Jan,
Good points, but the most important thing about frame angles is their relationship to the ground.
I always measure the angles with a dial or digital protractor with the bike sitting on the ground or with a set of wheel in a bare frame with a headset installed.
I've found lots of bikes and frames that have one set of angles in a bike stand with the top tube level but on the ground, the top tube can change a few degrees which changes the affective frame geometry.
In relation to frame geometry, the head tube angle, fork rake and to a lesser degree, wheel base length affect the handling of a bike. Anything that changes these can affect the handling.
Things like the angle of the rear dropouts and the wheel position in them can a affect the head tube angle by several degrees!
I remember back in the 70s one bike manufacturer advertising that you could adjust the handling of their bikes by moving the rear wheel forward or back in the dropout to change the head tube angle.
If the headset has been changed, the thickness of the crown race and lower headset race can also change the head tube angle.
A front end collision can take some of the bend out of the fork blades making the forks longer thus reducing the head tube angle. It can also bend the frame and increase the angle.
The seat tube angle has more to do with the rigidity or stiffness of the bottom bracket and rear triangle area.
Ride comfort is a whole different discussion that involves many other variables.
Chas. Colerich Oakland, CA USA
Jan Heine wrote:
<snip>
> Most other methods aren't accurate enough. After all, 2 degrees make a
> huge difference in the bike's handling. (For our book "The Competition
> Bicycle," we measured all the featured bikes, dating from the 1890s
> until the 1990s. All angles were between 66 and 75 degrees, a spread of
> only 9 degrees. So even without seeing your bike, I can tell you its
> head angle is somewhere around 70 degrees, plus or minus 4 degrees.)
>
> However, the angles of your frame don't tell you all that much about the
> handling - you need the fork offset (rake), and that is tricky to
> measure. Ideally, you remove the fork and put it on an alignment table,
> with V blocks. For our Bicycle Quarterly tests, we made a square that is
> clamped on a dummy axle, then aligned with the fork steerer. Scribed
> markings let us read off the fork offset. The problem is aligning the
> square with the steerer axis - easier on older bikes with the fork bend
> near the bottom and very hard on bikes with straight fork blades that
> are "bent" only at the crown.
>
> Jan Heine
> Editor
> Bicycle Quarterly
> 2116 Western Ave.
> Seattle WA 98121
> http://www.vintagebicyclepress.com