Took off the safety feet and put on spikes. The guy from the shop who sold them said they were the best thing on grass, but window cleaners used them on concrete and they weren't good on that/dangerous even. I felt the safety feet would slip on grass so went for this option as am on grass a lot. Best decision? He did say the safety feet were better on concrete as they had more surface area to stick. Are the spikes in the right position. They would line up with pre-drilled holes. Do people swivel them around when moving to concrete? There's a lot of spike under the bottom to stick in concrete, so I don't know how people can do that.