Bike Forums - Chain rub

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I need a little help on my new build. I'm running SRAM rival shifters and rear derailleur, force front and a FSA Gossamer 50/34 compact crank. Cassette is 11-27 sram 10 speed. When I'm in 34-11 and 34-12 the chain seems to rub on the inside of the the larger front chainring. That can't be normal, right? I can't figure out a way to get rid of the rub, any ideas?

As a general rule, you should not be crossing your chain from 34 to 11.

The only thing that could change that is a BB with a longer shaft to move the righthand crank/chainrings to the right, BUT:

When the chain is on the 5th cassette cog (counting from left to right), and the 34T chainring, does the chainline look straight?
If so, then take the advice of the previous poster and just avoid combinations that result in extreme cross-chaining.

On the other hand, if the 5th cog appears offset to the right from the 34T chainring, a BB with a longer shaft may be in order.

Edited to add: Doh! I was thinking in terms of a square-taper BB. That crank has outboard BB cups, so it would require spacers to move the right bearing to the right.

The chainline is straight when in the 34T and the 5th cog on the cassette so I guess I'll avoid the combos that rub. This is my first full build so forgive me if these are stupid questions but, is this a problem will most bikes or an I using the wrong cassette and crankset combo?

Everything you need to know:

http://www.sheldonbrown.com/gears.html

Quote:

Originally Posted by Sheldon Brown

Try to avoid the gears that make the chain cross over at an extreme angle. These "criss-cross" gears are bad for the chain and sprockets. Especially bad is to combine the inside (small) front sprocket with the outside (small) rear sprocket. This noisy, inefficient gear causes the chain to wear out prematurely.

And:

http://www.sheldonbrown.com/gear-theory.html

Quote:

Originally Posted by Sheldon Brown

A chain drive is one of the most efficient mechanical linkages there is. This is true as long as the chain runs in a straight line, but derailer gears don't always run the chain in a straight line. As you select gears that run the chain at an angle, the efficiency drops precipitately as the angle increases. For this reason, you should favor combinations that allow the chain to run nearly straight. Generally, the large outer chainring shouldn't be used with the largest rear sprocket, because of the sharp angle it creates.
Similarly, the small inner chainring shouldn't be used with any of the smaller rear sprockets.
The small-small combinations create extra problems besides the bad angle. The smaller the front chainring, the greater the pull on the chain; the smaller rear sprockets only engage a very few teeth with a very few chain rollers at any one time, so stresses are concentrated

Your experiencing a very common problem with compact cranks, particularly on frames with short chainstays. The large diameter difference between the large and small chainrings is the cause.

The only real "cure" is to avoid the small-small combinations. They aren't needed anyway so there is no loss in utility by avoiding them.