Aren’t you sick of rectangular examples?
I know I’m.
Dealing with simple decks which have rectangular cross section makes perfect sense.
And it's simple – ideal to understand the basics.
The enclosures below don’t make this harder, after all these are non-structural parts.
Now that we know the basics, can we get creative?
Structural and Non-structural parts
Structural parts hold the external loads and your structure together.
On the other hand, non-structural parts can be removed, and everything will remain in one piece (perhaps not as pretty).
Examples?
- Internal frames in cars are structural, these are covered with thin metal sheets which are not.
- Esk8 enclosures, which cover or hold the electronics, can be removed and the deck will remain as strong and stiff.
- On airplanes, the “no step” writing on the wing is usually written on non-structural panels which will cave in once stepped on.
If you want stiffer and stronger deck (or anything really), it makes sense to play with structural parts only, and you can do this in several ways:
1 - Add Material
The simplest one is just adding thickness and material, thicker deck means larger moment of inertia, and this means stiffer deck.
Just remember the equation from here:
I=h³·b/12
Adding material also means adding weight, which is less appealing.
2 - Adding Thickness Without the Material
This is more interesting.
The idea is this: put your structural material as far apart as possible and fill the gaps with lighter, non-structural one (or at least less material).
Classic example is the I-beam:
Photo by Christian Fregnan on Unsplash
Lets compare the moment of inertia of I-beam and a rectangular one:
For:
b=40mm
h=15mm
t=2mm
I(rect)=h³·b/12=15³·40/12=11250mm^4
I(I)=h³·b/12-2·(h-2t)³·(b-t)/2/12=15³·40/12-2·(15-4)³·(40-2)/24=7035mm^4
(*for the I-beam subtract the moments of inertia of the two blue rectangles)
With this small example we keep 62% of the stiffness with only 30% of the weight, compared to the rectangle!
Doubling the thickness to h=30mm will bounce the stiffness to 300% (compared to the original) and still remain very light - 35% of the weight (compared to the original rectangle).
This is the case with integrated decks, they are hollow inside and still very light.
I-beam is obviously not the only example (classical one).
Recent, very cool one, is LaCroix Hypertruck (no affiliation).
Look how the material is removed from the inside of the hangers, leaving only the essential, structural, material on the outside.
3 - Separate Structural/Non-Structural
Separate completely load bearing structure (the frame) and the non-structural parts which have their functions.
Like in the example of my own esk8:
All the stiffness and the strength come from the 3 carbon tubes and the 3d printed PLA enclosure carries all the internals.
The advantage here is the simple combination of different materials or form factors.
The moment of inertia of a hollow tube is also known and is calculated in the following way:
I=π/64·(OD^4-ID^4)
"The moment of inertia equals pi divided by 64, times outer diameter to the fourth minus internal diameter to the fourth"
But
You might noticed that anything beyond simple thickness change can rapidly become complicated, and winging it with rectangles and circles might become frustrating. Maybe even impossible.
Luckily, there is a tool that can help with that, it's called Finite Element Method (FEM).
You might even used it already, it involves meshes and bright colors, and I can tell you all about it next time.
Enjoy,
Dani
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PS
I’m doing something new here.
A short series of posts dedicated to designing your own, as strong as you want, structures.
I’ll cover the basics, so you can do the rest.
Let me know what you think: dani@dosimplecarbon.com