few Qs on frame geometry

[Chris N]

It depends what you want to do.  On a 'race' bike a steep seat tube, longer reach and lower bars effectively rotate you forwards and down around the BB.  This is a 'faster' riding position.

If you're trying to replicate an existing bike, you shouldn't increase the stem length to make up for a steeper seat tube angle.  The horizontal distance between the BB axle and the saddle is important and if the frame/seatpost combo doesn't let you get far enough back, swap the post for one with more layback.

[Chris N]

Also, in these situations I find it very helpful to draw your existing bike to scale and overlay the new frame geometry to compare them.  If you draw it half scale you can get most of the front end of a bike frame (Put the BB somewhere near the bottom-left corner and work left-to-right towards the head tube) on a piece of A3 paper.

[MacB]

It's easier to use the free CAD program here:-

The Bicycle Forest :: BikeCAD

If you're intending keeping the same saddle to BB relationship then I find it easier to envisage the front triangle as pivoting around the BB as you alter the seat tube angle. For any given effective TT length then the slacker the ST angle the shorter the reach, and vice versa. Put another way a slacker ST angle means more of the TT is behind the BB.

As Chris mentions the saddle to bar drop matters as well, worth measuring up your existing bike precisely.

[saturn]

Thanks all. I know the measurements & angles of the current bike, I've got the CAD drawing from when it was built. I'm now using it a reference for a proposed frame to build up into a winter bike for next winter, haven't found a suitable frame that's exactly the same so I'm trying to figure which is is the closest and which are the least worst compromises (tight budget so can't go custom for this).