Originally Posted by
DannoXYZ
The frame flex unflexes on a dead part of the pedal-stroke when you're not exerting as much force. This typically is around the bottom of the stroke before the downstroke on the other side. The unflexing opposes the downstroke-force on the other side and gives you a little more force to push the pedal against. It's kinda like the end of the wrist-flick in squash.
Titanium is very elastic compared to steel, closer to CF. It springs back smartly and returns all the energy. If it didn't, the frame would get hotter and hotter as you ride and melt.
Titanium is much less stiff than steel, much less stiff than carbon fiber, but about 60% stiffer than aluminum. Bike frames are designed based on stiffness and weight, and never get close to the elastic limit of the frame. That said you can build a stiff or a flexible frame out of steel, carbon, titanium, or aluminum. It is just easier to build a light and stiff frame out of carbon as you can vary the "tube" shapes and junctions to provide the ride that you want