I built an inexpensive chain guard for my bike. Getting your pant leg or shoelaces caught in the chain while riding is at the very least annoying, and can potentially be very dangerous. The usual pant clip does keep your pant cuff out of the chain, but does not keep shoelaces safe.
I ride a fairly heavy bike, and realized I never used the largest gear on the front derailleur. I decided to replace it with a smooth plate, which acts a chain guard. I made this at TechShop.
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Step 1: Mount the Chain Ring on the Lathe
The existing big chain ring was a 44 tooth one, but I never rode fast enough to shift up into that range. I found a 48 tooth gear in my parts bin, and decided to repurpose it.
Since I had access to a metal lathe at TechShop, I decided that was the tool I wanted to use. I used the four jaw chuck on the metal lathe, since this crank set uses four spokes to locate the gears. It is important to center the part when using a four jaw chuck, hence the dial gauge on the left in the photo. Centering is done via small adjustments to each of the jaws, measuring the runout between adjustments. This was by far the longest step in the process.
Make sure the chain ring is held in tightly, but not too tight or the gear will flex out of round.
Step 2: Cut Teeth Off of Chain Ring
Move the cutter in close and turn on the lathe. Slowly slice the teeth from the outside of the chain ring. It will be pretty noisy initialy, as it is an interrupted cut. When the sound becomes uniform, you know you have cut all the teeth off. Stop the lathe, inspect your work, and unmount it when you are satisfied with the cut. I cleaned up the edge with a small file.
Step 3: Replace Old Chain Ring With Toothless One
Next step is to remove the existing large chain ring from the crank set. Using a 5mm hex key, carefully unbolt the chain ring. Be careful as you remove the last bolt, as the largest chain ring and the middle one will both be loose.
Remove the old chain ring, and place the toothless one in its place. Replace the bolts and tighten them down, ensuring both center and outer chain ring are seated properly.
All done! Now, go for a test ride, without worrying about shoelaces getting caught in the chain.