Decadence

Hi, I just made an impulse gumtree purchase of a full-carbon bike (I've never owned a carbon bike before), and just noticed a bit of a bulge where the seatpost clamp goes over the seat tube.



It's sorta hard to see, but it appears the seatpost clamp was overtightened a bit, causing what at first I thought was a crack under the paint. I've been trying to eyeball it to see if there is any visible cracking in the fibres, and it doesn't appear to be cracked.

Is this anything I should be worried about?

nomadmax 

I'm no engineer but that doesn't look good

dsbrantjr

Hard to tell from the angle of your photo, but it looks like the slot in the seat tube tapers in toward the top, which could indicate an undersized seat post. While it doesn't look great, I don't think that a failure at that point will be hazardous. I'd ride it and monitor it for crack progression.

mrt2you

i agree someone used a to narrow seat tube or over tightened it.
if you look closely you can see a horizontal crack below the black and to the left of the cutout. i would worry about that.

shelbyfv

Check that the current seatpost is the correct size.

Crankycrank

To my eyes it's impossible to say for sure with that photo. Could be a crack or just sloppy CF construction. I say take it to a shop where they can give it a good look in the flesh.

Andrew R Stewart 

Is the separate clamp fully seated, looks like it has drifted up and off the ST a tad. Or maybe the frame was designed for a clamp with a longer skirt. Was the frame purchased through a seller that you can stand across a counter and show the possible problem to? Did the purchase have a return policy?

Unlike metals carbon structures don't typically (ever IME) have a stretch or malleability after the resin has set up. So removing the post should let the ID return to the factory created one (whatever that is). Far easier to judge the post's fit being the intended one compared to a metal frame where the ST binder area could stay reduced in if a smaller then intended post was fitted and tightened. Andy (who will not yet stand up on the soapbox looming on the horizon)

AnkleWork

Worried enough to chat about it, but not worried enough to disassemble it and look closely.

2cam16

I wonder if it also has something to do with the seatpost clamp/collar in the wrong orientation? As far as I know the slits are not suppose to be aligned.

mpetry912 

2Cam brings up something important - the clamp slots can pinch.

the clamp you show is too far up on the seat boss. Ideally you'd put the clamp bolt in front so the clamping pressure is evenly distributed.

seat post diameter might be off as well, suggest a measurement.

Mark Petry
Bainbridge Island, WA USA

Decadence

Thanks for the replies everybody. I loosened the clamp and found that the seatpost has a small amount of play. It's probably only a millimetre or two but it is visible, and easy to feel when rocking the seatpost side to side.

Anyway, here are some more photos - a front-on shot of the clamp slot as well as the shape of the downtube (and seatpost by extension):





I'm guessing that seatposts on carbon bikes, like those on alu or steel bikes aren't supposed to have even a millimetre of play... does anyone know what type of seatpost I should search for if I want to find one that is the correct size?

Cheers

Decadence

After some searching, it seems that the bike comes with a proprietary seatpost

Which begs the question - if the seatpost that came stock with the bike isn't the right fit, how might this have come to be?

edit: more pertinently - since it would take considerable time (possibly for naught) waiting to find that specific part to appear for sale, and potentially not even solving the issue (since the seat-tube could be incorrectly sized) are there options for creating a shim?

Andrew R Stewart 

Aero shaped post... have fun! That there's a mm or tow of slop isn't a good sign. I take it that the frame was offered without it's specific post? Did the offering list a post or mention this aspect? It is easy to claim the frame/post are not well matched. If the post is the "proper" one then it's a sign of poor design and/or manufacturing, not typical of better brands these days. I'll take the lack of answering my questions about sourcing and such means that this frame wasn't bought from a shop with people that you can go back to.

Shim- Sure but not the best solution, though perhaps the only viable one if the post is the "intended" one. Shim stock is quite available through various industrial suppliers like https://www.mcmaster.com/# There's the time old method of trying soda pop can material but the Al these are made from do tend to tear and not hold up as well as brass or steel.

Good luck. Andy (who's off to the Philly show to see nice bikes and meet friends)

shelbyfv

Providing basic info will save time for you and those hoping to provide you assistance. What is the bike? What seatpost was in the bike when you bought it- brand and size. What seatpost do you think should have come with the bike when new? Otherwise you'll just get 10 more replies of "maybe" this or that.

Decadence

It was a full bike purchase, not just the frame. The seatpost/seat tube is proprietary (ie. no other seatposts fit besides stock/official ones) so it seems to indicate bad design/manufacturing as you pointed out. It is a GT Expert with upgraded groupset + wheelset, and as mentioned in my original post was purchased through gumtree, so no refunds or returns.

I think I'll junk the frame, sell the forks, groupset and wheels

TiHabanero

"I think I'll junk the frame, sell the forks, groupset and wheels"

Wise, very wise decision.

AnkleWork

That's rather drastic and wasteful considering you don't really know whether or not there is an actual problem. If it was made that way, why not just ride it and monitor for problems? [rhetorical question]

Crankycrank

Why not try to put a clear epoxy coating (or 2+ coatings) on the seatpost and build up the thickness until there is a snug fit in the seat tube?

Decadence

Looking inside the seat tube, it does appear that there is a small crack (about 7mm). I tried securing the seat clamp to the point where the seat post won't move up and down, then tried moving the post side-to-side and I can feel the base of the post hitting the inside of the seat tube...

Maybe I'm worrying about it too much but tbh it feels like a time bomb waiting to go off

Decadence

I'll try this. Should be better than a shim since it will go the entire length of the seat post instead of just a section of it.