Originally Posted by impolexg

Bikes:
1986 cannondale sr1000 in 58cm, aluminum
2009? lynskey r330 in 58 (large?), titanium
2012 kona raijin mtb, 29", size large (made by lynskey for kona), titanium

As we all learned in physics the stiffness of a tube is linear with the moi, which roughly grows with the 4th power of the diameter. So tube diameter is the thing to measure. For each measurement of minimum outer tube diameter in mm. I'm providing the triple of <cannondale, lynskey, kona>:

Top Tube: 35, 33, 35
Seat tube: 32, 35, 36
downtube: 45,44,44
seatstay: 19x26,15,19
chainstay: 25.5, 20.5, 19

Of note, the r330 has manipulated tube shapes, they have diamond profiles 'n stuff. I just measured min diamter because it was easy and the difference is so small. The cannondale on the other hand has massively ovalized seatstays, their short diameter in direction of bike is 19mm but the long diameter across the bike is a whopping 26mm. The canondale seatstay difference was so large that I left it in the table.

Using the "press foot on pedal and eyeball it" technique the cannondale is very resistant to flex. Like really solid, not a spring. With same wheels the r330 is very flexy, boing-boing. The kona mtb with a 140 float 34 suspension fork and maxxis dhf 2.3 in front and a vittoria mezcal 2.25 in back was surprisingly resistant to flex, its flex seems to be much closer to the cannondale, and that is including some pretty significant tire flex.

Stiffness is complicated because all the components effect each other but it seems like the lynskey seat stay diameter is the source of the flex here, and the cannondale's wide seatstays are why it feels so sprinty (even though it is aluminum which is not as stiff as titanium).