lgraba

How close do you have to be on seat post sizing? I need to replace the seat post on my daughter's bike, and I've measured the existing post at 1.24 inches, or 31.5 mm. When I go online and lookup sizes, I see 31.6, 31.4, 30.9. When I remove the existing seat post and measure the seat tube inner diameter, I get 1.25 inches or 31.75 mm. This would suggest I get a 31.6, but would that be too tight? If I drop down to 31.4 or even 30.9, will that be too loose? What is the common wisdom on this?

CACycling

You should really try to get a replacement of the exact diameter. First, look closely on the original seat post. Quite often the diameter is stamped in somewhere below the minimum insertion line (it may be very lightly stamped so look really closely).

If the size isn't stamped on, carefully measure the existing seat post with a good set of calipers to see what you come up with. Seat posts are sized every 0.2mm for the most part so it is highly unlikely that 31.5mm is the actual diameter of the one you have. Good luck.

sivat

The alternative is to take the frame to a shop that has a seatpost gauge. Put the gauge in until it stops, and you have what size post you need. .2mm too big and you will have to force the post in, with the possibility that it will never come out. .2mm too small and the post will slip.