The Dooly  by Woodys has two 7/16" Flat-top"  runners mounted on a plate for each ski. Four contact lines on snow surface help decrease darting of sled as it makes own groove in a trail. When in a turn, only one of the Dooly bars is in contact with the snow or ice. A Dooly with 4", 6" or 8" of carbide on both runners provides the same turning power as a single runner with 4", 6" or 8" of turning carbide. Sold individually.