Saving a cut down frame, making it useable again.

Recently had a little 18” uni come my way for $10. I thought it would be handy for unicycle Hockey since my usual uni is a homemade 28” that is a little unwieldy in the tight game. It only weighs 4kg and will hopefully be able to be carried in a backpack (with some fold up pedals ) as I ride my 28” to and from the games. This particular uni had been shortened in it’s past by hacksawing the frame’s seat tube. This meant that it had no seat post clamp and no way of tightening it if there was one. So I called into play what I had learned making my 28”, the main frame of which was a direct use of the forks off a 40 – 50 year old bicycle (the frame was rusted through but the forks were fine). It took a couple of hours to do.
I will be including all sorts of information with this so that, hopefully, anyone, no matter their experience level, will be able to do this. Please excuse my pictures. I took them with my old ipod.

So here is the little uni with a seat post that won't stay in. The frame had been cut off roughly with a hacksaw...and not square.

The items to be fitted: A 40cm long seat post (I’m just under 6’ tall ) but because it’s a 22.2mm diam. post, only 1 seat mount was available: Nimbus double bolt seat clamp: metal based uniseat I was given last year and it’s original rubber cover and foam.

Don’t forget the safety gear! It’s not as much fun unicycling if you can’t see (not impossible, just not as much fun), and if you go deaf as you get older you won’t be able to provide witty comebacks when someone asks “where’s your other wheel”!

Put the seat clamp you’ve got against the side of the tube and mark (on the back of the frame. Check crank orientation) a point about 1 – 1.5cm below the bottom of the clamp. Use a ‘centre punch’ and put a dint in the middle of your mark to give the drill bit something to locate on.

I made sure I got the top of the tube level. Do this with a ‘square’, as in the photo, moving it around the tube until you can see where the edge is highest, and then file that area a bit then re-check, file, re-check, file, etc. Little bit by little bit so you don’t go too far.


File off the inside edge so the seat post will move smoothly.

The drill bit I’m using is called a ‘step drill’, and was one of three in a cheap set. These are a straight flute bit which means it doesn’t tend to ‘dive’ as much when it breaks through the work piece. It can grab though, so make sure the workpiece is either heavy or clamped down. I have some interesting scars on my left hand from where a small sheet of steel, with pointy corners, got caught by a bigger step drill and spun. Wasn’t nice. The steps on my three range from 3mm diam. to 20mm diam. in 1mm or 2mm steps and are very useful for putting large holes in metal with a minimum of fuss.
The hole I’m drilling here is to the 7mm step.

Get a file (chainsaw file works quite nicely) and take off any burrs on the inside. Here you can see I’ve marked out the slot. To get this nice and easily, hold a marker as below and slide your hand down the tube. This also works a treat for freehand ruling up of pages and other things.

Now carefully hacksaw in the slot. Guide the start of the cut with the side of your thumb, and be careful not to cut into the opposite side of the tube or through to the other side of the drilled hole. I finished off the raw edges with a gold, oil based, paint marker ( I had it laying around ) to protect it from rust.

I tapped a drill chuck key into the seat clamp gap (making sure the bolts are loose) to make it easier to then tap it on to the frame. When doing this use a wood, rawhide or rubber mallet to protect the clamp or, failing that, a piece of wood between the clamp and hammer.

And finished.

The seat base is fairly extreme in it’s bend, and as I like the back to be as flat as possible (and I don’t do fancy tricks needing to pop the seat out and around) I needed to move the mount points as the 22.2mm seat posts have very limited adjustment.
This involved holding the seat base firmly at the position I wanted and dotting in the wanted bolt positions through the adjustment slots. Use a centre punch in the middle of each dot. Drill holes that just let through the thread on the bolts but not, on these bolts, the square top of the shank just below the head. A round needle file or, better still, a square needle file (which I couldn’t find) is used to carefully put 4 ‘corners’ into each drilled hole. Do this bit by bit as you want the bolt’s squares to just fit making it far less likely that the bolt could turn as you tighten them up.
Putting in the extra holes may have weakened the base but as I said, I’m not one for stunts and it’s a cheap uni so stunts would probably break it. So I’ll stick to hockey, and learning to idle and ride backwards, and a bit of hopping.

And done, with the cover and padding back on. Didn’t twig that now the cover (which used rubber tabs that hooked over the bolts to hold it on) wouldn’t be lined up for proper fastening. Oops. Will have to work out an alternative method if it shows a propensity to come off.

And the finished unicycle.

The funny thing about this is it cost $10 for the unicycle, and $30 for the long seat-post and clamp. Ah well.

And the 28” I mentioned.

Hope you've enjoyed this.

Oops! Forgot to mention using the file again inside once you’ve cut the slot in the frame tube to remove burrs.

Great stuff, Duncan! I love your approach. But what’s going on at the bottom of the 28’er frame, at the point where the fork ends meet the bearing holders? And did you buy a unicycle hub for that or adapt something?

G’day LargeEddie,
Love your avatar by the way. I’m finishing a frame for a 46" wheel at the moment.
Anyway, back to topic: I cut a couple of lengths of angle iron, drilled 2 holes for the bearing mounts (awful things from a bearing supplies place. Their bearings never fitted properly. I’ve now got good mounts and bearings from Go figure) and a hole for a bolt to go through and attach to the original axle slots on the forks. Sat the forks on a level surface, cut a small steel ‘web’ to go down the back of the fork to the bracket, and had it all brazed on. A bit messy but has worked fine for 232kms so far. Once I’ve worked out the best way to set up the proper uni bearing mounts I’ll change it over.

I had got a 20" uni with a 36 spoke wheel, second hand, for $25. I used the hub, cranks, seat post and quick release clamp from that, got an oldish 28" rim, spokes and tyre from a cycle shop, and used the seat from my original 20" uni that I bought when I was 21, 24 years earlier. The seat never clamped really tightly to the different seatpost and tended to sink down at the back. This is when I discovered that it was more comfortable like this, so got another seatpost clamp, bent a steel rod in half and made a rear support to keep the seat where I like it.

Hope that answers all your questions.

  Here's a close up pic of the bottom of the forks.


Thanks! I took the illustration from a 19th century patent. I should actually go for a ride dressed like that one of these days. :slight_smile:

Excellent. I do hope you’ll post about it when it’s ready to roll, and fill us in on where you sourced the wheel and tire.

Yep, not the prettiest but if it works it works.

It does, thanks. Keep 'em rolling!

Great explanation on the seat post. I also like your creativeness with your 28" uni.

Thanks Unicyclist Lou, Glad you like it.