The clamps in the Hobie Cart Post Collar Clamp Kit help reduce direct loading of the cart on your kayak's bottom/keel. If the keel of your boat rests against the support bar on your kayak cart, it can leave an indentation. These collars mount on to the cart posts and distribute the weight against the scuppers which are stronger than the keel. Fits all year-models of Hobie plug-in carts. See the Resources tab for instructions.
excellent article, (Hobie) best brand by far
works great, keeps bottom off of the crossbar, better than tennis balls