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Sheldon Brown schreibt das:
Dishonest Frame Sizing
When you see a small frame size that features a higher bottom bracket than the other sizes of the same model, you're seeing dishonesty and deliberate deception.
This isn't nearly as common as it used to be, since the abandonment of a slavish dedication to the level top tube, but used to be widespread.
Here's how it works:
Let's say a given model comes in 25, 23, 21 and 19 inch frame sizes. A shop might have a 21 and a 23 on the floor. A small rider comes in, tries to stand over the 21, but it's too tall. She figures, well yeah, that's too tall, but the 19 ought to be two inches lower, and I'm sure I could fit a bike two inches lower, so I'll order one.
However, the 19 inch frame has a 3/4" higher bottom bracket, so the standover height is actually only 1 1/4" lower than the 21" model! Now, if the manufacturer really cared about customers who need the smaller frame size, they'd install shorter cranks on the 19". This would actually permit them to _lower_ the bottom bracket of the 19" size, since there's less of a pedal strike issue with the shorter cranks.
The problem turns out to be basically that the wheels are too large for the rider. However, there are marketing difficulties in selling a bike with smaller wheels, so that's an ignored option.
If you want to build a 19" frame with full-sized wheels _and_ a level top tube, you wind up with an itty-bitty head tube and steerer. This creates issues with the headset and handlebar stem, so they can't go below a certain minimum head-tube length.
That's where the cheating comes in...if you raise the bottom bracket, you can make a frame you can call a 19" and still have the level top tube and a reasonable head tube.
As I mentioned, this is no longer that common, since the sloping top tube eliminates the problem, but short riders looking at older used bikes should beware this scam.
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