# Is a 1/8" AR400F steel plate strong enough, for 4,000 lbs?

I have a Ford F350 cab & chassis and a gooseneck trailer. For the gooseneck hitch I want to use a steel plate. The steel plate will be BOLTED to the frame, above the rear axle And the gooseneck ball will be welded to the steel plate.

The distance between the frames is about 34". So the plate Is going to be 34"long x 8" wide.

The trailer is 16,000 lbs GVWR. And a maximum tongue weight of 4,000 lbs (at 25%)

A regular A36 steel plate, 3/8" thick, will bend under load. (A36 mechanical properties: Brinell=112, Tensile=58,000 psi, Yield=36,000 psi)

So I'm thinking to use a 1/8" thick, high strength steel ASTM 514 / AR400F (Tensile Strength=184,000 psi, Yield Point=150,000 psi, Elongation = 23% in 2",Brinell Hardness = 360/444)

Is a 1/8" AR400F steel plate strong enough, for maximum 4,000 lbs?

If not, what do you think about AR400F 3/16" thick ?

• Issues to consider: higher strength means greater brittleness and susceptible to fatigue cracks. Elastic deformation is preferable to plastic deformation. You need to consider that velocity increases inertia with mass and resistance to second moment of area forces is essential. A kingpin and floating plate is a better way to manage the forces than a tow hitch ball for very good reasons. Apr 18, 2019 at 22:34

I could see a moment when an overloaded trailer with defective brakes is rushing into the cabin.

You need to consult the trailer's hitch manual, or a professional engineer.

My estimate is you need a minimum 5/8" thick plate with manufacturer recommended geometry and anchored at a the correct distance.

This is a safety critical motion-dynamics problem which needs due deliberation as to the possible forces which may be encountered during acceleration, turning, hitting a pothole, braking, jackknife possiblity, limping due to unbalanced load, and many more issues. It's not just a motorcycle trailer.

• Does it have to 5/8" high strength steel, A514? Or it could be 5/8" A36 ? Or it doesn't really matter, as long is 5/8" ? Apr 18, 2019 at 17:40
• For twisting torque, and shear strength they behave similarly. I would use the A36, because of ductility. But again the bolts, details of connections, possible adding of stiffener ribs, especially under the ball which taper diagonally to 4 corners. A detail study of your car's chassis and see if there is double plated areas indicating reinforced spot for load support is prudent. Apr 18, 2019 at 19:00
• Ok than, I'll go with A36 and some reinforcement underneath. Does the width make a difference? I was planning on using 8" wide plate. Will a 10" wide plate make a significant difference? Apr 19, 2019 at 3:23
• I think so. My concern is the gooseneck connection. I see for lateral stability usually they use disk turn table, like in semi's horses. I assume you know what you're doing. If you load anything with high center of gravity you need to turn slowly. Apr 19, 2019 at 3:39
• Ok, I think I'll go with 5/8" and 10" wide. And 4 bolts on each side. Thank you very much for all your help Apr 22, 2019 at 13:54

What do you mean "bend"? Permanent plastic deformation or: deflect under load and spring back when the load is removed? The elastic deflection is the same for both steels (at the same thickness). The high strength steel will resist plastic/permanent deformation better but I can't calculate the thickness. However, when the specification says the A 514 is weldable, they mean when a welding engineer develops a procedure (low hydrogen, etc.) for a welder experienced with the material. So, if you are welding it may be tricky and even torch-cutting should be done with caution. Some middle ground is ASTM A 516, or A 515 very common, weldable plates that come at a few strength levels between A 36 and A 514.

• Yes you are right, 'bend' wasn't a good word. I guess 'Permanent plastic deformation'. Basically, if it will have enough strength to safely tow the trailer? Apr 18, 2019 at 2:39
• The steel plate will be bolted to the frame. And only the ball will be welded to the plate, if it makes any difference. So you are saying that welding on A 514 is really challenging ? And probably very hard to find an experienced welder that knows how to do it? Apr 18, 2019 at 2:56
• The one ball I examined was 4140 chrome -moly ; the hard,cracked weld caused the truck to wreck. Apr 19, 2019 at 15:12