Originally Posted by
TejanoTrackie
My formulas do not make any assumptions regarding the amount of flex, only that there is more flex with a shallower head angle, all else being equal. Are your measurements based on a frame only, or a frameset which includes a fork ? Bicycles frames are basically trusses, where most of the vertical load is carried axially by the frame members and there is very little bending flex. So, even the most flexible frame is going to have a quite rigid ride. If you really want a soft compliant ride, you’ll get a lot more benefit from larger tires inflated to lower pressures. The only time you are aware of a frame’s flexibility is when you apply power to the pedals, which results in torsion and out of plane flexure of the frame members (mostly the downtube).
Here's
another bike flexibility report from the 1990's, this one on fork lateral and longitudinal stiffness. For a given load (47.5 lbs), the measured amounts of longitudinal flexing of the forks ranged from 0.130" (for a very stiff Schwinn Paramount steel fork) to 0.185" (for a Trek carbon fork). So a difference of 0.055" from stiffest to most flexible. (The article notes that the Trek fork was something of an outlier compared to all the others.)
How one could use those figures to calculate the range of flexing of a given fork according to head tube angle, I don't know. I don't have the math skills. And I don't know whether it's safe to assume that the effect of head tube angle on fork movement is linear for differing loads.
You do apparently have the math skills, though, so if you can come up with some rough figures, I'd be grateful. My guess is that the effect is minimal, but that's just based on my own experiences, including the fact that my favorite race geometry bike has a straight fork built with oversized aluminum blades and a 1 1/8" steerer.
[Edit] Just realized that this is an unnecessary digression in this thread. Sorry!