Originally Posted by
unworthy1
if your measurements are accurate, a 28mm seat tube (Metric tubing) with PROPERLY sized 26.8 seat post means a wall thickness of 0.4, which is very thin-walled. This is in the neighborhood of Reynolds 753 tubing, lighter stuff than 531DB or Vitus 172 DB,
AFAIK. Typical seat post for Metric 531 DB should be 26.4 (or sometimes 26.6) and note there's no butting in the top of the seat tube.
So I am still in doubt of something being correct in this here scenario: either the OD of the seat tube, the size of the seat post, or that the post fits that tube.
But I will stand to be corrected...
Seat tube diameter is approximately 28mm. Depends where you check and there is, of course, some marigin of error, even with digital calipers
Didn't get close to 28.6 anywhere on that tube, so I'm going with 28mm.
According to the catalogues and corrective information on this forum, the top tier of Motobecane bicycles from around 78 / 79 should have 26.6mm seatpost. But, I did ask about Motobecane seatposts before and got some answers here:
Motobecane C5 / C51 1979 / 1980 seatpost diameter
Very good info from Verktyg there.
The seatpost currently in this blue Motobecane is a modern, but cheap and cheerful seat post I got based on the measurements by the previous owner. I have ordered a nicer Stronglight aero, but wasn't sure if it's going to arrive on time and ended with two 26.8mm seat posts. I have some seat posts in various diameters, so I will be able to test this seat tube. But, the current seat post seems to fit pretty well. Whether this would be measurement stated by Motobecane when the bicycle was being sold in a shop, I don't know. If anything, it would seem that it might actually be undersized. I'm not basing this on any scientific or measured qualities, I'm basing it on the tactile feedback when installing it. I will run some more thorough tests and measurements next week, for now I needed the bicycle on the road, so the only thing I did was to gently go with a rounded file around the inside of the seat tube, but that didn't do anything to the actual wall thickness.
The thing is, how well these seat posts I have have been measured before the stamping. What was happening to the seat tube during production of the bicycle? Did they choose the right seat tube or grabbed one from another source, because of some mix-up? How the bicycle was used throughout the past 43 years, was it ever knocked down, was the seat tube ever hit with something heavy? These are all questions I wouldn't be able to answer, so my attitude right now is:
1. Take a setpost that's close enough and see if it fits.
2. To loose? Grab one with a larger diameter.
3. Too tight? Grab one with a smaller diameter or if it's just a bit too tight, a piece of sanding paper.