There's a couple of places that a set of these are helpful,like Tony says at the bend of the rails in front and the spot between the wheels that are ahead of the rear axle and the next set on the outside of the rails.This spot seems to be where the sliders wear because this is where the riders weight is focused.As far as where to put the set at the bend of the rails just have a look and see where the most pressure on the sliders are and put your wheels there.