Started using my phone for navigation on longer gravel trips, needed a phone holder that fit my stem. This setup fits my 90mm relatively normal stem that came with the BMC I am lucky enough to own.
Stem: https://tinyurl.com/y3bdrnls
You will need:
Spring: Closed end tension spring, 1/4 inch around and 1.5 tall. I found them in a junk drawer, I think they came in a mix pack from home depot.
Coat Hagner wire Thin, cheap coat hanger from the dry cleaner. Cut two pieces. 3 inches long each.
Zip Ties: I used the thinner type and they fit nice on these parts.
Assembly is pretty straightforward:
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Print the parts, one of the stem hugger, two units of the cradle. Print with supports on the hugger, and orientate with the arms pointing up. Cradle needs no supports. I used PLA+ at 50% fill.
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Remove the supports on the bottom side of the hugger, sand/scrape smooth. Test that part on your stem.
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Give the arms a dry run on the main body to test that everything slides nicely, then remove.
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Thread one of the coat hanger arms into the little hole in the body of the cradle, when the tip of the wire gets to the arched opening, thread it through one of the springs, then keep pushing the wire into the next opening. Add the second spring. Push the wire flush. The springs should now be trapped by the wire and dangle there in their slots like wind chimes for you.
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Slide the fingers on the first cradle arm onto the main body, repeat the same coat hanger wire threading maneuver on the second arm.
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With your whits about you, stretch the second arm over the stem hugger body, insert those two rails on the cradle arm into the hugger body, and allow the springs to pull each half together. That is your phone holding action.
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Zip tie the assembly to your bike stem.