This blog post will cover the creation of a finished, functional part: a 3D printed skateboard truck, printed in Onyx reinforced with carbon fiber. Let’s go.

What is a Skateboard Truck?

 

A diagram of all the parts of a standard skateboard truck.
‍A diagram of all the parts of a standard skateboard truck.

The trucks of a skateboard are the interface between the deck and the wheels. At its simplest,each truck consists of a baseplate (which is bolted to the board) and the hanger (on which the wheels sit). It also serves as the steering mechanism for the skateboard, translating leaning to one side into turning. The magic of the steering mechanism relies on the interface between the baseplate and the hanger, which forms a triangle. The hanger is free to rotate around one edge of this triangle, with bushings acting as springs to re-center it when the rider isn’t leaning.

 

Note how the axles turn in response to leaning forward or backward.
‍Note how the axles turn in response to leaning forward or backward.

Why 3D Print a Truck?

First, let’s specify something: I am not printing the entire truck, only the parts that would normally be made out of aluminum (the main parts of the baseplate and the hanger). The axle I am making myself out of 1144 “fatigueproof” steel (more on that later), and the hardware and polyurethane bushings are off-the-shelf.

The aluminum parts of the truck are typically cast in steel or sand molds. That means that in order to prototype new shapes, a new positive must be made, then a negative cast (presumably in sand for a prototype), then the new positive cast in aluminum. This turnaround time could potentially be shortened by simply printing the prototype overnight then assembling it in the morning. This project build explores that possibility while showing how the Mark Two dramatically eases the process of fabricating by hand.

Making the Hanger

The first step of this project was, of course, to model the truck. In my case, I used Fusion 360 because it allowed me to combine sculpted shapes and parametric features. I dimensioned most everything off an Indy 139 Silver truck (Independent Trucks are a well-respected standard in skateboarding). The only exception was the bulky part of the hanger, which I sculpted manually to be a little extra stocky to provide more support in bending.

The beginning of the sculpted shape for the hanger.
‍The beginning of the sculpted shape for the hanger.
The completed CAD model of the truck in Fusion 360.
‍The completed CAD model of the truck in Fusion 360.

 At first, I printed just the hanger, and printed it in two parts with two small mortise-&-tenon joints to keep them together. My hope was that a two-part design would allow me to route fibers in two different planes, giving the truck extra strength. Unfortunately, my lackadaisical joinery made for a joint that could split under the formidable forces of skateboarding. While it never broke, the joint flexed too much for my needs. So, I went with a single body for the hanger. And, as I mentioned before, that hanger needed a steel axle insert.

The split caused by faulty joinery can be seen on the right side.
‍The split caused by faulty joinery can be seen on the right side.
The mechanical drawing for the axle.
‍The mechanical drawing for the axle.
As an aside, here’s a fun fact about skateboard axles: They work because they use mixed units. Normally, bearings fit nicely on shafts because they are precision machined with very tight uni-directional tolerances. However, they are also significantly more expensive, and that cost isn’t justified in a piece of hardware that’s going to be beat up and replaced in a matter of months, if not weeks. Instead, the standard for skateboard trucks is to use a 608 ball bearing (8mm ID x 22mm OD x 7mm W) on a 5/16” axle to allow looser tolerance on the shaft diameter. Why does this work? Because bearings are likely to be within 3 tenths of a thou of their nominal (~.3150”), so your allowable tolerance in your shaft is now .3147”-.3125” = .0022”. That’s about an order of magnitude looser than the tolerance for a typical drive shaft. Now it’s much easier to cheaply manufacture the axles and ensure that the bearings will easily slip on and off repeatably.

To keep costs low, I bought lengths of “Extra-Strength” or “Fatigueproof”1144 medium carbon steel rod. I chose this steel for a few reasons: 125,000 psi yield strength, very machinable, no need for heat treatment, high impact resistance, and very low cost on McMaster. Seems perfect for a hand-made axle.

Because I didn’t cast my axle into the hanger or use a special ribbed axle (like many truck manufacturers), I notched the shaft into the rest of the hanger to constrain it axially and rotationally, creating a kind of sideways keyway. Cutting the keyway turned out to be very easy with the help of this custom printed tool:

 

The jig for the sideways keyway on the bandsaw.
‍The jig for the sideways keyway on the bandsaw.

Using this keyway jig, I could hold the rod against the band saw table and in the vice with no worry of slipping, as well as using the the cutout in the holder as a guide to get the right keyway dimensions without having to slowly creep up on my measurements. I simply rough cut with the band saw then filed until I hit the nylon of the jig.

Then, still using the holder, I cut the threads on each end of the axle, followed by putting the rod in a drill and lightly sanding the sharp edge off the thread. Voila, axle complete.

 

I prepared the 3D printed part of the hanger by simply ripping off the supports that I could access and drilling out the supports inside the hole for the axle. I then had to hammer the axle into the hole, and to do so I wanted to grip the printed part firmly in a vice. To do this, I needed a soft jaw to hold the front of the hanger, which was fairly easy to model in CAD and then perfectly easy to print over night. Once the axle was inserted I just hammered in the key (also printed out of carbon fiber-reinforce Onyx) and the hanger was ready to ride!

The custom 3D printed fixture for holding the truck in the vice.
‍The custom 3D printed fixture for holding the truck in the vice.
The fixture works perfectly.
‍The fixture works perfectly.

Assembly

The baseplate was much less of a process to make because it involves no steel reinforcement. All I had to do was print it and insert the off-the-shelf pivot cup. I then put the rest of the truck together the way you would any skateboard truck: insert kingpin, slide on bushings and hanger, then crank everything down with a nylon insert locknut.And that’s the entire build! A little hand machining made simple with the help of some 3d printed jigs and fixtures, along with a functional 3D printed truck. If you have any other projects you want to see done with the Mark Two, let us know via TwitterFacebook, or LinkedIn!

Myles Cooper
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August 4, 2016
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