Best material for thin drop brackets ?

[Ding]

What is the best material for thin (1/8") drop brackets? I know the strength of the different (actually can look them up online), but don't know which material is the best for forming. The drop brackets I need extend 5 inches below the tunnel, so pretty sure I need a brace that attaches to the outside of the running board (over the heat exchanger).

Materials I am looking at include . . .
•316 Stainless (annealed) - is it strong enough?
•7075-T6 Aluminum - can it be formed without becoming weak?
•17-4 PH Stainless (annealed) - way too expensive?
•Some form of steel - which one and how to resist rust?
What do you guys think?

Any fabricators out there?


Here is a crude sketch . . .

 

[dhkr123]

Brace won't help since the running board flexes. In fact, the rear suspension cross shaft is responsible (via the drop brackets) for stiffening up the tunnel. Unfortunately, it does nothing for the running boards.

What are you meaning when you talk about "forming"?

More seriously, I would definitely go thicker than 1/8", by double.

 

[arteeex]

The stainless materials you're looking at will have about the same elastic modulus (near 29,000,000 psi). The 17-4 is a stainless that can be hardened but there is no benefit for this in the application.

You'll need more than 1/8'' in the lateral direction. It would be smart to add ribs and/or fold the edges to increase the lateral stiffness. I wouldn't expect a lot from the gusset. It will stiffen the running board way more than the bracket itself will gain.

Aluminum will need the same attention. In order to increase lateral stiffness it needs thickness in that direction. You can effectively gain thickness without adding much material by using ribs here too - or starting with a thicker piece and cutting pockets.

With either material the risky spot is the transition zone at the interface with the tunnel. My suggestion is to buy brackets from someone with track record and hope they've done the homework.

 

[Ultrafrozen]

I'd stick with 1/8" or 3/16" SS (any kind is good - 304, 316) and form vertical ribs at the front and rear edges. Have a look at commercially available drop brackets and you'll see what I mean. You can also add a formed angle and install it above the running board to stiffen it.

See if you can add a lateral stiffener (crosser) that will tie the two drop brackets together. Maybe one will fit across where the old mounting holes are without getting in the way.

 

[kinger]

What are you worried about flexing? The plate cant bend in or out because its tied to the shaft, the only strength you need to worry about is the shear strength of the bolt shearing a slot in it.

The skid could flex side to side inside the tunnel as a unit but your side braces would help with that or as mentioned the ribs would help the plate (another simple option is weld in small angle iron to it, because you probably dont have a forming maching laying around LOL.

Some quick math and if the hole in the plate for the skid was 3/8" diameter then tau = 4F/pi*d^2 = 18,117 lb-in

So you need a material with at least 75% higher then that

SS 304 has ultimate tensile strenth 90,000 so that would work
Alum 6061 is 45,000 and would probably oval the hole over time.

I would have to use stainless if it has to be 1/8"

 

[arteeex]

Check your units. And, remember, you use about 50% of the UTS (depending on the which DET you're applying) when considering shear loads.