My experience matches your numbers. I've run 52-42 front with 14-28 FW with Campy NR RD, but you have to get the chain length just right. You can shift into large-large or small-small without jamming or dropping the chain, but these combos do not run smoothly.
So to decrease the small ring to 39T, one would have to decrease the large ring to 49T as you say.
The chainstay length and whether the DO's are horizonal or vertical also play a role in just how wide a gear range you can get to work with the NR RD.
Regards,
Jerry Moos Big Spring, TX
John Betmanis <johnb@oxford.net> wrote:
At 10:13 AM 12/18/06 -0800, Vladislav Luskin wrote:
>Though Campy doesn't tell us so, we know that we can use a 42-28 low gear
with a Nuovo Record rear derailleur. My question is can I run a 39-28?
>
Per Sutherland's, NR max freewheel size is 28 teeth and maximum total capacity is 24 teeth The latter is the difference between the smallest and largest freewheel sprockets plus the difference between the smallest and largest chainwheels. So if your smallest rear cog is 14, you can't have a chainwheel bigger than 49. Now, that's the rule, but I don't know if it takes into account that you won't be using the big/big and small/small combinations. I have 42/52 on the front and 14/28 on the rear, which is the "max allowed" according to the rule and no noise or problems even with the small/small combo.
John Betmanis
Woodstock, Ontario
Canada