Ill disagree the harbor freight one are fine for my TT. The rubber and groved edges help it catch the tire and ground better especially on paved/ concrete surfaces. The hard plastic ones don't grip as well and can slide. Chocks are 2 fold , to stop you from rolling away is number 1, the second is stabilization. With the HF ones im not rolling away. I do use 2 sets of chocks on my TT with the levelers it dosent rock at all. You guys with the bigger tires and such might need diffrent stuff. People sware by the x chocks for stabilization. But they are not really chocks to stop you from rolling away. Oh, side note, the hard plastic yellow ones used to have an experation date on them. Lol.