Seat post tolerance in a frame

[ishaw]

My gt lightning cane with a 27.2 post fitted and I recently bought a ti post to replace it with in the sane size.

However, it doesn't feel right to me. When I tighten the clamp up, it feels like it is taking up more slack than I'm happy with, the seat tube split almost touches when clamped tight enough to hold the post.

The one it came with isn't as bad, but to me still feels like too much.

I've read that the lightning frames sometimes required a 27.4 post.

Am I needing this obscure size or will I be fine with a 27.2?

My verniers imply the seat tube is 27.5 at the top.

 

[Splatter Paint]

Drinks cans are often 0.1mm thick. Cut up a drinks can and make a nice long shim, then try and fit the 27.2mm post with the shim - if it goes in with ease then it is time for a 27.4mm post.

I’d aim to leave a 5mm gap on the shim for compression and don’t let the shim go below the top of the seat tube, they can be a pain to get out! I’d grease the outside of the shim, but not between the post and shim so it’ll preferentially slide out the frame.

SP

 

[d8mok]

I had a right mess around with mine

27.2 Thomson is too small. Tried shimming it with a can and yes it works but slipped over time and i was worried about it causing issues.

I then purchased a 27.4 Thomson which was too big.

I then went back to the standard 27.4 one and it fits great but isn’t blingy.

 

[ishaw]

Hmmm, not the kind of news I was hoping for, as want to fit something nice.

The post it came with was a ritchey but it weighs a ton and is heavily marked.

The ti post I mentioned is no good size or otherwise as it creaks from the head so that's going back.

Is there a way of confirming one way or another what post size this frame needs? Could get expensive if nothing fits.

I don't really fancy shimming this frame.

Why did they make ti frames with odd post sizes. This could be 27.4, my dbr axis tt is 27.0 and dynate h 30.2. Whats wrong with a standard size?

 

[d8mok]

The issue was that the new 27.4 Thomson actually measured slightly above. And the 27.2 was spot on to size.

The standard one measures a tiny tiny bit below 27.4.

 

[Splatter Paint]

It could be tolerances on the posts themselves. Is your 27.2 actually that size?

I only suggested the shim as a test for the 27.4.

 

[ishaw]

Sure, appreciate it was to test, will give it a try and also measure up some 27.2 posts and see if they are as they should be.

I can only measure the top of the seat tube with the verniers, but with them reading 27.5 I suspect that suggests a 27.4 post is in order.

Will report back once I've had a chance to measure and test fit a few.

 

[2manyoranges]

This can be a very frustrating problem. But as you imply, it's vital to get to right. The mechanics of getting it wrong are awful - too small a post exerts extraordinary forces on the top of the seat tube, and if that's extended above the TT-ST junction then it's a recipe for a cracked frame. And spec sheets for frames can be very misleading - over time I have had some weird departures from the stated size, particularly on Clockworks and P7s. If you can't access seat posts to borrow, then I had resorted to buying VERY cheap posts on eBay just as 'checking' posts - worth the 3-4gbp which they cost to get it right.

 

[2manyoranges]

and one solution is a USE 27.4 shim and USE post - shims are 9.99 in 27.4 on eBay. Nice light high performance solution.

 

[mk one]

Its not the frame thats the issue, posts are different diameters, regardless of them having the same size stamped on them!