Originally Posted by
busdriver1959
Just a small correction. The 531 seattube is .9/.6. Not .9/.7. The OD is 28.6, subtract 1.2, (.6 x 2). The ID is 27.4. The correctly sized seatpost is .2 smaller. That's how you end up with 27.2.
+1 I recently had the time and inclination to measure the ID of one of my 531 ST with Starrett inside gauges (aka "snap gauges") and that's what I found: 27.4mm ID on a 28.6mm OD tube. Measurement taken about 30mm below the seat lug.
Makes sense as 27.2 post in a 27.2 tube would drag all the time IF no manufacturing tolerances got in the way. A wee bit of 'play' would work much better.