Post #2071650

2026-05-06 09:31 UTC

Obligatory not an engineer. As part of my vanlife prep, I've built a calculator to handle the deflection calculations on long metal posts (because I want a hammock instead of a bed and I need to build a custom stand to fit my layout--let me know if you need/want more details). I was originally looking at 40x40mm extruded aluminum for weight and deflection values I'm getting out of my calculator, but I just ran some numbers on unistrut and... Can someone double-check what I'm seeing here? I'm looking at an ~86-inch length of metal, with a point load of 400 lbs and clamped support (which I believe is the best "simple" calculation for my intention). For 8020.net's 4040 (40mm) extruded aluminum, I'm getting 10.174mm of deflection. For UnistrutOhio.com's P1000 (1-5/8" or 41.3mm), I'm getting 0.063mm of deflection. Is that realistic or have I messed up my calculations somewhere? #AskFedi #AskEngineers #vanlife

Replies (3)

@pockets@lgbtqia.space 2026-05-06 09:37

I'm also doing weight calculations on this. (I'll do pricing too, eventually, but waiting until I'm closer to making a purchase before getting quotes.) 86 inches of 8020's 4040 is about 11 pounds. 86 inches of UO's P1000 is about 13 pounds. That is a small enough increase that it's probably moot. And unistrut is supposedly significantly cheaper than extruded aluminum. So If I can get significantly more strength (less bending/deflection) out of a cheaper but only slightly heavier material, that seems like an easy and obvious choice.

Open ##2770507
@pockets@lgbtqia.space 2026-05-06 09:42

For further reference, I found an easy-to-follow formula over on MakerHardware.net, and I built a calculator inside Google Sheets to run the numbers for me. I did double-check my outputs against MH's and got the same results (give or take rounding errors) to confirm my calculator was working correctly. https://makerhardware.net/knowledge-base/tools-and-calculators/aluminium-extrusion-deflection-calculator/

Open ##2770508
@pockets@lgbtqia.space 2026-05-31 08:02

To answer my own question: Yes, steel really is that much stronger that aluminum. It's also heavier, and not insignificantly. It's also cheaper, and not insignificantly.

Open ##3132188