumb, then leakage can occur. Welded doors are flexible and can be warped to match the plumb of an opening. A bonded door cannot be warped out of plumb due to its superior stiffness. This inability to ‘bend’ a door to seal it into the opening and the difficulties experienced in attempting to provide an accurate opening led us to rethink the methods used to hang a door.

Our process for attaching doors to the trailer involves pre-assembling the door to a door jamb and then welding the pre-hung door assembly to the frame of the trailer. This design maintains a uniform gap between the door and its sealing surface all around the door allowing the rubber seal to have equal compression with no gaps contributing too much better water resistance than our previous designs.  Even though the door seals to the  jamb, a trailer is a dynamic assembly subject to flexing as it is towed. Making a truly water tight door would require using economically unfeasible mechanisms such as those used to seal bulkhead doors on ships. Therefore, our door jambs are designed to channel any water that does happen to make its way past the seal away from the interior of the trailer and back out to where damage to the trailer’s interior is avoided.

We also designed the hinges to be an integral part of the door and jamb. The hinge assembly utilizes a tab on the hinge butt which is mechanically locked into a groove in the door and the door jamb. This mechanical interlock combines the strength of the hinge butt with the strength of the door and jamb to achieve exceptional strength without the bulkiness of previous designs.

The hinges on nearly all horse trailers are of the strap type. Strap type hinges are mounted like a strap across the face of the door and onto the trailer. This puts the direction of forces against the door perpendicular to the hinges. In technical terms, the hinge leaf is subjected to shear forces. The leafs of strap style hinges must be very large and thick to withstand the forces applied to them. Large hinges, however, can pose a danger to horses as they protrude a significant distance out from the side of the trailer. Strap style hinges are used because they are readily available and are relatively easy to install.

Our hinges are oriented so that they are parallel with the direction of force. In technical terms, the hinge leaf is subjected to tensile forces. The differences between an object said to be in shear vs. in tensile can be compared with attempting to break a Popsicle stick in two. If one were to grasp one end of the stick in each hand and bend it until it breaks - it is broken by shear forces. Grasping one end in each hand and pulling in opposite directions until it breaks is using tensile forces. If you examine the hinges on the doors of your home you’ll see that each of them is oriented so that any force applied to the door applies tensile force to the hinge leaf - not shear. We feel that this design lends itself very effectively to horse trailers, particularly the sides where horses may be tethered.