Tips From http://www.butlercc.edu/engineering/en115/en115_basic_treb_design.cfm

Object	Design guidelines	Material
Counterweight	75 to 100 times heavier than projectile	should be adjustable
Arm	angle should be about 45 degrees when cocked; upper part of arm should be 3 to 5 times longer than the lower end	pvc pipe
Base and framework	must be heavy enough to support arm and counterweight and for precision, yet light enough to have a high material efficiency	pvc pipe
Sling or rope	should be slightly shorter than upper part of the throwing arm (for starters, that is); too short will release the projectile too early, higher trajectory; too long will drag the ground, lower trajectory	any material
Sling release pin	a more hooked prong will hold the sling loop longer than a straighter one. ie a prong less hooked or in line with beam gives an earlier release, higher trajectory;a prong more hooked or forward-pointing gives a later release, flatter trajectory	metal or other material
Projectile	a heavier projectile tends to pull the loop off the prong earlier than a lighter projectile does. heavy projectile gives earlier release, higher trajectory; light projectile gives later release, flatter trajectory	tennis ball-provided
Trigger	you need some way to release the projectile in a repeatable way	any material
Trough	the projectile needs to slide down some type of guide; friction will be important here	any material
*Base wheels-optional	wheels added to the base may increase distance; repeatability(? )	any material