As far as I know where older 531 is concerned, there is no such thing as a "double butted" seat tube. Seat tubes can be straight gauge or single butted and are only butted on the bottom. There are standard gauge and heavy gauge seat tubes. The heaver seat tube will take the smaller seat post. For the most part French (metric dimention) seat tubes run between 26.4 and 26.8. Most common is 26.4 and 26.6 from what I've seen. I've seen some frames that use as small as 26.0 seat post, like my 1940's Follis touring tandem stashed under the house awaiting restoration.
Can't explain all of the variations; but like someone said "hey, they're French". That explains everything.
Brian (I don't have anything against Frogs... they taste great!) Baylis
La Mesa(Tastes like Chicken),CA
> I think we discussed this a while back, and one of the framebuiders said that
> you can't determine seatpost size from tubing type alone, as tubes are often
> reamed for a certain size post. I haven't seen any 26.8mm posts on PX-10, but
> I've seen a lot of 26.4 and 26.6 posts on PX-10s and other French bikes that all
> used the same 531DB metric gauge tubing. All the 531 french bikes of the era
> should have had the same seat tube OD (slightly smaller than English gauge 531),
> so I don't know if there was a variation of 531 wall thickness or just reaming
> that accounts for the post sizes. To reveal my ignorance, aren't some seatubes
> only single butted rather than double butted? I'd think a single butted tube
> might be butted at the BB end where forces are greater, but I don't really
> know. Could metric 531 have varied over the years between single and double
> butted seat tubes or as to the ID of the butted section? The only pattern I
> have noticed is that French bikes with 26.4 posts are more common in the early
> 70's and less common in the 80's, but the pattern is far from consistent.
> Jerry Moos
> Eric Elman wrote:
> > Riddle me this Bike-Friends.....
> > My 1968 Peugeot PX-10E has an original 26.8mm Simplex seat post
> > My 1972 Peugeot PX-10E has an original 26.4mm Simplex seatpost
> > My 1977 Peugeot PX-10LE has an original 26.6mm Simplex seat post
> > All three have Reynolds 531DB French decals, all three have 1/2 chromed rear
> > stays.
> > Why would they have three different seat post sizes????????????
> > Eric "always finding something to keep me perpetually confused" Elman