Last week we completed our loads calculation for the skateboard frame.


A good idea is to check first if the results make sense


Let's look again at the pictures from last week, only the long frame for now.


What are the results at the edges of the frame, the wheels?

We can see that the shear force at both sides is equal to the reactions - 60 kg.

This is important, the values at the edges should make sense, in our case these are the forces only.

What happens with the bending moment at the wheels? It's zero.

Simple supports can't hold moment, so again these results make sense.


Now let's look at the general shape of the curves:



The curves change only if something is happening from the outside. Legs or reactions from wheels all can do that. So it does make sense.


Finally, let's just get the numbers from the curves and use them later for the actual design:



What's up with the numbers in this table?


If we're making the shorter board, then we have to worry about the 20.6 kg.m in the middle (shear forces are way less interesting or important).

It's clear that if we have bigger load we'll need bigger and heavier structure.


Also, if we want to go as light as possible, we'll design for this load only and we'll not go for higher values.


We can go even further and reduce the frame size/thickness towards the wheels, where the moments are smaller.


How do we go from numbers to structure?


We need some basic idea which will be the baseline for future ideas.


Let's go with carbon


I want to use carbon tubes to build the frame.



Actually, it's carbon/epoxy, a composite material, very light and strong.


Carbon tubes?

They're available in various sizes and are an existing product. So, these are relatively cheap.

They are very popular in RC modelling, various drones, and aerospace.


Why don't we adopt these for longboards?

We just need to find the ones that work for our needs.


Carbon fiber products start as a woven fabric of carbon tows. Tows are bundles of tiny carbon filaments, they are usually counted in thousands; 1k, 3k, 6k, 12k.


See this Wikipedia picture for example:


The most popular is 3k. Going from simply carbon to composite material is usually messy. It involves epoxy and specialized equipment.


Several years ago I made my share of glider wings and RC gliders. I'm not an expert, but I made several nice parts, like this set of wings for example:


This time it's different


We aren't laying a whole new frame out of carbon fiber.


We're using carbon tubes which are already made and use them in the build.


The basic concept looks like this:


This is version zero.

A lot of work ahead to make it ride able.