Seat post tolerance in a frame

[Bullpup]

Interesting thread. I have a Thompson Elite 27.4 that actually measures 27.3!

 

[Woz]

Ishaw is confronting this:

...So the 27.4 trial post arrived today and it is very tight. Fits in the top fine but the further it goes the tighter it gets, I have to push pretty hard to get it down, and I was worried it would get stuck. It could nearly be ridden without the clamp being tightened, not had a post this tight in a frame...\

Which is a worry. And I can see how challenging that is. My take? Most likely someone with more enthusiasm than understanding whacked in a too-large post into the seattube, flaring the top. Once it has flared it is well nigh impossible to reduce the diameter. I have experienced this many many times, and it is most frustrating.

If it’s just a blob of stuff from welding, then wire brushing using the specialist wire brush/reamer is good thing to check out and eliminate.

Remedies?

Get away with a Very Robust seatpost clamp and really tighten the bugger up.
Brutal and cheap.

Bite the fashion bullet and use a USE with long shim, as long as shim extends beyond seattube-toptube junction
Aesthetic compromise - about 50gbp in total. More if using ti USE.

Cut the seatpost to a length where it marginally binds but clamps properly at the top - as long as this means enough insertion. In this configuration will only ever be rideable by you or someone with same inside leg.
Cheap and will do the job.

Take the frame to a ti specialist and get a proprietary collar fitted.
Undoubtedly expensive and poor company will mess up frame.

I think that’s it - anyone else?.......

That's it. What we don't know is how deep down in the seat-tube it starts to bind. The seat-post cluster isn't exactly friendly,
there is a lot going on there, and the triple triangle design probably adds more potential seat-tube distortion. Ti to boot, so that will
be a bugger of a job which I wouldn't recommend. Wouldn't recommend reaming to 27.4 either as an option.

I will add that you can indeed cut the bottom of the seatpost at an angle which could help insertion depth too ie. longer
at the front, shorter at the back.

I think if I had this problem, I would go down the shim route. AL 27.4 -> 25.4 reducer and taper the bottom by judicious sanding.
https://www.sjscycles.co.uk/seat-posts/262-humpert-seat-post-shim-254mm-id/?geoc=FR
When the fit is about right with grease, I would consider a more permanent bond material between the seat-tube and shim.

 

[pete_mcc]

Guys, FFS, talk about the blind leading the blind! This is a real downside of forums, where all sorts can post irrespective of their knowledge of cycle mechanics. You end up with lots of "advice" that isn't worth a cup of cold tea, and the odd well-informed post getting lost in the noise.
No one, absolutely no one, has mentioned that simple and effective piece of kit ... A SEAT POST GAUGE. It costs about £20 from the likes of SJS and will tell you in 5 seconds what the correct seat post size is for your frame.
The seat cluster lug (assuming you have a lugged frame) or top of the seat tube (for a lugless frame) is almost never round because of heat distortion during manufacture. (Sometimes a diligent frame builder will ream it to be nicely round after building, but that's unusual.) So stick the gauge in, let it find its natural position, and read off the required set post diameter. Now you know what the correct seat post size is supposed to be. But be aware that no manufactured article is ever completely round and there are always manufacturing tolerances. This is why measuring with a Vernier is effing hopeless ... it will only measure the diameter at one point on the diameter but the bloody thing ain't round! Never mind, a decent manufacturer can be reasonably relied upon to get this more or less right. So get a seat post of reasonable quality in the nominal, measured size and get on cycling and enjoying life.
Phew!

So you didnt read the bit about the seattube being wider at the top and narrowing meaning a post will initially fit but becomes progressively tighter. So a conical seat tube size gauge will only measure the top, which is likely flared and so will not give an accurate measure of the true seat post size. Also if by your suggestion that no top of seat tube cluster is round then any standard conical seat tube gauge will just sit naturally at the narrow point so will be as effective as a vernier.
The irony of those who choose not to see accusing others of being blind is palpable.

 

[old_coyote_pedaller]

So you didnt read the bit about the seattube being wider at the top and narrowing meaning a post will initially fit but becomes progressively tighter. So a conical seat tube size gauge will only measure the top, which is likely flared and so will not give an accurate measure of the true seat post size. Also if by your suggestion that no top of seat tube cluster is round then any standard conical seat tube gauge will just sit naturally at the narrow point so will be as effective as a vernier.
The irony of those who choose not to see accusing others of being blind is palpable.

Seems like his comprehension skills are lacking, several of us and now you have pointed that out, but.........

 

[Woz]

Interesting thread. I have a Thompson Elite 27.4 that actually measures 27.3!

You could make a fortune selling it to Ishaw

 

[hookooekoo]

I did a few measurements a while back using vernier calipers. To get a good clamping action, the post has to be very close to the seat tube diameter. A difference of as little as 0.2mm may be too much to get a good clamping action.

 

[2manyoranges]

Woz good point re triple triangle.

You are right - there’s a lot going on there and there might be a pinch point around the crossover welding. Ishaw can you measure where it’s pinching, but inserting by hand until it pinches, then record top of seatpost with tape, pull out and measure externally against tube. One option is to sand the seatpost! So it’s 0.2 or so smaller at the bottom than the top...

Another guess - and it’s just a guess...that the following might have happened...frame is completed and goes to QC - they have had a lot of expensive ti rejections and everyone is getting jumpy. This frame has a pinch point around the welding on the lower joint on the seat tube - where the seat stays cross. Supervisor says ‘put in a smaller seat post and crank up the seat clamp, then we don’t have to junk the frame....’. Plausible.

 

[Woz]

I did a few measurements a while back using vernier calipers. To get a good clamping action, the post has to be very close to the seat tube diameter. A difference of as little as 0.2mm may be too much to get a good clamping action.

Yeah. You can get really mullered cutting up various beer cans to find the right thickness.

 

[2manyoranges]

And another thing. It’s also worth trying different top clamps. I have a couple which are useless, and bind up. Hope always good. Salsa good too.

 

[Peachy!]

Oh oh and my 2p…
A tip I picked up whilst fighting in the Sudan..
27.2 post with a wrap of adhesive backed aluminium foil tape a little higher up where the clamp is. The 27.2 will easily get past your lower “bump” or whatever is making the seat tube a tight fit whilst the tape will be helping the clamp not over work the top of the tube.
The tape stays in place as it’s adhesive backed and doesn’t crush as it’s aluminium. You can also trim it when you have the hight correct so it won’t show.
Bit of a working hack, but then this is retro..