box rear frame?

[79fj40moneypit]

Ok I have been working on my 62 frame and I removed the inside layer of frame in the rear because of some rust delamination and the crossmember same thing rust,but the rest of the frame is clean.

Now I want to strenghten the rear frame, but I will be building a rear bumper , I could get some 1/2 plate sheared and slide it inside the frame and bolt it in, then take a 1/4 wall 4x6 tube cut 2 slots for the 1/2 plate and slide it on and weld it.

Should I put another crossmember in the rear frame, box the rear frame or would that be overkill?

 

[AfricanToy]

got any pics?

 

[Tinker]

If you're planning a stout bumper with at least 1/4" mounting tabs, that should be enough. The stock bumper is like tinfoil, so the rear crossmember was functional.

I left my old sorta-rusty crossmember in, but hogged out the center hole so my receiver bike rack mount would stick through it. My bumper is 3/16" C-channel, & the mounts are 3/8". 1/2" grade 8 fine thread bolts replaced the rivets.

 

[ntsqd]

If you do box it in, use metal as close to the stock frame thickness as is reasonably possible. Anything thicker is not only excess material, it also creates a bad stress riser.

The only reason to ever go thicker than 1/4" on a bumper tab is because the tab extends thru the bumper and has clevis holes in it. Even then there are better ways that don't result in carrying extra steel that only subtracts from your mpg and doesn't do you any good.

 

[Randy88FJ62]

If you do box it in, use metal as close to the stock frame thickness as is reasonably possible. Anything thicker is not only excess material, it also creates a bad stress riser.

The only reason to ever go thicker than 1/4" on a bumper tab is because the tab extends thru the bumper and has clevis holes in it. Even then there are better ways that don't result in carrying extra steel that only subtracts from your mpg and doesn't do you any good.

Can you give an example of a better way than tying 1/4" thick steel to the frame for tow points? I would hate to buy too much plate if there is a better way when building and designing a rear bumper.

 

[79fj40moneypit]

I thought that tabs for rings had to be 1/2 thick, I know that 3/4 would be overkill (easy for me to get).
My thought(not that i'm right) was to run it(1/2 or 3/4 plate) thru the box tube right and down the inside of the frame and bolt it to the frame.

 

[ntsqd]

Consider what you are bolting to. How thick is the frame rail? Is making the bumper tab thicker than that going to do any good?

If all of the typical engineering calcs are done for a bolt shear type design I think that you'll find that it won't work. But it does work. So bolt shear isn't how it functions. Clamp load is, it generates friction between the bumper bracket and the frame rail. Exactly how a crankshaft drives a flywheel, it isn't the bolts in shear, it's the clamp load friction between the flywheel and the crank flange that makes things go.
So the more bolts the better, but they don't need to be huge. Four 5/16" bolts torqued in place would be better than two 1/2" bolts even though the 1/2" bolts have more total cross sectional area. The reason is that the greater number of smaller bolts are necessarily spread out more and have more area clamped together. Though there is no reason that it couldn't be four 1/2" bolts. The bigger the bolt, the more torque it can take, so the greater the clamp load is.

When you see a bolt hole that has been worn egg-shaped it is because the clamp load failed and the bumper was then held in place by just the shear of the bolts. The problem with this is that the frame rail is too thin, there isn't enough bearing area, and that is how egg-shaped holes come to be.
So use lots of bolts with heavy flat washers & locking nuts, and torque them!

A trick sometimes used to increase the bearing area of a hole is to weld on a washer. This is best used when the existing hole is too small and will be drilled anyway. Use a washer for the next smaller size from the desired finished size, weld it on, then drill through both. This locally increases the bearing area of the hole without unnecessarily increasing the thickness of the rest of the part.

Ideally the bracket would be about the same thickness as the frame rail, be gusseted for the lateral forces, and tie into the frame rail on two surfaces that are perpendicular to each other. Check out Fourrunner's bumper thread for an example of this type of bumper bracket.

 

[ntsqd]

For shackle tabs there are some things to consider at the shackle pin hole. Bearing Area (pretty much what it sounds like), and Tear-Out (splitting the thinnest section(s) around the pin hole.

Different steel alloys kind of muddy the water here. If the tab metal is of the same strength as the shackle pin, then so long as the tab is at least as thick as the diameter of the pin and has one pin diameter between the edge of the pin's hole and the edge of the part, the tab will be strong enough. Since the tab metal is not likely to be as strong there needs to be more of it, but only around the hole. If the tab behind the hole has more cross sectional area than the cross section of the pin then it is roughly as strong as the pin.

The simple solution is to build up the tab around the pin. Weld on thin plates to build up the thickness of the tab around the pin hole. Somewhere around here there is a pic of a short section of tube welded into a tab. This can also work if the dimensions of the base tab plate are carefully laid out.
Could make the tab from something thick and then mill off what you don't need, but the built-up tab has the potential to be stronger than a billet part.

 

[ntsqd]

Some concept model pics. The first and last models have the middle plate at 1/4" thick. The center model has the pair at 3/16" thick. Not sure if that makes any difference, but it might give an idea of scale.
Built-up tab.
Double Plate tab.
Bushed tab.

 

[Eporter]

Very informative! Your words and Solidworks(?) visuals are great for explaining things!

I don't have an engineering degree. I have a design degree. I think I need an engineering degree as well.

 

[ntsqd]

Thank you.

It is SolidWorks. Work has decided to go to the Dark Side (Pro/E). I need a new job.

 

[Randy88FJ62]

Pro/e compatability is not the best when comparing to Solidworks although it gets the job done. If I had to choose I'd go with Catia V5 but it's 5 times as expensive as Solidworks...

ntqsd, what do you do for work?

 

[79fj40moneypit]

ntqsd, Thanks for the info, Although I am not a engineer by any means I see was your saying on the thickness of the frame vs the thickness of the bracket. I will rethink all of it. I'll run 1/4 out from the frame and thru the box tube plate both sides at the pin and a bracket to bolt to the bottom of the frame.

thanks for all the info

 

[ntsqd]

Welcome.

The hard part for me is not turning it into a huge "requires calculation" sort of thing, but still getting to a more efficient design that can be built at home.

Job:
Advanced Propulsion Technologies, Inc.
EcoMotors

Very informative! Your words and Solidworks(?) visuals are great for explaining things!

I don't have an engineering degree. I have a design degree. I think I need an engineering degree as well.

FWIW I think that those with some math orientation would benefit the most from just two Engineering classes, Statics and Strengths of Materials. Those two clases combined with one of the Machine Design text books, either Shigley, or Spotts, or Juvinal would serve 90+% of the home fabricator's needs.