Smith Machine Guide Rail Thickness

The lifting bar on a smith machine slides up and down guide rails. Although you theoretically lift vertically, the reality is that you exert force differently. This can cause the guide rails to flex.

Too much flex may cause the lifting bar to bind.

Generally, the rule is the thicker the better, but it has to be solid steel. When using bearings, it has to be hardened steel to stop the bearings from eating into the guide rails.