TL;DR: the result is here.
My 6 years old son made a lego experiment. The piece in paper is supposed to be rotated fast so that we can see the motorcycle inside the flame wheel.
![](https://konubinix.eu/ipfs/bafybeibiafnujw2vorusqrde76gscpggwyh646zxylbsiqlhvopqb57mji?filename=a.png)
Yet, the piece being in paper, it broke easily.
![](https://konubinix.eu/ipfs/bafybeia5osta33hx2g2wx5fyvlzarc7mjz2fpazlp4dewlspver477wc3y?filename=a.png)
Let’s try to print a part in which he could glue the disks.
![](https://konubinix.eu/ipfs/bafybeieu3mkpzadskjsbembgxh24yrt6ddfjf4hu46ufjrrfty44naqzmy?filename=a.png)
![](https://konubinix.eu/ipfs/bafybeiafytdlfhx7464nf75yhjrtrtm334lx4epq3lzfb5dthdduzx6lpm?filename=a.png)
The axis should fit in a square of 4.5mm X 4.5mm.
![](https://konubinix.eu/ipfs/bafybeiglodbwpupyifixkliydxqzzpocawdanuqldsedlu7ikl5sbkwnwq?filename=a.png)
Actually, the square axis appears to be bigger than that, so I may want to try 5mm for a start. It is smaller than the diagonal of the axis (4.3 * sqrt(2) = 6.08) so it should be ok.
![](https://konubinix.eu/ipfs/bafybeib7fmvignm2o5pchvznocyzbug4szrmynpnlp6bfpy2rdhaum5lla?filename=a.png)
The square axis should be about 1.9 cm long.
![](https://konubinix.eu/ipfs/bafybeigm5kp4tjfgtdvg3un6l4tkg2fd4icsnf367m32ufi5y65rssr6km?filename=a.png)
And the disks should have a diameter of ~5.9 cm
For the axis to get into the piece, I need to make a hole into the disk supports. The disks supports may actually be a cylinder of length 5mm.
Result
With a few minutes of blender, creating a cylinder, two cube, a few resizing and a few boolean operations, this was an easy one.