Joined: Nov 2006
Posts: 19,810
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From: NJ, NYC, LI
Bikes: 1940s Fothergill, 1959 Allegro Special, 1963? Claud Butler Olympic Sprint, Lambert 'Clubman', 1974 Fuji "the Ace", 1976 Holdsworth 650b conversion rando bike, 1983 Trek 720 tourer, 1984 Counterpoint Opus II, 1993 Basso Gap, 2010 Downtube 8h, and...
Charles,
You probably don't have to worry about the fork or the seat stays. The clearance issue is at the chain stays.
650b rims with 42mm tires (Hetre), the widest point of the tire is around 320 or 325 mm from the axle (I measured without taking the wheel out of the frame, hence the imprecison) and the tire is round, so the wide part is large.
On my 650b conversion, a Holdsworth 531 special from about 1976, I measured this as 45mm. And it's enough. I have fenders and the wheels turn and it's all good, but the clearance on either side really is as little as 1-2 mm.
You can get more clearance by hammering a piece of steel pipe between the stays. On my Lambert I used the head of a hammer and beat it toward the bb with another hammer. I'm sure it weakens the stays, but I figured it doesn't weaken them enough to really matter.
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