One large-diameter wheel machined from solid aluminum bar. No castings, no thin walls, no excuses. The running surface is precision-ground and hard-anodized until it is harder than the track it rolls on and completely immune to rust, even when the door lives right on the beach. https://www.hunepulley.com/product/door-roller-hardware/plastic-screen-window-sliding-wheel-roller-accessories-for-aluminum-window.html