Hydros is delivered with three shims, Zero, One.Five & Two.Five. Shims are needed to adjust and control the Angle Of Attack (AOA) of the back wing. Shims help dictate the amount of lift we will achieve from the back wing. The bigger the AOA the most Lift we will have, the smaller the AOA the most speed you will achieve. These should be chosen in relation to the size of the front wing that is being used.