A spring-loaded RC plane wheel that really works

prusaprinters

<p><strong>What it is:</strong><br>This is an RC airplane wheel with spiral spokes, capable of absorbing some of the landing impact. To satisfy a broad range of applications, I made four different rim types and five different tire shapes (all interchangeable); in addition the wheel is fully scalable (proportionally and unproportionally) to fit arbitrary model sizes and styles.<br>The idea of using spiralized spokes as a shock absorber is neither all new, nor is it mine - but all attempts to construct a spiral wheel for RC airplanes that I've seen so far don't really work (either too stiff or too prone to sideward wobbling), so I decided to design my own which, as yet, never failed to work as it's supposed to.</p><p><strong>What it does:</strong><br>Spring-loaded spokes can serve as a bumper/buffer to avoid fuselage damage from hard landings, yet they are not beneficial if the landing gear's legs are already elastic enough to take the landing impact without further measures. Keep in mind that a spring is not a shock absorber. Also keep in mind that an airplane that jumps back up after hitting the ground is practically uncontrollable in that situation, so it makes sense to keep the model's “jumpiness” at a reasonable level.<br>For those who don't need suspension at all but like the design, I have also added a rim variant without springs. This one can be used like any regular plane wheel.</p><p><strong>Choices to control spring resilience:</strong><br>To make the spring feature “work” means to keep the spring feature from working “too good.” ;) &nbsp;Obviously, a spring so weak that it is pushed down simply by the model's weight is not a good idea, so different models require different springs.<br>For this purpose there's three different hardness grades to choose from: “Soft”, “Medium” and “Hard”. These three give you a (coarse) choice of hardnesses, however, the choice of material is just as important. Don't use an unflexible or brittle filament like Greentec or any kind of carbon-enforced filament for the rim. For example, very good choices are Armadillo, PA6, PC, or PA12 (order sorted soft to hard). Not as good, but usable in narrower limits, are PETG, ABS, or ASA.<br>And, by the way, don't use an overly heat-sensitive filament like PLA - if you do so, at least protect the hub with a bushing of PETG, ABS, or PC (or simply a piece of carbon tube). With ground contact at landing speed the wheel suffers an extreme acceleration resulting in friction resulting in punctual heating within a split second.</p><p><strong>Scale the wheels as desired:</strong><br>First, choose the tire shape you want to use. All the tire files have a “mm” declaration that tells you the diameter of the wheel if scaled to 100%. Take this as an indication for the scale to print the tire as well as the rim with.<br>Note that scaling the wheel affects the spoke's material thickness as well - if you scale overall size to less than 75%, always use the “hard” rim type (or use a .25 mm nozzle). The spokes should always be at least two printing perimeters thick in order to correctly connect to the rim. To illustrate this: In the slicer's preview, a correct spoke-to-rim connection has to look like this:</p><p><img src="https://media.prusaprinters.org/media/prints/117975/rich_content/7152f612-276a-46e7-80e2-042948108f22/bildschirmfoto-2022-01-16-um-170636.png#%7B%22uuid%22%3A%22783b895f-b33e-4237-a486-c9ebcf625a73%22%2C%22w%22%3A1200%2C%22h%22%3A950%7D"><br><br>And <strong>NOT</strong> like this:<br><br><img src="https://media.prusaprinters.org/media/prints/117975/rich_content/cc887509-8861-472b-9e7d-59ecd48d36a8/bildschirmfoto-2022-01-16-um-170551.png#%7B%22uuid%22%3A%2269bfa7d6-51e1-43df-a923-9791f1c4b3af%22%2C%22w%22%3A1200%2C%22h%22%3A950%7D"><br><br>The wheels may also be scaled unproportionally by the Z axis only (if you like wide-base tires or narrow “oldtimer” style wheels). The only important thing is that tire and rim are scaled by the same value.<br><br><img src="https://media.prusaprinters.org/media/prints/117975/rich_content/1b0e9a2f-fbf3-482e-93f4-b663b27f4af8/3-z-scales.png#%7B%22uuid%22%3A%22e1807b44-f658-4411-909b-13f815f8a53c%22%2C%22w%22%3A2500%2C%22h%22%3A1500%7D"></p><p><strong>Printing the tire:</strong><br>Use some soft TPU filament - my personal recommendation (in respect of softness as well as weight) is Varioshore TPU from ColorFabb, but (though heavier) any filament with a hardness of Shore A95 or softer will do fine. Do <strong>not</strong> try to use any “hard” filament like PLA or PETG - you would not be able to slide the tire onto the rim.<br>My recommended basic settings are:<br>Wall thickness: One single perimeter of about 0.7 mm<br>Layer height: 0.15 mm<br>No supports!<br>Infill: 10 to 12% of infill are needed to support the wall structure. A closed cell infill structure like Grid, Triangles or Cubic will make the tire more taut (quite like “air-pumped”) while an open cell structure like Rectilinear or Gyroid will result in a softer/flabbier, even shock-absorbing tire. Of course, infill type and amount are not critical if printing the “gearwheel” type tire.<br>Do not forget to use a glue stick or painter's tape as an underlay when printing TPU on a PEI base - otherwise it will stick “a little” too good to the buildplate.<br>Always print one tire at a time - either in an indivual or sequential print job.</p><p><strong>Printing the rim:</strong><br>If you want to use the spring-loaded version: For most applications, the “medium” hardness variant is the best choice to start with.<br>In most cases 100% infill are adequate. Use either concentric infill or a lot of perimeter lines - the hub will have to be rebored later, and concentric print lines will result in better rolling properties than other patterns after reboring.&nbsp;<br>To print a pair of spring-loaded rims, you should print the file mirrored for the second wheel. However, in order not to waste filament and energy, I recommend to print one rim first and wait with the second one after the (successful) stress test (see below).</p><p><strong>Post processing:</strong><br>The rim's hub has to be rebored after printing to fit your axle and offer a smooth inner surface. There is a 5mm countersink on the protruding side of the rim. Use this to center the drill correctly. I recommend to stay close to the axle's diameter (e.g. for a 3 mm axle use a 3.2 mm drill, not 3.5 mm). Drill slowly - at least slowly enough not to melt the plastic or overstress the springs.&nbsp;<br>Slip the tire onto the rim. No glue needed. Finished! Note that you can always remove/exchange/reuse the tire.</p><p><strong>Stress testing:</strong><br>Mount the wheels to your plane and perform a hard landing. Just kidding. ;)<br>Better idea: Put one wheel (rim and tire) on an axle (drill, pole, etc.) and push it down on a bathroom scales with two fingers.&nbsp;<br>First test: The spring travel should not be used all the way down until you reach about 4-5 times your model's weight.<br>Second test: The spokes should not break unless you reach about 10 times your model's weight (no, you don't have to find out <i>when</i> it will break).<br>If the wheel stands both tests, you can rest assured that it is adequate for your model.</p><p><strong>So what if the test fails?</strong><br>Keep in mind that the above test criteria are hard - harder than required for many models. Chances are that your landing gear's legs could never take &nbsp;a load as high as the test requires for the wheels. If so, you will have to define for yourself what grade of stability is needed. However, if stability is to be increased:<br>First, you might consider to choose a different rim type. The three variants supplied give you but a coarse choice, so this is rather unlikely to be the right/only action that is to be taken.<br>Second, consider printing the wheel at a different Z scale. This, in contrary to the above, is a very sensitive way of adjusting hardness. A Z scale below 100% makes springs softer, a Z scale above 100% makes them harder. Of course, when using this method, you will also have to print the tires anew.<br>Third, rethink your choice of filament material for the rim. If you want something “indestructible”, have an eye at Ninjatek's Armadillo or PA6.<br>Fourth, write me a comment that additional hardness grade steps for the rim are needed. I did not want to mess up the file set with too many options, but of course it's no problem to vary the spokes as desired.</p><p><strong>Finally:</strong><br>When mounting the wheels to your plane, use a little oil inside the hubs to lower friction when rolling.<br>And, if you feel like making my day would be a nice gesture, you're surely welcome to take a photo and post it as a make.</p>