Originally Posted by impolexg

I completely agree that stiffness is a perception of the rider. One factor you left out is the frame size. Bigger frames have longer tube segments so more leverage to flex. A smaller lighter rider on a smaller frame will have a totally different experience than a tall heavy rider on a large frame.

There's a pernicious myth that 6/4 is significantly stiffer than 3/2.5. You don't need to believe me and can verify by looking up alloy properties from all over the internet. Look up modulus of elasticity.

6/4 is 20% stronger but 0-7% stiffer than 3/2.5. That stiffness difference is small potatoes compared to what is achieved by a small change in tube diameter (remember tube stiffness grows with the 4th power of the diameter, it laughs at a puny 7% change.)

The extra strength of 6/4 means its possible to build a lighter frame from it at same strength but that is only by making the tube walls thinner which will make the frame more fragile than a thicker walled 3/2.5 bike.