Quote Originally Posted by Dave Kirk View Post
Hey Hansen,

Thanks for writing.

The orange bike uses normal OS tubes (1 1/8" top and 1 1/4" down tubes) but the Big Blue Bike (BBB) uses the XL stuff.

Yes it's perfectly acceptable to use a top tube as a seat tube assuming the butt lengths work out properly for both good solid clamping of the seat post and a long enough butt at the BB end. The seat tube in the BBB is 953 and is most often used as a downtube. It has long butts so both ends work out well and it's silly thin in the middle to keep the weight in check. It works very well.

One could use a heavier wall and end up with a more common post size but the tubes are more than strong enough even with the very thin walls so I'm opting to cut down the weight.

What do you think?

Dave
Sounds good to me! I guess in the end since there's no way to use a 27.2 post in a 31.7 ST without either a shim or a pressed in collar using a 30.0 or a 30.6 post really isn't much difference.
I'm contemplating using a 31.7 ST for my next project but being that it would be 54cm I think I would end up with no more than 80mm for the butt length at the BB and 50-60 mm at the seat lug end. Is that sufficient assuming use of a clamp on front derailleur?

Thanks,
Hansen