Seat post tolerance in a frame

[raidan73]

I joined a new forum once (not this one) and my in my first post I was well and truly caught in the baited trap a long standing member had set. I looked like a bit of a clown but ate some humble pie and joined in the good natured ribbing I received. I stuck around for several years and really enjoyed my time there. I think that was probably the first forum I ever joined. From then on I decided to take a more measured approach when signing up to a new one (not that I've joined many) and made an effort to get to know the locals, rather than ride into town with all guns blazing. Since joining Retrobike I've discovered that I know more than some people about some things but way less than a lot of people about other things. I've been here long enough now that I know who the go to guys are for particular topics or brands. Sometime I give advice and sometimes I ask for it but I never assume that just because I know how to solve a problem that it's the best, or only, way to solve it.

Lots of good advice and discussion points here

 

[ishaw]

Right, time to try and clear this up a bit.

1. Many, many lightning owners report the seat post size as 27.4. This was also 5he size of the og post as far as I'm lead to believe.

2. There is no factory shim.

3. The diameter of the seat tube appears to be 27.4.

4. A 27.2 post is too loose

5. I can tighten a 27.2 post in there and it holds, i just don't like it.

6. The 27.4 post I have is tight. However, it goes in nicely initially, gets tight/binds a bit, then moves down more freely.

My conclusion is that perhaps there is a bit of tube distortion around the triple triangle 23kded area? It still needs a 27.4 post.

I'm no stranger to ti frames and less than common post sizes. I own a sandvik dbr axis TT which takes a 27.0 post, a raleigh dynatech torus at 30.0 and then there's my GT team RTS at 26.8.

Ultimately I want the right size post for the frame. No amount of measuring sticks, voodoo or otherwise is going to convince me that it is not a 27.4 id seat tube.

However, I would be interested in the under sized Thompson if it is black. Worth a go to see if it fits better.

I will try to clean up the inside of the tube, there could be gunk or something that is causing the binding.

As you were....

 

[d8mok]

On point 2 - I believe some did have a factory shim. I sold a 16 inch frame which had a shim which certainly looked factory fitted and didn’t come out.

 

[hookooekoo]

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.

It should be possible to insert a seat post at least 100mm without encountering resistance (a USE seat post shim is 96mm). If the seat tube isn't a constant diameter for at least 100mm, it probably needs reaming out. I'd guess the reason for this is that during welding, a small amount of distortion occurs, and the tube is no longer perfectly circular. And in any case, tubes are probably not manufactured with particularly high tolerances, so reaming should always be the final step to get the correct internal diameter. Maybe they just forgot to ream it at the frame factory?

 

[hookooekoo]

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..

Great idea!

 

[2manyoranges]

Hookoo - Ishaw’s frame’s ti so will have been made to high tolerances and almost certainly not reamed out since ti is brutal on tools and frame fabricators hate wasting time, angst and tool edges on ti.....

 

[hookooekoo]

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

LOL!

In my case, I found that one layer of drink can was not quite enough, and two layers was slightly too much and made the fit too tight. Assuming a drink can thickness of 0.1mm, that means a 0.2mm jump is too great.

I'm going to try Peachy's aluminium tape idea.

 

[hookooekoo]

Hookoo - Ishaw’s frame’s ti so will have been made to high tolerances and almost certainly not reamed out since ti is brutal on tools and frame fabricators hate wasting time, angst and tool edges on ti.....

TIG welding causes highly localised heat, and unfortunately that means you're probably going to get distortion, especially in thin-wall tubes. It won't be visibly distorted, because the effect is quite small, so it won't look sub-standard, but it's enough to impact the seat tube circularity.

I'm not sure about Ti frames, but the final steps on steel would be reaming the seat tube and head tube, because they require a precise internal diameter for the seat post, and the headset. I'd be surprised if Ti frames are never reamed in any way.

I'm aware that Ti can blunt tools, but surely that's just a reason why Ti frames cost a lot, rather than an excuse not to ream the areas that need to be highly circular and a precise internal diameter. It's not much use paying for Ti if you get a frame with a worse finish than steel. And poor seat tube finish would be a worse finish, because all good quality steel frames are, or should be, reamed.

 

[captaincowelly]

Regarding solving the issue,I like Peachy's ally tape idea.Have used it for similar issues,works well.Otherwise,if the 27.4 seatpost is still binding after a good cleaning of the inside of the seat tube,I'd personally start sanding the bottom of the post to fit.Would take some work and a bit of skill but if you give the post a very light dusting with some primer then insert it you should see where the tight spots are.Sand/file away that area,repeat the process.

 

[old_coyote_pedaller]

I've done the seat-post sanding thing, plus sanding inside seat-tube with emery paper wrapped around a brush shaft.
Can't remember what frame it was but there was no doubt as to the size of post/tube, they just didn't like each other.
The post would go so far in then get way too tight, sanded bottom half of post and that depth of seat-tube, took a while but it all fitted together properly in the end.