I’d assumed that I’d use a 30.9mm seat post with it, but now that I’m at the point of reaming, I’m questioning if that’s correct. I haven’t reamed it yet - just thought I’d double check here first because it seems a little too tight. Is that the right size or should I be looking at something smaller?
Yes, the tube is meant to use a 30.9 post. And yes that will result in a fairly thin tube at a high stress area. It wouldn’t be my first pick for a seat tube.
You could add a shim and use a 27.2 post. Maybe then you wouldn’t have to ream it.
That’s actually meant for a 30.6 which has always been my beef with Columbus on these. 30.6 is few and far between on choices but it means you aren’t removing any real material with a clean up ream. If you reamed this to 30.9 you’d end up with a 0.4mm wall thickness below the taper. Thompson and BBB are the most easily available posts.
Thanks @DEVLINCC! Appreciate the insight. I’ll have a look at what’s available in 30.6mm but I do already have several 27.2mm seatposts kicking around and I may just go with the shimming idea, which seems safest and also gives me lots of seatpost choices.
I did go looking for a source to back up my claim but did fidn what I was looking for initially. I do remeber seeing it written but was also taught by Dazza at Llewellyn Custom who helped develop a seat post to suit these tubes from WP Composti a few years back. I’ll see if I can find a confirmation to follow due dilligence.
I prefer a 27.2 seat post myself. Gives a much nice feel in the saddle and way more options and styles to choose from. Plus you can play around with teh styling of teh shim to dress the bike up.
Beautiful! Lots of inspiration there! Don’t go digging around on my account. Based on what I see when I look at a 30.9 post, I think you’re probably right. Thanks again for the guidance.
No worries. I still couldn’t find a definitive answer other than directly from Dazza price/stock list. He worked with Columbus to refine a lot of their catalogue sizing so I feel like he would be a reliable source of data for it.
One place I worked, each frame got an aluminum reducer, light press-fit with Loctite, to become “one” with the frame. Used 27.2 posts, and any reaming that had to be done was in soft alu.
I have had experiences with bikes with a shim that was loose, and I didn’t like that as much. Like when you raise the seat and the shim moves up in the frame instead of staying put.
The only downside I see with the press-fit alu shim is if a frame repair needs to be done there, it will be difficult to remove. With a steel or Ti frame you can melt the shim out, but that will of course ruin the paint if it’s a painted frame. I consider that an acceptable downside since if you’re removing the shim to do a weld or braze repair, you’ll be repainting anyway.
there’s an cheap aluminum seatpost reduction sleeve for 27.2 post for that 30.6/30.9 inner diameter, they also sell it in the web
but personally I made machined custom steel then braze it, more robust and add some reinforcement to overall structure. Ream it down after post brazing
