Originally Posted by
Slaninar
Short answer:
Try a 30.9, if you can't find a 30.8 seat post. If it slides in without using the slightest hand pressure (not forcing it), it's too big and you'll need a 30.8.
Another way would be to measure the frame width, or check manufacturer's specs - what seat post diameter was it designed for.
Long answer:
Everything is machined to some tolerance. So there's always a difference between a nominal size and the actual size (there's also a measurement error, just to make things more interesting). So your post might be of 30.9 nominal width, and the frame might have been manufactured for 30.9 - which is a lot more common size.
For even more details - how to measure, what the standards are, how to "round up" the measured diameters, I put it on a web page, with a few pictures and table charts for help, the link:
Seatpost diameter sizes (standards) - Bike Gremlin - Bicycles
Thanks. After doing a fair bit of reading. I found an old post from 2010 with the same question and they went 30.9 without a problem. I measured the post with calipers (analog) and it looked like 30.9. In the seatpost tube it fits loosely and there is a small amount of play.
I looked at the Thompson from CRC and they only have one inline post at 30.8. I wanted with layback.
I found a 30.9 Truvativ T40 for a great price. Ordered that and waiting for it to come in.
Cross your fingers