Yes the relaxed seat tube angle is for getting Knee Over Pedal Spindle in the ball park with short cranks. While I don't think that a strict KOPS is required its certainly helps in getting in the ball park. Bike manufacturers use steep seat tube angles for short people, A, because its a dodgy way to shorten the top tube specifications without really shortening the front centre distance and B, because when you are using cranks that are too long for your legs then it does help to stay on top of them as it opens the leg angles. The number 1 downside of this is that riding with such a steep seat tube angle (and corresponding lack of saddle set back) is that a phenomenal amount of weight is placed on your hands/wrist/arms which makes riding a bike VERY uncomfortable.

Yes, short cranks are more expensive yet in the scheme of things they are cheap compared to the cost of the frame. Short cranks for people with short legs greatly reduces the leg movements which reduces leg rub. It takes the stress off the knees and when you combine this with the relaxed seat tube angles required for KOPS then the riders weight is taken off their hands/wrists/arms making the whole riding experience MUCH more enjoyable for the small rider.

When I first started thinking about and working with these issues I thought that I had come up with something new. Then I came across some OLD small bikes. All of a sudden there everything was, short cranks, low BB's, relaxed seat tube angles and short front centre distance. So, nothing new under the sun. We USED to know how to build bikes for small riders. The trouble is that this knowledge was lost in order to make bikes cheaper by using less different sized parts for different sized riders and less variations in frame designs to accommodate different size riders.

Anthony