Wind Loads on a trailer

[fourbtgait]

In building my Benroy that removes from the utility trailer, I am looking/calculating the wind load and bolting to the trailer.
In my research I have found 2 different formulas that produce a wide discrepancy.
To make it simple, I negated the effect of the tow vehicle and any curves. frontal area then is 20 sq. ft., side area is 40 sq. ft. To make it simple.

One formula is F=1/2 x rho x V(squared) x A x C, which gave me at 70 mph on the front a force of 7,350 pounds pressure.
The other formula is F= A x P x Cd which at 70 mph on the front a force of 500 pounds of pressure.

Quite a variation in loads.
The single shear strength of a regular 3/8" machine bolt in 3/4" C-D plywood is 2,560 when torqued to 200 ft pds.

thoughts?

[drhill]

Rule of thumb for wind loading is 30 pounds per square foot of frontal area at 100 mph. I know you won't drive that fast but ever drive into a headwind?

Anyway, I have a 5x10' benroy that is fairly easily and fairly regularly removed from the trailer. I have a total of 8- 1/4" bolts holding it down. The bolts are installed at the front corners (to angle braces in the frame) and to the second to last cross member. The bolts are in pairs and are welded to a short piece of metal strapping. This allows me to insert the bolts from above and then go below and install the nylock nuts without the wife holding a wrench on the heads. I call them wife saver bolts. Have a look at my album, there are pictures of the bolts in there.

[fourbtgait]

Yes, the first calculation produced 25 pounds pressure per square foot at 70 mph. Then I happened on the other formula which produced 367 pounds. I was tempted to use 5/16 bolts, going thru plywood is what lowers the bolt shear strength. My bolts go thru the side wall which does have 6 oz glass skin.
Thank you for your reply.

[KCStudly]

... when torqued to 200 ft pds.

200 lbs-ft of torque will fail (most?) 3/8 inch nominal fasteners. My reference lists 31 lbs-ft as the dry torque for a plain SAE J429 grade 5, 3/8-16UNC carbon steel bolt. Reduce that by 15/ct (multiple x 0.85) for zinc plating, so 26 lbs-ft. Reduce by 20/ct for cadmium plating (x 0.80), so 25 lbs-ft.

[fourbtgait]

Sorry. 200 inch pounds which is 16.66666667 foot pounds.
APA report number E825E. Though dated 1997 fairly applicable still.

[aggie79]

I wouldn't sweat the calculation.

Keep in mind that there is only a single 1/2" or 5/8" diameter hitch pin holding the ball mount to the hitch receiver, and that structural failure in the plywood - bolt heads pulling through - will occur long before the bolts experience shear or tensile strength failure.

I'd recommend using 4-6 bolts with the largest washers you can find installed between the bolt heads and the plywood. (On my teardrop, I used trailer deck screws at 6" on center along the outside frame rails.)

[Shadow Catcher]

There are different grade bolts with differing characteristics, easily obtained are grade 8 and there are others available. In figuring wind loads, it is not static on coming trucks head winds... I was concerned that our solar panel held on with 3M VHB tape might be a bit of a problem until I talked with a 3M engineer. On our trip west three years ago we hit sustained 40 to 50 MPH head winds blistering heat and the back wash from big rigs.