Mountain Biking - Top tube length formula - what you think?

Hi there,
I came across this formula in REI website. just wondering what you guys think.

How accurate is this formula to figure out the top tube length?

(Torso length + arm length) / 2 – 6" = rough top-tube length
Reference: http://www.rei.com/online/rei/learn/noDetail.jsp?URL=rei/learn/cycle/clfitf.jsp

I know personal preference and intended use are big factors in deciding these questions, but my point is, does this formula give a general idea of what to look for.

thanks.

any response, comments, suggestion regarding this formula? is it just a gimmic or what?

Doesn't sound too reliable to me. In order for it to be accurate you would have to be pretty accurate on your measurements considering that a difference of .5 cm can make a pretty big difference in fit. Also the formula does not take into account your prefered riding preference. Do you (the rider) prefer to be stretched out or set up more?

There are too many variables to take into consideration to be able to put any faith into a formula like that.

Also, I don't like "rough top tube length". That pretty much tells me how innacurate it is. There is generally only about 1 cm difference between bike sizes. For example a 14" Ellsworth Enlightenment has a 21.25 cm top tube, a 16" Enlightenment has a 22.25 cm top tube, an 18" Enlightenment has a 23.50 cm top tube. A "rough" top tube length wouldn't really help out too much.

i wouldn't say its entirely unreliable, the "rough top tube length" helps you choose amongst the frames out there, like the website says, that can be adjusted/fine tuned by varying your stem.

what it fails to mention is that the top tube length is one thing but the size of the cockpit is also affected by the angle of the top tube. i.e. a cockpit sizing formula should calculate not the top tube length but the horizontal length of the cockpit between handlebar and seat, note that this is also affected by the angle and height of the seatpost, which varies with your preferred riding style/position.

i wouldn't say its entirely unreliable, the "rough top tube length" helps you choose amongst the frames out there, like the website says, that can be adjusted/fine tuned by varying your stem.

what it fails to mention is that the top tube length is one thing but the size of the cockpit is also affected by the angle of the top tube. i.e. a cockpit sizing formula should calculate not the top tube length but the horizontal length of the cockpit between handlebar and seat, note that this is also affected by the angle and height of the seatpost, which varies with your preferred riding style/position.

I always just assume (bad word, I know) that when top tube is mentioned it means "effective top tube" and not "actual top tube". With compact geometry actual top tube is pretty much worthless.