Is there any kind of simple computer application available that will allow one to tweak different geometry parameters with a real-time visual representation (just a "stick-figure" frame would do), with complete resultant dimensions/angles specified on the fly (so you could see, for example, how lowering the bottom bracket would affect other dimensions)? Such a program would be a big help when submitting a custom frame order, as well as being just plain fun.

Yup. Look here: http://javaboutique.internet.com/bikeCAD/

Yup. Look here: http://javaboutique.internet.com/bikeCAD/

also try http://www.bikeforest.com/CAD/bikeCAD.html

Anyone know why the left side of the view is cut off and how to correct this?

My only other comment is that it's too bad that neither software includes a reach dimension that would demonstrate how two frames with different seat tube angles and TT lengths can fit exactly the same. This is the most often overlooked and misunderstood aspect of frame sizing.

Thanks, that's just what I envisioned! Wish it had BB drop rather than height, though. BB height is such an imprecise, hard to define quantity (based on tires, rider weight, etc). I'd like to design a cyclocross frame, which I'd be putting all manner of tire sizes on.

The software lists the tire diameter and allows it to be changed. The BB drop is just the tire radius (1/2 diameter) minus the BB height.

The effect of droping the BB should be obvious. If for instance, if the BB has an 8cm drop instead of the common 7cm, then the top tube will be 1cm closer to the ground (assuming the same c-c frame size). Most builders probably ignore the minor difference in chainstay length, but FWIW, it's simple to calculate. To keep the center of the tire in the exact same position relative to the BB, the chainstay would be 2mm longer. The head tube length is independent and would not necessarily have to change, but if the same saddle to bar height and steering tube spacer setup is desired, then the head tube would be made 1/sinH shorter, which is about 1.05cm (might as well just say 1cm). Lowering the BB doesn't affect the fit of the bike, but will lower the center of gravity and may improve cornering, but reduces pedal/ground clearance.

Well, I just got CMG's link working, and that version uses BB drop! I'm in heaven.