Quoth Marcus Helman:
>I know most of the literature says don't use the small-small or big-big
>combinations. Doesn't this generally refer to triples and/or modern bikes
>with wider spacing? I would think that on bikes in the CR timeframe, with
>double chainrings, cross-chaining would not be as big of an issue, unless
>the chainstays were really short.
Yes, but...
There are two issues involved. One issue is the _angle_ of the chain, and, indeed, this is greater with triples and with short chainstays.
The other issue, however, is sprocket _sizes_ and this is, in my opinion, _much_ more important to the wear issue than the angle.
Consider a bike with 52 and 26 chainrings, 14 and 28 rear sprockets. The criss-cross gears 52/28 and 26/14 give exactly the same ratio...but...
The chain tension is inversely proportional to the chainring size, so there will be exactly _twice_ as much chain tension when you're pedaling with the 26 ring as with the 52.
Meanwhile, back at the hub, assuming the chain wraps halfway around the sprocket, you're lucky if the chain is in contact with 7 teeth of the 14 tooth sprocket, vs. 14 teeth with the 28.
Thus, the 26/14 combination applies twice as much pull against half as many rear sprocket teeth, compared to the 52/28 combination. This is what kills sprockets and chains prematurely!
Large/large is likely to run a bit rough, and to cause somewhat increased wear to the sides of the the chainring teeth, but the small/small is the real killer.
Depending on chainline, it is often perfectly reasonable to use the
large/large combination. F'rinstance, my Hetchins is set up with the
50 tooth ring lined up with the middle of the cassette, so I can use
all 9 rear sprockets with it...but I only use the 5 or occasionally 6
largest rear sprockets with the 28 tooth ring.
>I only have 12 speeds, I'd like to use them all.
Not a good idea, unless your small chainring is larger than normal.
Sheldon "Small/Small=Abuse" Brown
Newtonville, Massachusetts
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