Tight Seatpost

[zigzag]

one of my bikes has slightly undersized seat tube diameter which makes inserting/adjusting seatposts hard work (e.g. when packing the bike for airplane journeys). i wonder if there is a way to increase the internal diameter by a tiny bit? the ideal tool would be a long rod (~35cm) with a high density foam cylinder on one end and sanding paper stuck to it - attached to an electric drill. any other ideas?
the frame is carbon btw.

[mrcharly-YHT]

Unless you use carbon seatposts, I wouldn't sand away the inside of a CF frame.

Use aluminium seatposts and slightly reduce the seatposts instead. Much easier, cheaper (if you bu%%er it up) and safer. Bit of fine wet and dry, sand away.

[zigzag]

i use cf seatposts too (alloy ones are too rigid and heavy), therefore i'd like to modify the seat tube. i think increasing internal diameter by 0.05mm would make the difference i'm after.
i've been using a plastic shim with a thinner seatpost which works well, however i'd like the seat tube to match the blueprint ideally.

[The French Tandem]

.05 mm in diameter is just .025 in thickness. If you sand halfway on the frame, and  halfway on the seatpost, that's just a loss of .0125 mm in thickness, something I would not worry about. I would just do it by hand, with a lot of patience. Forget about the electric drill, or you will run the risk of eating away a too big chunk of material.

[Biggsy]

Or try some alternative seatposts.  One post can be looser than another even when it's supposed to be the same size, due to differences of manufacturing tolerances.

[zigzag]

Or try some alternative seatposts.  One post can be looser than another even when it's supposed to be the same size, due to differences of manufacturing tolerances.

i've tried two, and both are tight in a similar way. those same seatposts fit just right in another frame.

[zigzag]

.05 mm in diameter is just .025 in thickness. If you sand halfway on the frame, and  halfway on the seatpost, that's just a loss of .0125 mm in thickness, something I would not worry about. I would just do it by hand, with a lot of patience. Forget about the electric drill, or you will run the risk of eating away a too big chunk of material.

i was thinking that a drill on low revs should do any damage. another idea i just had is to use a track pump shaft and somehow attach a soft abrasive ball/cylinder onto its end.

[Chris N]

A flex-hone in the right diameter and abrasive material might work: http://www.brushresearch.com/brushes.php?c1=1  I've used them on stainless steel bores in the past and had good results.

[Jurek]

I was thinking of exactly the same as that ^ but couldn't remember what they are called.
Used to use them extensively in the workshop I first worked in after leaving college, to achieve accurate bores.

[zigzag]

is there a place in/near london where i could take my bike for this sort of job?

[Jurek]

I have my doubts, as doing what you are asking is erring into bodge territory.
It isn't really the territory for an outfit specialising in carbon fibre lay-ups.
Nor is it the territory for someone equipped to cut/shape/form metal.
It falls somewhere between the two, and I can envisage workshops equipped to do either of the above, backing away. Rapidly.
That's not to say that it cannot be done.
Embrace the challenge.
If I were stuck with the problem, I'd probably go for the flex-hone brush one size up from the bore you are trying to achieve, and just be very judicious with how much you use it.
Spin a little.
Measure a lot.
Interested to know how you get on.

[Chris N]

Agree with that. And use a fine abrasive if you've got the choice, so you remove material more slowly.

[Jurek]

Heh!
I was about to append my post with 'See what Chris n has to say about the above' or words to that effect.