Shortening a frame

Due to the added height of a new seat adapter, I have to shorten the seat tube on my chrome Yuni frame. Tools I own that might be of use include a hacksaw, a drill and a file. With the limited repetoire of tools I possess, would it be wiser to enlist the services of a metalworking professional? If so, how much does that sort of job (i.e., cut off the seat tube, drill a hole, cut down to the hole) tend to cost?

Thanks all,
andrea

Just a humble suggestion. Perhaps it’ll be better to shorten the seat post instead of the seat tube. It’s cheaper to replace a seat post than the whole frame if something goes wrong.

To cut down the length, try using a pipe cutter. It gives a cleaner cut then a hacksaw. To cut a slot, it requires a bit more work. I sent mine to the workshop to have them mill out the slot to save me all the trouble

Cheers

Re: Shortening a frame

I just did this to a Yuni frame a weekend ago, for the same reasons. It’s pretty simple and you’ve already got the right tools.

Step 0) Take EVERYTHING off the frame.

Step 1) Mark the frame for the right cutoff point. If you like a nice square cut then wrap a piece of paper around the tube and line it up before you draw on the frame. I didn’t bother, as I knew that later I would have the chance to file it square, so I just scored the paint with a knife all the way around.

Step 2) Measure the current slot height. Measure down from your line and mark the bottom of the new slot. Make sure it isn’t to the left or right of center.

Step 3) Measure everything again. Last chance to get it right.

Step 4) If you have a drift punch, then lightly dent the frame at the bottom of the new slot. DON’T put a real dent in the frame, just a little poc mark so the drill bit won’t wander. I didn’t have one so I filed a tiny pocket for the drill bit to get started in.

Step 5) Select a SHARP drill bit that is as thick as the slot is wide. Chuck it in your drill. Find some mineral oil or other cutting oil. Vegetable or olive oil works just fine.

Step 6) Set the frame down horizontally on a soft surface, slot side up, and secure it somehow. I laid it down on a pine deck and put my foot on it.

Step 7) Brace yourself so the drill doesn’t wander, put a few drops of oil on the frame and SLOWLY drill a hole where you want the bottom of the slot to go. SLOWER is better. Take your time; the frame isn’t going anywhere. If you go too fast the bit will heat up and get dull. The oil is to keep it cool and flush out the debris. Oil frequently.

Step 8) Find some other way to brace the frame, oil the hacksaw blade, and SLOWLY saw through the seat tube where you drew your line. Again, take your time and use lots of oil. This part took me about 10 minutes.

Step 9) Now hacksaw the slot out. Use two cuts, on on either side of the slot, and alternate between the two slots so that you are sawing against relatively solid metal.

Step 10) Using the file, carefully clean off all the burrs. Take your time and sculpt the frame a bit. Make the top nice and square, round off all the sharp edges, etc. This part should take you about a 1/2 hour or more.

Step 11) Make sure the inside of the tube is smooth so it contacts the seat post all the way around. Use your finger and feel for any trace of a bump or burr on the inside of the frame tube. Even if you did everything else wrong, get this part right. If you have a half round file then use it to smooth the inside the tube. If not, then either find someone who does, or get very creative. Wet/dry sandpaper works just fine but it takes a while.

Step 12) Clean up. Make sure there are no shavings or grit inside the tube.

Step 13) Put a thin layer of grease all over the inside of the tube and the new cut edges to prevent corrosion. Put the unicycle back together.

Step 14) Have fun riding it.

Re: Shortening a frame

Cyberbellum has some good advice. Here are a couple of my thoughts.

First, if you have a good relationship with a bike shop you might be
able to use their saw guide (like a miter box for tubing) to get a
perfect cut when you shorten the frame.

Second, it mat be very important to make sure the bottom of the slot
is round. Sharp corners concentrate stresses and can lead to
cracking. I might even drill an oversized hole at the bottom of the
slot.

Ken

Re: Shortening a frame

Cyberbellum has some good advice. Here are a couple of my thoughts.

First, if you have a good relationship with a bike shop you might be
able to use their saw guide (like a miter box for tubing) to get a
perfect cut when you shorten the frame.

Second, it mat be very important to make sure the bottom of the slot
is round. Sharp corners concentrate stresses and can lead to
cracking. I might even drill an oversized hole at the bottom of the
slot.

Ken

Thanks very much for the well-laid out instructions and suggestions - exactly what I was looking for! I’ll give it a go as soon as I have access to a vise.

Peculiar: yeah, I think I am going to look into a manual pipe cutter for my frequent seatpost hacking. In this situation, I just can’t put the saddle down low enough on the frame for me to hit the pedals, so I’m going to have to adjust the frame.

  • andrea

Is the slot’s sole purpose having something to tighten?

I never thought about it before

Yup. Without it the only way to get the seatpost to stay is to crimp the frame around it. That’s a one-way frame adjustment. It’s also absurdly easy to do with “modern” seat clamps. A small ledge around the inside of the top of the clamp to prevent it from sliding down would be a nice touch.

The slot should be at least one diameter long, and preferably closer to 1 1/2 or 2. Any longer and you start to lose strength in the frame. It can be any shape as long as it doesn’t close up when the clamp is tightened or have any sharp inside corners. (Outside corners, or points, are just fine. As mentioned above, inside corners concentrate stresses and start cracks.) As I was typing the above message I got to thinking about how a lightning-bolt zig-zag slot would be cool.

Be VERY careful with the vice. It will make your nice round frame oval in am instant. I had access to one and chose not to use it.

Those ledges are a nice touch on the 3 bolt clamps Darren sells ($9 cdn)

I just shortened my 28" Sun frame an inch. Basically followed cyberbellum steps. It was pretty easy.

Only problem I had was that found I purchased the wrong diameter post clamp. So I tried to save the original SUN clamp which was welded to the frame. Luckily, turns out that knocking the clamp up and down lots of times finally broke the weld. I am able to re-use the clamp.

Thanks for the help.

Re: Shortening a frame

“Chrashing” <Chrashing@NoEmail.Message.Poster.at.Unicyclist.com> writes:

> Only problem I had was that found I purchased the wrong diameter post
> clamp. So I tried to save the original SUN clamp which was welded to the
> frame. Luckily, turns out that knocking the clamp up and down lots of
> times finally broke the weld. I am able to re-use the clamp.

Don’t do it! The Sun clamp will crimp the frame and dent the seat
tube. It is worth the $5 cost for a decent clamp.

Ken