Frame design, in many ways, was much simpler when I was building back in the 1980s.
Top tubes were always level; it was not acceptable, to me or my customers, to build a frame with a sloping top tube.
Once a rider had established his correct frame size, he set the handlebar stem about 4 or 5 centimeters above the head bearings.
Then he set his saddle to the correct height and he was for the most part, good to go.
The handlebar stem could be adjusted a centimeter up or down as the rider wished.
The other point that made everything simpler was the fact that a person could go buy a frame of any make, in the same size, and the seat to handlebar height ratio would be the same.
If the top tube length varied slightly it could be corrected with a longer or shorter stem. Handlebar drop never even entered into the equation because it was automatic once you had the frame size right. The level top tube was in fact a point of reference.
Many years before I started riding, the wheel size for a racing bicycle was set at 27 inches diameter, or 700c as it is known today. This means that there is a fixed distance from the ground to the bottom of the head tube. This is always the same for any size frame with 700c wheels.
On a level top tube frame, it doesn’t matter that the builder changes the bottom bracket height. If he raises it, and the seat tube length remains the same, then he also raises the top tube and the head tube becomes longer. This is because the bottom of the head tube remains in a fixed position.
The rider’s saddle height is measured from the pedals (Or BB center.) to the top of the saddle. So although the rider is sitting higher because of the high bottom bracket, because the head tube has become longer by an equal amount, the seat to handlebar height difference always remains the same for any given size.
Each frame size will have its own saddle to handlebar height difference, which increases as frames become larger, decreases for the smaller sizes. On a modern sloping top tube frame, raising or lowering the bottom braket height will not necesarily alter the head tube length.
With today’s design, the bottom of the head tube is still in a fixed position, but the top of the head tube can be anywhere; it is not governed by a level top tube as it used to be. There seems to be no standard point of reference between the different manufacturers.
Where today’s design has an advantage it is in building very small frames. The bike pictured top left, is my personal bike; it is a 51 centimeter. (Center to top.) You can see in the picture that if the position of the bottom head lug is fixed, a framebuilder can only lower the top tube another 2 cm. and the lugs merge. To all practical purposes a 49 cm. is the smallest level top tube frame he can build.
The only way to go smaller is to shorten the seat tube by raising the bottom bracket. This really goes against the requirements of the rider, because the last thing a person with short legs needs is to be higher from the ground.
I was asked just this week how would I go about designing frames for women. If the woman was 5’ 4” or taller it was no problem; I would just build according to the customers measurements as I would for a man.
If the woman was less than 5’ 4” then it was not so much a case of building a frame to fit, but one of how small can I build this frame? There is not only a limit to how short can I make the seat tube, but there is a limit the how short one can make a top tube.
The whole problem is fitting two large wheels into a frame that has reached the limit for those size wheels; it restricts what you can do. Smaller wheels are available but rims and tires are limited to a much narrower choice than for the standard 700c.
Frames are now sized like tee-shirts; extra-small, small, medium, and large. My advice to female under 5’ 4” would be to buy a frame in the smallest size possible. There will be no problem with the seat tube length, and there is now a far wider range of handlebar stem lengths and angles than were ever available in my day.
With this wider range of handlebar stems, it should be possible for most riders to dial in a near perfect position. The only problem as I see it is that another dimension has been added to the equation, and that is handlebar drop.
In the old days one only had to concern themselves with saddle height and reach, drop took care of itself with the correct size frame.
I always maintained that a rider’s arms (On the drops.) should be in direct opposition to the legs. Choose a combination of drop and reach that will achieve this.
Back in 2007 I wrote an article which included a chart that gave a drop measurement and was based on my old fit philosophy from the days when top tubes were level. Some people have found it useful.
Your comments and input as always are appreciated.