Wouldn’t it be cool if your R/C car had automatic continuously variable transmission? It’d probably a lot faster for the same motors and power consumption, as well as better at going through rough terrains, up steep inclines, etc.
There may be a way.
Imagine something like a torsion spring (a concentric spiral spring made out of a long, flat metal sheet). It’s turned from its center by the vehicle’s motor. The outer rim of it is coated with something with a lot of friction, such as a rubber, and it powers a belt. The belt turns a nearby wheel that powers the wheels of the vehicle using gears.
The trick here is that the direction in which the spring spirals outward is the same direction it pulls the belt in, so, if there’s more resistance to turning from the belt, it will naturally cause the spring to expand by it being pulled open, and if there’s less resistance from the belt the spring will contract. The spring expanding will increase the ratio of its size to the size of the wheel the belt powers, thus increasing the mechanical advantage. The spring contracting will decrease the mechanical advantage.
So, when the vehicle is going through rough terrain, there will be more resistance from the wheels, hence more resistance from the belt, therefore the spring will expand, thus increasing the mechanical advantage (like putting it into a lower gear). The same will happen when the vehicle is accelerating or going uphill. If the spring is configured well, the “gear” the car is in will always be exactly appropriate for the conditions the car is driving in and how fast it’s going.
There’s one more part of this mechanism I haven’t mentioned: when the spring expands and contracts, it’ll take up more or less of the belt length, so you have to modify the path it’s going through accordingly. The easiest way is probably to have a separate wheel on the belt’s path that keeps tension on the belt by being able to move farther or nearer on a spring, like multi-gear bicycles have.
Oh, one other thing: Alternatively, the motor could power the non-expanding wheel which would then turn the torsion spring, rather than the other way around. I don’t know which is better.