Steambow Stinger II Damper Side Rail

Update: Added the softer string stopper file. The other one was a bit stiffer, depending on the material used.

This is a Steambow side rail set with string stops that I designed for fun. This is best used on the Stinger II Compact, which really doesn't need string stops. Please note that Steambow does not recommend using anything other than the OEM side rails due to the pressures and forces that this part may experience on the standard version. For that reason, you should never use this with a bow limb above 90lbs, unless it is on the Compact where there won't be any pressure. I have not personally tested above that, and would not recommend it anyway. Steambow already makes a part that can handle the extremes. As a printed part, the overall strength is dependent upon the print parameters you select when printing. The crucial part is the stopper itself, which MUST be made of TPU.

Use at your own risk and avoid dry fires. I am not liable for misuse. While the side rails are not a safety item, extreme care should still be taken!! The rails must be printed with full supports due to the angles involved. The 55lbs bow limb doesn't really require string stops either according to the manufacturer, but this part still works with it and can definitely withstand a dry fire or two when printed out of the right material. An accidental dry fire with the rails installed once caused the TPU part to split at the layer lines, but the limb block, string, and limb, and tips were unharmed. I printed more parts and was back testing. Minimum arrow weight is far less important with the stops installed, which is a nice side effect.

I did not notice a difference in accuracy in my personal testing with these side rails. That was not exactly the intent in the first place, but your results may vary. I didn't specifically test for that. My main design objective was for it to survive an accidental dry fire from a 55 or 90 lbs bow limb. I believe the rails could withstand a single dry fire at most with the 90lb limbs. I would expect the rails to become damaged and unseat from the bolt holes at the layer lines or the arm may even snap after that. Part failures were usually unspectacular, which is a good thing in this case. 

Print the rails "outer arm up" with a minimum of 2mm walls and 50-75% cubic infill (or higher) for best results. Use PETG or better. Polycarbonate or other tough and/or composite materials are even better. Note that Steambow's polymer of choice is a carbon fiber filled polymer, so Nylon-CF should also be a suitable material. I do not recommend to print this out of PLA! It does not have the impact strength and may snap unpredictably or violently since it is quite stiff. It is not well suited for these types of stresses. I ran FEA models to see where the weak points were on the rails, which was a great engineering exercise. It led me to make some of the design choices I made. The hexagons are mostly just decoration though. Materials that have some give are a little better for this task.

After printing, the support material should carefully be removed. If your print is not perfect, you may consider sanding the surface that will mate up against the Stinger body. There is still a lot of room for improvement on this design. So while I learned a lot, it may not be a fully optimized design. For me, it has been pretty solid.

And be sure to print multiple TPU stoppers at a time, as those are the key to making all of this work. You want each stopper to be identical, or else accuracy could suffer. Be sure to select a random seam so that it is easier to pre-compress it evenly. You want to test the TPU dampers by compressing them a few times to make sure too much effort isn't required. They tend to come off of the printer a little stiff. The rails can't take the forces involved without the TPU “spring”. I know from firsthand experience. While this is functional, I definitely still prefer the part available from Steambow that comes with the M10. It is way better, but this was fun. Just be safe.