Optimum steerer tube diameter is a balance of stiffness and energy absorption.  Our R&D shows that 1 3/8" is ideal.  Smaller diameter -- 1" 1 1/8" and 1 1/4" -- are more flexible, but no lighter.  Large diameters are too rigid to easily absorb enough impact energy, leading to overbuilt frames and forks.  The result; heavier and stiffer riding bikes. 

Our 1 3/8" steerer tube has set the new standard.  The benefit is enough added stiffness to give precise handling without being too rigid or uncomfortable.