Originally Posted by
Andrew R Stewart
Anthony- Why is this (in bold) a requirement? I do feel that the body needs to be centered (weight wise) over the pedals but as one pedal moves rearward (the one in front because of the short cranks) the rear pedal moves forward, so their center of balance is still the BB. If anything I've found that with shorter thighs and feet that a slightly steeper seat tube is a possibility.
Also fitting really short cranks is a wonderful concept if both the availability of them covered a wide selection of brands as well as the rider being comfy with that choice of leg movement. IME many riders are already use to 170mm arms as that's the standard in most current small adult road bikes (for many unfortunate reasons). I've known customers (at the retail shop) who don't like even a 5 or 10mm arm length change (let alone a 30mm one). True there are places for such a short arm (or even two different length arms on the bike) but these cases, for adults, are very uncommon and not usually found as a solution (that works well).
I do agree to place the BB drop such that whatever arm length produces the desired cornering capability (for road bikes). Andy.
My guess is if using cranks in the 140 mm range or less, the seat tube angle may need to kick back for pedal to knee positioning. A guess.
For a rider 5'-1"... 150-155 mm cranks have been useful to me this was for an offspring and leg to torso length ratios are different often.
I do agree that if you drop the bottom bracket as allowed by shorter cranks, all the better for the bike's handling.
If using 571 rims I would design to have a wide range of tire sizes to be used. I went with 650c as it gave a road bike look, like Dad's machine. I used a stamped bottom bracket shell as it allowed for easier angle manipulation. Short point stamped lugs for the same reasons.
I would draw it out on paper too if bikecad is not working for you. I use a typical CAD program and have the parts at hand to measure off from, no guessing that way.