if you buy a new green glide there is a reason it has a radius on the glide, it allows air to flow past the fiber and into the vac slot, as glides wear down and it becomes flat you will move less air. thats one of the reasons interlink has the zig zag pattern on its glides, to help; minimize that lock down. i however have a machine shop available so when a glide gets worn down alot i machine it so the slot is the first part to touch the carpet. because it allows air to flow accross the fiber pulling more water up with it and increasing airflow. flat glides will still work on cut pile, just not berber and cgd as well.