Last week we were choosing the tubes which will be the main building blocks for our frame. Since then I have a small update. I looked into additional tube with standard cross-section – 12mm ID and 16mm OD. This one is more common then the 18mm tube and with better pricing. Here is the updated table:
In the table you can also see the resulting factor of safety, for the 16mm tube we get around 4, when our requirement was 2 minimum. We can go higher but it means bigger and heavier structure which is not desired. So, the tubes satisfy the requirements and they also slightly lighter then the 18mm tubes.
Now we go to the actual hardware design. The tubes are ordered, and I wanted the end units to be ready as soon as the tubes arrive. The obvious choice for me was to 3Dprint the end units. The challenge here is the printing material. It is PLA which is very weak compared to carbon or aluminum. The challenge is to design the units in a way that they will not be stressed by the loads, in our case the person who rides the board. Luckily, we already have these loads.
Notice that the bending moments are reduced significantly towards the ends of the frame, now if we can find a way as to not stress the unit, we will get what we want. This is what I came up with:
The carbon tubes run through the entire length of the end units, the central tube is longer. The trucks are standard with standard interface. What did we accomplish with this design? Once the frame bends, the end units will rotate around the wheels, there will be no relative deformation in the end unit itself, therefore no stresses. This is still needs to be verified with analysis and possibly tests, but so far I’m happy with this design. Here is a nice time lapse of the print itself:
Currently my aim is to make the final parts from Aluminum. This will require a redesign and material reduction, but the first tests will be with the printed units.