I appreciate everybody's input. Thank you.
I'm aware of the admonition against large/large and small/small combos but I will always want to use them, and if the chain wears out more quickly it's OK by me (126mm rear space, 6 speed). Rubbing the lg chainring is too annoying though; can't live with that
CS are about 44cm long.
I'll try moving the spindle to the right as Dale mentioned, but I DO like a narrow tread so I'm sorta boxing myself in as to the full range of solutions, aka longer spindle.
The real fix (I hope) is not screwing up the BB alignment (or skewing it intentionally a skosh the other way!). It took me a two frames to figure it out, but my BB fixture is pretty sloppy and allows the BB to find different skew angles at will, every time its installed. Well, not any more since I started using shims to firm it up. I may spring for one of better design, and come up with a better way to check for perpendicular on the surface plate - I'm equipment challenged in that department.
Is it not that uncommon to have to move the chain line to the right to combat this sort of thing (even with the BB square)?
Thanks again, John Clay Tallahassee, FL USA
http://healthcarefunding.blogspot.com/
Single Payer IS the compromise.
> From: Hilary Stone <hilary.stone@blueyonder.co.uk>
\r?\n> Subject: Re: [CR] Chain-rub With A Very Wide Range Double
\r?\n> To: "John Clay" <jmedclay@yahoo.com>
\r?\n> Cc: "Classic Rendezvous" <classicrendezvous@bikelist.org>
\r?\n> Date: Sunday, June 27, 2010, 4:00 PM
\r?\n> Correct chainline is important to
\r?\n> minimising chain rub. Short chainstays exacerbate the
\r?\n> problem of chain rub on the large chainring... With longer
\r?\n> chainstays the problem will not exist - so movingn the wheel
\r?\n> back in the dropouts might help. However you certainly
\r?\n> should not use the small to small or large to large
\r?\n> combinations.
\r?\n>
\r?\n> Hilary Stone, Bristol, British Isles
\r?\n>
\r?\n>
\r?\n>
\r?\n> John Clay wrote:
\r?\n> > I have discovered that when on the small ring and
\r?\n> smallest two cogs the chain rubs the inside surface of the
\r?\n> large ring. It's a Stronglight 99, 48/28 with shortest BB
\r?\n> spindle projection on right side possible. Is that a
\r?\n> known issue with wide range double arrangements? I haven't
\r?\n> calculated the geometry (will undoubtedly do so), and I know
\r?\n> that on this particular frame I skewed the BB alignment a
\r?\n> skosh, in a direction that would exacerbate this issue
\r?\n> (fortunately I know why). Still, I wonder if this is part
\r?\n> and parcel of such a large difference in the diameters of
\r?\n> the two rings. Anyone?
\r?\n> >
\r?\n> > John Clay
\r?\n> > Tallahassee, Florida
\r?\n> > USA
\r?\n> >
\r?\n> > http://healthcarefunding.blogspot.com/
\r?\n> > Single Payer IS the compromise.
\r?\n> >
\r?\n> >
\r?\n> >
\r?\n> >
\r?\n> _______________________________________________