Bicycle Mechanics - What's the "ideal" chain line?

I wanna change to a slighty wider bottom bracket to improve my Q-angle, but I think it will stuff up my gears.

I have a 'normal' double chainring set-up, and I'm still using 8 speed Shimano.

So, what cog should the inside or outside chainring line up with?

Thanks :)

the space between the 4th and 5th cogs should be a straight line to the space between the chainrings.

When you say "improve" your Q, you mean increase it. Most modern cranks have a wide Q compared to older designs. Narrower Q is more aerodynamic, and generally works better, but you may have wide hips, so Ill shut up.

Thanks.

Recently I've been straining my vastus mediali during really hard rides, so I'm just experimenting with a wider Q-angle and some increased supination to see what happens.

Instead of changing your BB, try some kneesavers from LooseScrews: http://www.loosescrews.com/index.cgi?d=single&c=Pedal&sc=Adapters&tc=&item_id=SC-PDA&id=623225112551

Nutbag's inquiry implies a very important (and overlooked) question, for which I do not have a ready answer -- how does an individual determine his/her own optimal Q-factor?

If one needs a wider stance and is having knee problems, to me the obvious solution is to change from a double chainring to a triple, thereby picking up some potentially theraputic lower gears at the same time.

My personal experience with Q-factors is that I have one road double with a 70mm BB shell (Bianchi), one 68mm road double (UO-8), and three 68mm triples, including my mountain bike. All of these feel OK to my knees. However, I did have a 73mm aluminum-framed mountain bike with a triple, which did occasionally give me some knee pains. Since this was my highest-Q bike, the stance MAY have been too wide for me. (I closely monitored and ruled out my initial suspects of saddle height, tilt, and fore-aft position.)

Thanks for all the info.

Tinker, those Kneesavers are a great idea, but I think it will be too big a difference :)