GFB Lightweight Underdrive Crank PulleyGFB lightweight pulley kits are made from aircraft grade 6061 T6 aluminium and offer faster acceleration through significant weight reduction on the crankshaft. Most of our kits are underdriven, which means the accessories are driven slightly slower to reduce parasitic losses. GFB pulley kits are designed to improve acceleration by reducing the rotational mass (inertia) on the crankshaft as well as parasitic loads from the driven accessories. The