This is correct, but it still has the problem of accurately (to the millimeter?) finding the vertical through the BB center. This requires a plumb line AND a frame that is level. Leveling the frame is not trivial unless you have a horizontal top tube. Since a 1 degree difference in seat tube is abotu 10 mm of error in setback, you need to level the frame to 0.1 degree, in rough terms. It's easiest with a bubble level.
On Tue, Oct 6, 2009 at 3:27 PM, Norris Lockley <nlockley73@googlemail.com>wrote:
> Lou Deeter is quite correct in his suggestion that the angles can be
> measured by trigonometry. Certainly the seat angle is the simpler of the
> two, but the head angle can be measured with a bit more fifling about.
>
> Most French and possibly manufacturers never used to give the seat angle,
> but refer to it by a measurement of the set-back, in millimeters from a
> vertical line through the bottom bracket to the centre-line of the top-tube
> to an imaginary line running along the centre of the seat tube.
>
> When I used to import Vitus Duralinox and TVT frames..that was all the
> information that I could get from the manufacturer. Obviously when the
> set-back angle is given in the catalogue or spec sheet its much easier than
> measuring it yourself.
>
> Many jigs used by these manufacturers actually had their jigs callibrated
> in
> millimeters to enable the angles to be set very accurately.
>
>
> Norris Lockley
>
> Settle (where it has rained continuously all day. UK
> _______________________________________________
>
--
Ken Freeman
Ann Arbor, MI USA