Oct 19, 2014

Motorized Ripstik: Preliminary CAD

Though "Ripstik" is a proprietary name, so I will need to come up with a better one eventually.

Ripstiks are great toys. They have small enough turning radii to travel in my dorm's hallways, and can actually handle cracks well enough to go outside. Their method of propulsion means I don't have to shift my weight to place my feet on the ground while moving, and carving in tight circles is fun. Casterboards also don't pretend to be efficient modes of transportation, unlike skateboards, so their silliness is fine with me.

If you don't know how casterboards work, here's a decent video (I don't intend on doing tricks with this board, sorry).

So I want to make a motorized version. To attach this motor, I want to incorporate it into a custom-made caster. And if I'm making my own casters, I might as well make the rest too, right? Lots of fun machining things.

I have some beautiful blue shim steel sheet that would be perfect for the deck, and I have some plywood to stick underneath it for stiffness. The rest of the materials I would have to buy, so I'm applying to techfair for funding. If they sponsor me, my project would happen through winter and be presented in February.  
shim steel has such a beautiful iridescent blue
yay techfair! (please fund me)
Plywood and metal composite works really well, actually. I could use 1/8" sheet aluminum and plywood for the ripstik chassis, since I have access to a metal brake and wood clamps. The strange geometries required are made possible through bandsaw and waterjet. And Solidworks has a great sheet metal tool, "flatten", that converts shells between 2D and 3D.

Front chassis and its flattened version

The overall shapes of my board closely mimic the contours of a real ripstik. My end goal is to achieve something that could be mistaken for a regular ripstik from far away, but is clearly custom up close.

Both front and back chassis are mostly hollow to accomodate batteries. Real ripstiks are only 0.5" thick, which severely limits battery space. I intend to use A123 batteries, so my ripstik requires a bit more than 1.25" thickness. Probably no one will notice.

There's a hole in the rear to accommodate the torsion rod that connects the front and back chassis. This hole is smaller than required, since I am still unclear about the capabilities of the metal brake. A proper-sized hole might have too little material to bend without breaking, and I might have to drill the actual hole after folding.

I am also currently lacking the bracket mounting holes to attach the deck. Working on that soon.

Back chassis and its flattened version

The back chassis is a little different. Acceleration pedal, back lights, and other hardware are going there, and the rear is raised to provide clearance for the modified back caster.

Casters on ripstiks are at a slight angle (~25 degrees), but adding an angle to the chassis would be unnecessarily difficult. My plan is to incorporate the angled portion into the mounting block half of the caster.

I'm still debating whether to make these unibody casters or to bolt on separate wheel-holding-armature-things (these really need a name) to the rotational part of the caster. Stresses on the wheel would likely break the connection points, but separating parts would make machining easier and waste less material. Either way I would have to carefully plan out the geometry, thickness, and material for these casters. Anyway, here's some preliminary CAD:

generic caster frame
Yes, I know some of these angles are not actually mill-able. But it gets the idea across. Thrust bearings for the casters are idealized as beveled cylinders.

front caster assembly with 76mm wheel

back caster assembly, missing sprocket and chain

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