Knex compatible Leaf spring that is modular so you can change the stiffness to your liking, the leaf spring length is a red rods length with two grey one way pieces on each end. i will be adding more leafs to the file and more different leaf spring lengths. You will need a soldering iron or somthign to melt/weld the plastic together, to assemble you the leaf spring you stack the 2 leafs under the leaf spring between the brackets and weld/melt it together but you can assemble the leaf spring together without welding/melting it but the leafs and the rod holes will most lickly slip off the leaf spring.