I’ve had to shorten all but one of my five unicycle frames. So based on that experience thought I’d write up a tutorial on shortening frames.
Normal tools for removing the wheel, clamp and seat.
Metal files, fine toothed. A flat ~1 inch wide, used to clean up the top of the neck cut off, and the lengthened slot. A somewhat rounded ~3/4 inch wide, used to clean-up the inside of the neck and slot. A small round, to clean up the hole after drilling.
Drill and bits, You’ll want a center punch, and a succession of 3-4 drills bits from small up to size of the original hole at the end of the slot.
Hacksaw, for seat post and frame neck. A pipe cutter will work for cutting a the seat post, but I’ve never used one.
Bench, I used one of those black and decker wooden benches which opens likes a vice and wrapped a rag around the frame. Sometimes I just held the frame down on top of the table on a rag. Holding the frame is tricky because you don’t want to crush or scratch it, and so it is never held very securely.
- Estimate the amount of neck to be removed.
On necks that contour wider (change in diameter) just above the welded fork arms, that contour sets the lower maximum extension of the slot/hole. On straight necks you’ll want the end of the slot to remain well above the area where the fork is formed.
Measure twice cut once, but many times its trial an error. If you have another unicycle, measuring from the axle to the bottom of the seat provides an estimated max length the frame with seat post clamp can be. Consider that you may want to lower the seat for longer cranks, or hoping, or may have a thicker seat etc. Also be aware that different style seat post clamps fit higher or lower on the frame neck due to different clamp lip designs.
Remove the wheel, seat post and seat post clamp from the unicycle.
Shorten the frame neck and post:
Hold the frame securely and cut the end of the neck off. Be sure to cut straight and square to the frame. File the cut to slightly rounded and smooth.
You probably need to cut the seat post down as well. I’ve always used a hacksaw but I’ve heard that a pipe cutter works nicely. (I don’t think a pipe cutter would work on the frame neck.)
- Drill and saw the slot in the back of the neck.
You are duplicating the original slot that was in the neck of the frame. This slot allows the frame to compress onto the post as the seat post clamp is tightened. The slot has a drilled hole at the lower end, this opening breaks the stress in the steel to prevent the cut slot from cracking along the frame.
If you cut of the entire old slot off the frame, be sure that you putting the new slot on the back side of the unicycle.
If you shortened the neck a small amount you may have some of old slot left. You’ll still want to drill and extend the slot that by as much as you cut off, it’s ok if the original hole ends up in the middle of the extended slot.
Center punch and drill a small hole to the new end of the slot. Start with a small pilot hole, and enlarge to a final hole size with 3-4 drill size steps. Then with a hacksaw cut the slot in the frame down to the hole. It’s important to not cut across the drilled hole. Before assembling the post into the frame you must file all the edges slightly rounded and smooth.