stiffer? a quick google search shows fiberglass' elastic modulus is 17.2 GPA, steel is 200GPA, so steel is 11.6x stiffer. to get the same effective stiffness, for a same length & width beam segment, the fiberglass spring will need to be approx 2.25x thicker (taller) to have the same spring rate as a comparable steel spring the way the equations work out.
1976 Spinnaker White Plymouth Duster, /6 A833OD 1986 Silver/Twilight Blue Chrysler 5th Ave HotRod **SOLD!*** 2011 Toxic Orange Dodge Charger R/T 2017 Grand Cherokee Overland 2014 Jeep Cherokee Latitude (holy crap, my daughter is driving)