All dolly sleds using skateboard-style wheels develop flat spots on the wheels very quickly. This requires the dolly grip to roll the dolly back and forth repeatedly to reshape the wheels until the bump from the flat spot is no longer detected.
The new Porta-Glide sleds have solved this problem because each of the 16 wheels has a different diameter. They are positioned on the wheel block so that all wheels are touching simultaneously, and, like the other systems, these wheels will flatten slightly. However, as they turn, instead of 16 flat spots hitting in unison, each of our wheels returns to its flat spot at a different time, making the spot undetectable.