When custom designing a frame, is there a particular stem length that is considered ideal, or is it depending on torso to arm length or some other factors?

I saw this post (http://bikeforums.net/showpost.php?p=5417474&postcount=5) which states that per Specialized, 100mm for 56cm and 110 for 58 is considered ideal, but why is that? Why not a longer top tube by itself?

I like to keep the stem length in proportion to the frame size so ideal in my world is:

<50cm frame – 90cm stem
50-53cm frame – 100mm stem
54-57cm frame - 110mm stem
57-62cm frame – 120cm stem
>62cm frame – 130cm stem

So there is no real science to it?

So there is no real science to it?

The common range of stem length spans from 90 to 130, with 110-120 being the most popular. The stem length has some bearing on how the bike steers since longer stems create a longer lever arm on the fork; shorter stems make the bike steer faster compared to longer stems. Some people also site the fact that with a longer stem there is more weight on the front end; lightweight racer types with low upper body mass might care about this. In the real world though, the common range all works fine; the amount of trail has more influence on steering speed than stem length and most riders have enough meat on their bones to effectively load the front tire. Because any stem within the common range seems to function fine, fitting the stem based on proportion seems reasonable – at least to me.

Special note: I’m not talking about picking the stem length for use on an existing frame based on some aesthetic point of view, I’m talking about designing a custom frame with a specific stem length in mind.

Needless to say, this is my point of view.

It also depends a lot on how flexible the rider is and how stretched out they want to be. And some people have longer/shorter upper bodies than others.

custom frame builders all have their own different theories on fit, this is just one of them.

http://www.hampsten.com/Company/design.html

Frame Fit:

A properly fitted bicycle is centered on the stem, which we think of in terms of sizes: small (100-105mm, for frames in the 50-54cm range), medium (110-120mm, for 54.5-57cm range), and large (120-130mm, for 57.5cm and up range). A bicycle built around the proper stem length – giving correct reach - will have optimal weight distribution and handling characteristics. We believe that the cure for a poorly fitting bicycle is not a longer/shorter stem but rather a well-built frame using the correct length stem.