You could try the sizing system at www.wrenchscience.com

There is no formula (that works). Reach is a function of three fit variables: Top tube length, stem length and saddle position horizontally relative to the bottom bracket. Torso length is only one indicator that will control those variables. I wish it were as easy as pluggin in a set of number to a formula....

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Tom Demerly
The Tri Shop.com

Not aware of any magic formula for top tube length. Functional top tube lenght can be modified by stem length or by adjusting the handlebars up/down. Putting the handle bars lower effectively lengthens top tube reach while elevating the handle bars does the opposite.

   As a layman I've read 100s of books, websites, forum posts, emails, etc., etc.... on the subject of bicycle fit. Many of them offer differently packaged versions of the same thing. Many are just downright confusing. Most will get you in a VERY ROUGH ballpark, like Yankee stadium with no outfield fence. You won't be hitting many homeruns by relying solely on the formula.

The reality is that no two people are exactly alike, even identical twins. Even on the off chance that measurements are the same, the likelihood that one is not as flexible as the other is inherently great. On top of that, tendon insertion points may be different, causing one rider to be more powerful in a more upright but less aero position. Fact is, there are hundreds of physiological variables that go into fitting on a bike. The only tried and true method is to use someone knowlegeable like Demerly and continue to fit with them over time as you dial in the bike. An added plus would be if they use a Computrainer or simlar device to give actual performance feedback in the positions. And even when you get all the variables exactly the same on two different bikes with regard to how your body fits to the saddle, crank and bars, some bikes will just not feel right due to how that equation relates to the distribution of weight on the ground and handling characteristics. That was the case for me and the Cervelo P3. I spent months at the local shop trying to find the right combination of parts that positioned me correctly and got that "just right" feeling. I never found it. I'm now on a Saber.

I'm of Tom's school, and was long before I came to these forums, that fit is overwhelmingly the deciding factor in one's performance on the bike. I don't care if you ride a $10000 Colnago Carbonisimo. If it doesn't fit, you'll be slower than on a properly fitting QR kilo. Fit the bike to the rider, not the rider to the bike (no offense to Giant, but you can't get there from here with 3 or 4 sizes without making compromises. Only a limited number of people will fit on those bikes PROPERLY).

(no offense to Giant, but you can't get there from here with 3 or 4 sizes without making compromises. Only a limited number of people will fit on those bikes PROPERLY).


Recently read that some of the Team Once members get custom sized frames if the standard three size option doesn't fit.

Yeah, I read the same article, cerveloguy. Funny, I don't remember Giant offering custom frames to consumers, even at additional cost.