The 3 most important things to know
Suspension design can be a complicated topic, easily worthy of a full time career. I’m not an expert but let me give you the need to know information to make a good design.
Get the Geometry Right
There are a lot of variables when determining the geometry, for example the length of the lower arm, the height of the upper mount, the shock attachment positions etc. Each one will affect the performance of the system. To size the length of the lower arm, first determine the desired ride height. If you are upgrading from a fixed gear use the original height as a starting point. I made a basic CAD drawing of the major parts: the wheel, shock, and lower arm. Here you can start to see how all the pieces will fit together.
After coming up with a rough length for the lower arm, adjust the shock attachment location on the lower arm to set the height. An attachment point closer to the fuselage will allow for more wheel travel however it will also give the wheel a longer lever arm thus compress the shock more easily. An attachment point further out will reduce the wheel lever arm but the shock becomes less effective as it lays to the side. I chose an attachment point roughly in the middle of the lower arm. Set the height a little taller as it will sag under load.
The next thing to consider is the size of the shock. The right size shock, will absorb all the energy from a rough landing. An undersized shock will bottom out and result in a violent bounce. An oversized shock won’t compress fully, wasting the unused travel. An oversized shock is also unnecessarily heavy. Also consider the height of the fuselage at full compression, will there be enough clearance to avoid a prop strike. It’s a little hard to estimate, but having too much wheel travel can be a bad thing. I chose a 1/8th scale R/C truck shock, I am really impressed with the quality for the price.
The stiffness of the springs and fluid weight are secondary considerations, these can be easily adjusted after the design is finished.
The print orientation is more important than the part shape
A very important consideration often overlooked, is how the parts will be made. For 3D printing the orientation must be taken into account. 3D printed parts are not very strong between the layers. I learned this the hard way, I spent a lot of time making a generous lug to beef up the joint. It failed when the pin ripped out through the layers. After that I printed the part horizontally and now the lug performs much better.
Make the gaps tight
One issue I had was too much wiggle in the parts. The wheels pick up a lot of shimmy when rolling fast, especially over the rough ground. If there is slop in the attachments the vibrations will cause the parts to rip apart. Its better to make the parts a little tight and sand them down then to have them too loose. I used a 0.4mm clearance between the lug and the mount gap. The fit is snug but loose enough to allow the parts to pivot without binding. Smaller parts will have more flex and a smaller gap is needed.
These are my best tips for making bush gear. If you want to download these parts, check them out on thingiverse.
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