2manyoranges wrote:
Reach is one of the most important measurements - bb vertical line to centre of head tube. Get that too short and as Ishaw says, nothing you can do will preserve the quality of the geometry and get you on the right position. And the 64-65 head angle and short offset forks go with 35mm stems. Sizing the stem above this to accommodate a smaller frame is a big no-no since it defeats the point of the geometry and wrecks the traction. It takes a while to settle into the feel of modern geometry, but going with something which has the same feel as your older bikes may put you on a bike which is too small.
I think you've hit the nail on the head with this comment - the reach could be the most critical measurement. (vertically above BB, horizontal measurement to the centre of head tube)
This is relevant to being stood on the pedals, rather than in a seated position. If your preferred type of riding is about being in the 'attack position', then the seat tube angle and effective top tube measurements become less relevant. If the sort of riding you do is more seated then the consideration of top tube length becomes more important as well as the reach.
When comparing modern geometry to retro, my thinking is you need to add the stem length (horizontal component not actual stem length) to the reach to make the comparison meaningful.
Eg: Modern bike size large: 18.9" reach + 1.7" stem = 20.6"
1994 19" Spec FSR: 16.5" reach + 3.5" stem = 19.8"
Wider handlebars will also add about another 0.5" to the effective reach, over a retro bar width, which I guess should also be considered.
The above sized bikes are/were aimed at a similar height rider. So is it to be expected that a modern geometry bike have a circa 1" longer reach?