The backer pads that come with a polisher when you purchase it are usually a hard rubber. They are supposedly flexible, but they are designed for granite, not concrete. They are too hard, and instead of flexing will just scallop the concrete. Even worse, if you try to use them as rigid backers, they don’t actually stay flat.
Our rigid plastic backer WILL stay flat, every time. And, plastic backers are less expensive than aluminum and won’t corrode, but still provide excellent performance and flatness. Arbor size is 5/8″ diameter, 11 pitch (5/8″-11).