As you can see from the attached pic King MUni-Man is starting to outgrow his custom 16" MUni. Fortunately for him I have a Yuni frame with a Monty trials setup almost ready to go. The problem is that the Uni is just a hair too large for him. I figure I can cut down the frame tube where the saddle post goes, taking off a couple of inches.
A friend of mine has a pipe cutter but it’s one designed for cutting copper pipes not the hardened steel found in a Yuni frame. Do you think I will ruin his tool? If so what tool should I use to top off the tube? Also, if I have to purchase a cutter what can I expect it to cost? I would use a hack saw but I’m sure I would screw it up by making the cut at some strange angle. Thanks.
I’ve used pipe cutters on chromoly seatposts and mountainbike steerer tubes and they worked fine. They do leave a bit of a ridge on the inside of the cut though and you’ll have to ream it out with a file.
A hack saw will work very well. There is a tool made for use with a hacksaw that enables you to get a perfect cut. It is made for cutting steerer tubes. Mine cost less than $30.
Depending on how much you cut, you may have to recut the slot in the tube.
You could start the cut with the pipe cutter to get a straight start and finish off with the hacksaw.
It’s not a big deal and don’t worry about it if it isn’t perfect. It’ll work fine.
If you can take the frame to a bike shop most have hack saw guides for cutting down tubing so you can insure a straight cut. I think a pipe cutter might just work though, I have used a pipe cutter to go through alot of seat posts and the material seems alot thicker and just as strong as the N2 frame.
If you are questioning breaking the tool though, go to a bike shop and get them to cut it down for you.
miter box + clamp + hacksaw + low grit sandpaper = straight cuts that are smooth on the inside. or if you have a friend with a sawzall (aka recriprocating saw) and a metal cutting blade, it’s much faster. more like 30 seconds compared to 30 minutes.
I’ve cut some very thick wall tubing with my pipe cutter. Since the inside of the seat-tube must remain clean and true, I would suggest using the pipe cutter to “score” a perfectly straight line around the seat tube and then use this line as a guideline as you saw the tube off
The tool that Frank A. mentioned is by far, IMHO, the best way to go. You will use it again and again to trim seat tubes, seat posts, and the like. The hacksaw requires little cleanup (a flat file for the outside edge and a round file for the inside). I performed this operation on my own MUni today. I use a Dremel with a heavy-duty cutting disk to cut the slot after drilling the hole mentioned by jagur.
Use a couple of round files, or some decent sandpaper wrapped around a pencil or dowel rod to clean up any burrs. Do not use a wood cutting miter box, it’s dangerous and you will ruin the blade. Sawzalls Rule! carjug
A pipe cutter should be fine. If for some reason you are too aggressive you might chip a cutter wheel (that is the one thing that might break), they are replaceable and available at most hardware stores for a couple bucks Don’t grab PVC cutting wheels by mistake (intended for plastic plumbing). Why re-invent the wheel? Pipe cutters are made to cut round hollow metallic forms at perpendicular angles. A no brainer. And yes you should expect a burr on the I.D. which should clean up quickly with a “rats tail” (round file). I have cut my own seat post down myself with such a cutter.
Even fully grown adults can ride 16" Unicycles. I think you would be better off putting the seat up a bit higher until he grows, rather than butchering the Yuni seat tube. I guess the Yuni frames don’t cost too much so it’s up to you, just seems like more hassle than it’s worth. I’m sure your son would love moving on to a bigger fatter setup, but the anticipation of waiting to grow into it could also be good for him.