Originally Posted by Rip Tide
No it's not stiffer. Glass is glass and the fibers are all the same
The s-glass is "stiffer" only because there is more fibers in a given area.
But the fact is that hardly matters. Modulus is just a way to describe stress.
What matters more is the taper and how a rod casts.
Glass isn't glass, there are different formulations used. Fiberglass is made primarily of silica (silicon dioxide, SiO2) but contains some percentage of other oxides that modify its properties for a particular application. Common fiberglass modifiers are calcium oxide (CaO), magnesium oxide (MgO), aluminum oxide (Al2O3) and boric oxide (B2O3).
Modulus does not just describe stress, it's the slope of a stress/strain curve -
An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region: As such, a stiffer material will have a higher elastic modulus.
While modulus isn't the same as stiffness, it does play a significant role. A one inch diameter member made with a material that has a higher modulus will be stiffer than a one inch diameter member made with a lower modulus material.
And of course, a half inch member will be less stiff than a one inch member.