Seat tube angle/top tube length relationship?

[Tetraptych]

I've never posted before, but I've been a long-time lurker, so I figured I might as well start posting while the board is new again. I even managed to resist the urge to use a smiley face.

Anyway, I had a pretty extensive fit done this fall by a very accomplished fitter (Ian from Fitwerx in Waitsfield, VT, Ian and Sarah are both excellent folks) with the intention that I'd be buying a custom (Serotta CIII) over the winter, but my finances are refusing to cooperate. Because of this, I'm most likely going to end up buying either a stock bike, or some used higher-end bike off Ebay in order to stay with budget.

When I got my fitting, they suggested a 57-58cm top tube with a 73 deg seat tube angle. I might be splitting hairs here, but I was curious just how much of a difference it would make if I deviate from that. I'm not all that concerned about fit, as I've been comfortable on some bikes that in retrospect were pretty poor-fitting, but I'm more curious about the effect on handling and weight balance if I went to a bike that had, for example, a 73.5 deg seat tube like a used Serotta CRL I saw on Ebay. I raced road bikes a bit in high school, then raced mountain bikes for a most of the '90's and I don't think I've ever owned a road bike that really fit me properly. I'm planning on spending as much time as possible on it, hopefully 200 miles a week or more when the weather permits, and really want to make sure I get something that fits me properly.

Just for some extra info, I'm 6' 1.5" tall, with shortish femurs and a longish torso, about a 33.5" inseam, and weigh around 190, although my weight drops to 180ish during the season.

Cheers,
Steve

[pbbob]

resisted the dancing chiquitas too!
all I can tell you is I have 2 custom steel bikes. one with a 72.5 seat angle and one with a 73.5. toptubes and seat tubes are the only measurements that are the same. one is for all day comfort the other for flat land speed. the other measurements and forks help make the bikes behave differently. my builder didn't recommend anything steeper because it would be to uncomfortable for my type of riding.
save up. nothing like a custom or 2 or ?

certified chiquita free

[Len J]

I think I can answer you.

First, I am assuming that you want to keep your current relationship of your Knee to the Pedal spindel (when in the 3 & 9 o'clock position). This means that the position of your seat relative to the BB will remain the same.

If you start with a well fitting Bike with a 57 CM TT and a 73 degree ST angle. Then to maintain the same fitting with a 72 degree seat tube angle, you would need a bike with a 1.2 cm longer Top tube (And you'd have to move the seat forward the same amount). Remember that the seat, bars, BB relationship is exactly the same, slacken the ST angle, lengthen the TT.

Conversly going to a 73.5 degree ST angle you would have to shorten the TT by .6CM (And push the seat back by the same amt) to maintain the same relationship

Hope this helps.

Len

[Tetraptych]

Len,

Thanks, that's pretty much exactly what I'm after. We've got two and a half feet of snow on the ground here in Vermont, so there's not a whole lot of cycling on, and I haven't had time to go skiing yet. Very easy to obsess over minutiae when you can't actually get on the bike. Ok, can't resist a smiley face this time, but beer is even better. :beer: