[reply]There is a thread a while back about how many sizes are needed to cover the range o cockpit lengths, probably 2-3 weeks back. I'm not sure what it was called.[/reply]
OK, I found it and tweaked it a bit, it was a question of the 3-sized TCR vs. the 6-sized Soloist. Here it goes:
There are a few different issues.
1) For your position, all that matters is the placement of the bottom bracket, saddle and bars. You connect these points by using a frame, seatpost and stem, and you could (if for some strange reason you would want to) use any frame you like as long as you find the correct stem and seatpost to correct the strange frame choice. In other words, you can fit a 7' tall guy on a small frame if you can find a 20cm stem and a 2' seatpost. Of course the handing of this bike will be awful, but the positioning is not the problem.
2) The real issue is, once you have determined the correct "points in space" for the bb, saddle and bars, which frame, seatpost and stem connect these points the best with regards the the handling that is achieved on the final product. If Giant has three sizes (S, M, L) and Cervélo has six (48, 51, 54, 56, 58, 61) and if an S is like a 48, an M is like a 54 and an L is like a 58, then the following applies (NOTE: I'm not saying these sizes are identical, this is just an example):
a) If you fit a 48, a 54 or a 58, you will fit just as well on a Giant (S, M or L respectively). They are the same sizes, so there is no difference.
b) However, if your ideal size is a 51, a 56 or a 61, you don't have those options on the Giant and you will end up with a compromised position, with a stem longer or shorter than ideal. You would ideally need a Small/Medium, or a Medium/Large, or an ExtraLarge Giant, but they don't make those.
3) The final question is, how many sizes do you then need. Why 3 or 6 size, why not 1, 12, 24, or 48? That's obviously open to debate, but it comes down to "resolution". By moving around on your bike, you will be able to shift your center of gravity a cm or two. On a road bike it's a bit more than on a TT/tri bike. So if you shift around that much, there is no point having frames in sizes that only differ 5mm from size to size. Your weight distribution then shifts more by moving around than by changing frames. Since obviously moving around on the bike to the degree that we normally do is perfectly acceptable from a handling point of view, it is not necessary to have frame sizes that differ less than that. That said, having them differ by 5cm is too much. So somewhere in-between is what you want to achieve. Say the delta between the shortest and longest toptube that 98% of the population will need is 8cm, those 8cm can be covered by 5 sizes if you change the toptubes by 2cm each time. We make the delta a a bit less near the ends of the range, and that is why we end up with 6 sizes.
Gerard Vroomen
3T.bike OPEN cycle