Wasn't sure where to put this, sorry. Roadies/ MTB?

How on earth do you know how much material you can grind off of the bits that you tune?
Experience?
Blind luck?
Finite element analysis?(please don't tell me it's this, my brain can't handle it)

definetly blind luck , thats how i do it anyway

Blind luck

it depends on how well you have listening on the sciens lessions in school

You can always calculate and simulate the parts in a computer, but it takes time to draw it, but everything isn't the same in practical situations.

I guess most of us who is tuning things have been working with metal pretty much and know what's holding up and not.

personally I wouldn't cut in carbon parts for no reason, there I would be more carefull, but I'm not afraid from using it.

Common sense, feel, blind luck.

Balls!

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http://worksrider.wordpress.com/

Small chunks at a time, test, analyse, step back and sleep, and repeat. An extra 0.5g in one run isnt going to help you much, especially if it causes something to break, or even worse, you to go OH SH*T