This is a simple, 3D-printable device that ensures a bike's chain stays on the crank-set (front gear). I use it on my 1x8 setup, as I used to have issues where my chain would jump off the front gear whenever I shifted the cassette (rear gears) too abruptly. The chain guide fixed all those issues!
- Connect the frame and spacer pieces to the seat tube with a hose clamp. (Everything is currently sized for a 1.25" tube, as that's what it is on my road bike. You will need to change the arc diameter for those two pieces if your bike has different dimensions.)
- Connect the frame piece and the chain guide with a 1/4" lag bolt and 1/4" hex nuts (I got some galvanized ones from Home Depot for a total of about $1) as follows, where ">" indicates tightening to the right, and "<" to the left.
- bolt head > frame piece < nut ... (space between tube and gear) ... nut > chain guide piece < nut.
- Use a brim when you print the chain guide piece. Mine had trouble sticking to the build platform when I didn't, leading to a tangled mess about halfway through the print.
- It may take a little fiddling to get it all aligned just right, so once you get it in the right spot, tighten everything down hard!
- The design might be improved a little by flaring out the corners of the slot on the chain guide piece, to account for the difference in chain angle between highest and lowest gear. But it works well enough for me now, so I'll hold off on making that change until I need to print a new one! :)
- I've included a photo of what mine looks like when it's installed. (Printed in gloriously ugly ["theft resistant"?] yellow plastic!) If memory serves, I used a 2.5" long bolt for mine. You may notice an extra hex nut to the left of the frame: I needed this to offset it, as otherwise the end of the bolt stuck out just far enough to get caught on the crank arm, but a 2" bolt was too short to reach the gear.