2mtr

on my Surly, with Shimano 105 derailuers up front and back, the darn thing rubs. The front is friction shift, so I adjust a bit. But in extreme combonation of gears (ie, large front cog and large rear cog, or the reverse), it will run crunching. I asked a mechanic, and he says I should never do this, as it is bad for the chain.
I've brought it into the shop five or six times, and each time they say it's good. But it never is, and I end up bringing it back in a week's time. What do ya'll think is up?
Feel free to label me an idiot if you must.

Astra

You answered your own question - don't use big/big or small/small combinations .

Chief

If you do the math, you'll find that big/big gives you about the same gear ratio as little/4th biggest, but you avoid the dramatic chain flex .... and so forth for little/little.
That's why the extreme gear combos are considered useless/unusable, and attempts to adjust the derailuers to precision are pointless.

2mtr

for history, if any one is using this as a refrence, the deal was that the front der did have enough torque to open up enough.