When locked it is firm, when unlocked it's a breeze to move on concrete floor. They are great and roll along just fine on the smooth concrete inside and on the brushed rough concrete in the driveway. Esta muy bueno.
When my mat goes in the maker it just slides itself to make it work without any troubles. Now when i need the occasional tool, it rolls out without a fuss.
I put 4 wheels on it and as soon as i turned it over and the weight on wheels 2 wheels broke off. The weight on the wheels caused the wheels to break off.