My previous version of these brackets was embarrassing. It looked good on paper (screen?), but just didn't hold the loads.

This post is all about improving the design.


What didn't work?

Mainly two things, first the sheet metal is not stiff enough right after the bolts, and it goes into plastic region very quickly. Notice how the picture and this movie look alike.

Look at the elevated stresses left to the deck bolts
Look at the elevated stresses left to the deck bolts

Second, the two parts just don't work together, and bend even in the axial direction.

Axial displacement
Axial displacement

Lastly, in order to manufacture those the lower bracket had to be cut across, and it's just, well, ugly...

This slit is just not cutting it
This slit is just not cutting it

Can we do something about it?


We should obviously stiffen the whole assembly, so we'll reduce the bending stresses around deck bolts and the drop down area.

To do that the moment of inertia must be increased. Remember this one? I talked about it here.

With the bracket as it is, the moment of inertia can be increased only by increasing the thickness. Then you're running into limitations of sheet metal bending.

We can change the material to steel, but who wants to do that? It'll weight a ton and will rust.

Alternatively, let's add a rib.


This is really just a vertical piece of material which stiffens the structure locally. The challenge is to make it work, w/o welding and w/o machining.

This is what I've came up with:

Two ribs are added and held together with the deck and truck hardware
Two ribs are added and held together with the deck and truck hardware

It took quite a few iterations to get the dimensions just right w/o extensive red areas. So, the new parts are bent out of 3mm aluminum sheet. I've added the material in the drop down area to help with the stiffness.

Here are some colors for you:


I've used my standard 120kg guy as the load, with symmetry boundary conditions (let me know if you need more info about this) on the half bracket. Besides some elevated stresses in the bolts area (which aren't important anyway), the rest looks very calm...

And I also managed to do it around two times stiffer:

Vertical displacement
Vertical displacement

I like how it comes out so far, next I'll try to make couple of these in order to test them in person.


What do you think? Is there a potential in these? What can be improved?


As always if you have any questions just let me know in the comments, and check out my Instagram page for more frequent updates.