Ride, Wrench, Swap, Race
Join Date: Jan 2010
Location: Northern California
Posts: 9,194
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
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I believe that 6s derailers will index accurately on a 7s freewheel, if perhaps less tolerant of extremes of the lateral positioning of the freewheel/cassette.
So I always try for getting the smallest cog as close as possible to the dropout without the chain actually rubbing or catching on the lower end of the seatstay (this only improves the chainline, as an added bonus).
It's when going to 8s that using an 8, 9 or 10sp derailer is pretty much a necessity if you want proper accurate indexing right up to the largest cog.
Don't be tempted to cold-set an aluminum frame's chainstays. They will crack before they bend and the cracks may be obvious or may be left hidden in the metal!
But it's ok to use a slightly-wider axle. 4mm is a lot but it seems like everybody gets away with it.
If you see the derailer twist or tilt as the QR lever is closed, that is a sign that the dropout alignment may not be playing nice with a wider hub spacing.