U-bolting on a crossmember - help please!!

[73 Dolphin]

On my 1500# trailer the cross member is clamped to the main frame. The cross member is first drilled to accept a U-bolt. The U bolt goes over the main frame and thru the holes in the cross-member, and nuts and washers are installed and tightened.

On my 3900# Tandem trailer, the cross member have a U-shaped bracket welded to each end. This is thru bolted to the main frame. I would think you can get such brackets and bolt them to the cross member and then to the main frame.

The problem I had was that the crossmember was the same width as the frame exactly. If it had been 3 inches or so longer either side it would have clamped easily. The frame is not wide enough to accept a bolt as far as I can see.

 

[73 Dolphin]

I don't think it's structurally strong enough for the rear corner of the trailer. At a bare minimum, I would drill through the center of the plate and into the bottom of the new cross member, then install a 1/2" bolt on each side. This will lock the cross member into the plate and keep the cross member from walking around, and keep the aluminum plate from bowing (to some extent).

If you're worried about rust/corrosion, slather the bolt and hole with 3M #3 aviation sealant, so the raw steel edges are coated in sealant.

You are probably right but I do not want to drill into the crossmember. If you look at the pictures you will see that I left the original crossmember in place and butted this one up to it. I jacked up the one bunk and leveled the original crossmember then bolted the bunk supports to the new crossmember. This way the weight is taken by the new and the old is still in place for added lateral strength. It is not taking the weight so much any more. I might even sleeve it at some point.

 

[Chris1956]

If the cross member is too short to use a U-bolt, which it appears to be, I would expect a trailer supply place would have a U shaped bracket to bolt to both the frame and cross member.

There is a large trailer supply place in Delaware, right near the U of D. They do mail order and have reasonable prices.

 

[Maped]

Clamping a cross member won't give you the strength you need, you should definitely bolt it. If you are worried about the holes you have drilled then treat the trailer like a boat and coat all holes and fasteners with 3m marine adhesive sealant to prevent any rust and electrolysis in case your mixing metal types. If you choose to stay with your current solution, pay close attention to the frame rails Everytime you're done pulling up the boat ramp to make certain they haven't spread. Also you should paint those ubolts and coat them with a sealant where they meet the aluminum plate or they will corrode within two year of getting them wet all the time.