When gluing RC car tires to a wheel, either foam or rubber tires, some drivers will leave a line of glue around the side of the tire. Then, they will sand that glue down, so it extends a specific distance from the wheel on all side. This glue on the sidewall serves to stiffen the side of the tire, preventing it from flexing (as much) when going into a turn. On a rubber tire, this glue may prevent it from folding under the wheel.
It is most often used in on-road touring car racing (with rubber tires), but also sometimes used in off-road racing with rubber tires or on-road racing with foam tires.
See our article on tire gluing for more information.
