Dale Phelps wrote:
> after reading the recent posts about 26.2 (1.031 OD) being "proper" for
> this vintage, I went out with my trusty Mit calipers and measured that
> bore several times and find that the nominal bore of the seat cluster
> up at the edge as well as down inside is ~1.020." I cannot imagine it
> to be distorted as consistently as it measures, yet it appears it
> should take a 25.8 post!
>
> Heavier tube? A Riviera? Or.....??????
I've never owned a Cinelli so this is conjecture, but no tube on a lightweight bike could ever be that thick. I'm certain Cinellis must take such a small seatpost because there is an added sleeve inside the tube to strengthen the seatlug area. A smaller post could just indicate a thicker sleeve was used on yours.
If I'm right about how they're made you should be able to feel for the bottom of the sleeve by reaching down inside with a spoke head or similar hooked end on a wire. Though if they machined a taper on the bottom of the sleeve, then the bottom lip of the sleeve may not catch the hook. You can also often see some features (like tube butts) inside a seattube by shining a flashlight in the bottom bracket while peering in at the seat lug, with everything clean and oiled inside -- the oil sheen bounces the light around in a useful way
This seat tube sleeve is a good design IMO, for making a bomb-proof reliable seattube-seatpost interface. Lots of bikes with typical single-butted seat tubes and 27.2 seat posts have problems down the road, and adding this sleeve improves reliability without adding much weight. The bike might even be lighter overall with the smaller seat post. I like how the reamer is reaming only on the sleeve, not on the tube, and when the reamer passes through the sleeve it hits only air below it. Reamer scrape marks can act as stress-risers that initiate fatigue cracks, as well as the problem of locally thinning the tube wall from reaming when there's heat distortion (which of course is there to some degree on every brazed or welded frame ever made).
I've used this method a few times -- silvered in a sleeve to repair a frame that was damaged because the seat tube was so thin at the top -- fatigue cracks for example. I've also used it to make nice looking lugless frames where a visible outer sleeve around the top of the seat tube wouldn't fit the aesthetics of the rest of the frame.
Internally sleeving the top of a single-butted tube mimics a double-butted tube. So you may well ask, why not just use DB for the seat tube? And a few builders do, but the main reason most don't is the problem of different frame sizes: Tall frames would get more butt at the top than they need, and small frames risk having all the butt cut off. Putting in a short sleeve just at the seat lug gets the job done reliably at minimal cost, in weight and in lire.
Mark Bulgier
Seattle, WA
USA