So, I'm getting a custom track frame built by Marinoni up here in Quebec. I haven't gone to get measured/fit, but I've ridden lots of bikes with knowledge of their geometries and know my fit pretty well. For the most direct comparison, I'll be moving from my 51cm Soma Rush (http://www.somafab.com/wp-content/up...chart_rush.jpg with an 80mm stem and 43mm rake fork) which I love a lot, but it's a little too big. It needs to be about 3.5cm shorter on standover, and maybe 1.5cm in reach. 650c seemed to be the sensible choice to get smaller without toe-overlap.
Here's the geometry I've sketched out, I'm mostly wondering if anything seems greviously wrong with this, or if there's something I'm not considering in the shift from 700c to 650c.
Top Tube (level) 51cm
Seat tube (C-T) 47.5
Head tube length 90mm
Seat tube angle 75.5
Head tube angle 72.7
Fork rake 30 or 38mm (have two forks I may swap in and out)
Trail 66mm or 58mm respectively
Chain stay length 366mm
BB drop 31mm
Things I have considered (quite possibly incorrectly) :
Chainstays: shorten them by 25.5mm at most (difference in radius between 650c and 700c)
BB drop: raise BB height/drop by at least 25.5 if keeping it the same
Seat tube: If BB height is equivalent between 650c and 700c, an equivalent length and angled seat tube should result in roughly the same standover height, so cut 3.5mm off 51cm and I'm at 47.5cm
Head tube: The axle to crown on the 650c fork is also about 25.5mm shorter, so the head tube can be 25.5mm taller, but to keep level with the 35mm taken off the seat tube, it in the end needs to be 10mm shorter.
Trail: I'm aiming for slightly increased trail/flop. My Soma is very 'twitchy', like, blows with the wind steering twitchy at 55mm trail.
Ahh!! Help! Please!