But, things are not so simple anymore with Trek's new compact geometry. The seat tube, which is as slack as a old touring bike, is placed slightly ahead of the bottom bracket, so that changes all of the old wisdom.

The laser lights that they use in the fitting stations at the new bike shops takes all of the guesswork out of optimum knee postion though. Instead of dropping a plumb line from your knee, which is not possible to do accurately by yourself anyway, the laser projects a vertical line that you can see in the mirror as you are pedaling with your body in a correct postion. It is easy to see where the "line" is relative to your knee and the pedal spindle at 3 o'clock .