OT bolt strength engineering ??

[greg g]

I am going to be doing the final install of my Studebaker Rear bumper in the next day or so. It is a Steel C channel 5 inch face, by 2 inch channel. This is welded to 3/8 uprights that were welded to the frame of the parts truck.

I removed it and cut some 3 x 1.5 C channel pieces to which the uprights are ow welded on three sides. The small C channels will be bolted to the underside of the frame rails. There is no frame cross member there but there is one about 4 inches forward along the frame. The holes in the frame are the factory ones that held the stock bumper mounts. The new assembly will be bolted to the frame with bolts 2 per side. What size/ grade bolt should I use that will be enough to tow about 2000 lbs with 200 to 250 lb tongue weight. I will be bolting a step bumper type reciever to the bottom of the 2 x 5 C channel. I was thinking 7/16 grade 5. probably use the same to attach the reciever to the bumper.

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[TodFitch]

For towing, you state 2000 lbs and you are using 4 bolts or 500 lbs/bolt. Double that for safety so you want bolts with at least 1000 lbs shear strength.

Doing a quick web search turns up this: http://www.portlandbolt.com/faqs/bolt-shear-strength-considerations which implies that you want bolts with 1667 pounds of tensile strength.

Bolts are rated in psi not pounds. Same web site says that grade 5 is 120,000 psi so you'd want a cross sectional area on the bolt of 0.013975 inches. Area is pi * r * r, and r = 1/2 d, so your diameter is at least 0.133392419827 or 3/16". I'd drop a four 3/8" grade 5 bolts in there and call it a day.

Disclaimer: The above is not a through analysis and if you get into problems using the above it is your own responsibility. But I think I've shown you a possible way to approach the arithmetic...

Edited November 15, 2011 by TodFitch

[greg g]

I know the shear strength is also a consideration for pulling and stopping, I guess if I look at the stuff that current trailer hitches bolt into and what they use, it should be plenty strong.

I will certainly respect your disclaimer. I don't believe a cross country suit would hold up in small claims anyway....

Thanks for the info.

Off to the fastener store in the AM. I don't think the offshore stuff the local hardware stores is competent enough for this application.

[Young Ed]

Personally I'd use nothing less than a grade 8 and the biggest size that fits in the holes in the frame.

[Greenbomb]

I'm with Ed. Well, not WITH Ed, but I agree with him.

[Don G 1947]

Cost difference between a grade 5 and 8 is next to nothing. Why even consider anything less than the largest 8 that will fit in the hole?

Don

[Chester Brzostowski]

Cost difference between a grade 5 and 8 is next to nothing. Why even consider anything less than the largest 8 that will fit in the hole?

Don

I agree grade 8 vs grade 5 is only important if you need to buy 100,000 bolts. Go for the grade 8.

Chet…

[Plymouthy Adams]

for my own personal reasons of considering sheer strength, I attached my receiver hitch to the frame using grade 8...I also chose 7/16 as minimum size hardware to be used.

[Don Coatney]

I would also find a bolt with an unthreaded shank long enough to penetrate both pieces of metal. By doing so there will be solid steel and no threads in the shear point.

[TodFitch]

I would also find a bolt with an unthreaded shank long enough to penetrate both pieces of metal. By doing so there will be solid steel and no threads in the shear point.

That is a good point.

And regard the other posts that followed my original one: While a 3/8 grade 5 ought to do it, it is certainly reasonable to use the largest grade 8 that will fit in the existing frame holes. If the holes are larger than 3/8, fine. If they are smaller, then I'd drill them out to at least 3/8.