What effect does seat tube angle have? I know that head tube angle affect steering, but does seat tube angle affect maneuvering somehow? As long as I compare a few models, they are between 72 and 74 degrees, and greater the frame smaller seat tube angle. So I guess difference by a few degrees doesn't have considerable effect.

Here're the comparisons.


Specialized geometry
Trek geometry
As for sheer maneuverability, seat tube angle may in some (rather extreme) cases affect wheelbase, which affects stability at variety of speeds. Also seat tube angle will have some minor effect on the position of rider's centre of gravity making it slightly easier to descend with smaller seat tube angle... I guess it would be an unnoticeable difference though.

But main aim of angled seat tube is above all to make pedalling more correct in a biomechanical way, thus more efficient..... and making it possible for a rider to reach the ground while still being on the saddle, say when they're stopping. For the right distribution of stresses within your joints you need an angle adjusted to your body - people with longer thighs will need smaller angle. In general too low angle encourages lower cadence and more forceful pedalling which isn't very good for your joints. What's interesting is that it doesn't work the other way round - it's possible to ride relatively safely with very high angles - it's usually 78-80 degrees for TT bikes. (I meant to put an article titled "Myth of K.O.P.S" for Sheldon Brown website, but the link doesn't seem to work)

Here is the link to Sheldon's Myth of K. O. P. S. (knee over pedal spindle) article.

Seat tube angle can be quite well made up for with fore-aft saddle position, so 2 degrees in difference can be easily neutralised. And the differences in angles between sizes may come from various sources:


it could be forced by the design, for example to avoid some weird top tube length or too short distance from the crank to front wheel.
it may also be a form of adjusting the geometry to body proportions of riders of different sizes.
or it could be done to lower the centre of gravity for a large rider, that for large sizes is pretty high anyway.


Or it could be a mix of the above, we should ask the designers.