Seat post size Btwin Triban Trail (any other information appreciated too)

[add595]

Hi all,
I recently rescued this Decathlon Triban Trail 3 from a salvage yard (no way was I leaving the poor thing there). I've no idea what seat post size it takes & can't seem to find any information online, does anyone know? Thanks in advance.

Also a follow up question, this one is 700c yet some I find online are 26"? Does anyone know why there were different wheel sizes for what appear to be the same bike/frame? Also any other info about these would be appreciated. Thanks!

 

[hamster]

Easiest is to go to a decent bike shop, they will have a conical seatpost gauge.
Alternatively buy yourself a cheap vernier and measure it. £14 from Screwfix or a plastic one will do the job for £3 on eBay.
https://www.ebay.co.uk/itm/38537111...gVpmjKAEExGtSKPd+qiGlcpV0=|tkp:Bk9SR8738t-WZA

 

[add595]

Easiest is to go to a decent bike shop, they will have a conical seatpost gauge.
Alternatively buy yourself a cheap vernier and measure it. £14 from Screwfix or a plastic one will do the job for £3 on eBay.
https://www.ebay.co.uk/itm/38537111...gVpmjKAEExGtSKPd+qiGlcpV0=|tkp:Bk9SR8738t-WZA

Thanks, I was hoping to find someone who had one that could check. I've checked it with a caliper & I think it's 29.6 or 29.8 but the top of the tube isn't quite 100% round so hard to get an accurate measurement. I've ordered a 29.8 to 27.2 shim (I've got several 27.2 posts already), I'll post again once I've tried it

 

[add595]

Update: 29.8mm shim fitted but there's quite a bit of play which is weird as in the meantime I borrowed a 30.0mm post from a friend to try out and found it to be too tight? Upon measuring the "29.8" shim I found it's actually 29.6! Conclusion: 29.8 is the correct size post required.
I'm gonna order a better quality shim or just order a 29.8 post. I'll have spent more on seat posts/shims than I have on the bike itself at this rate