Capri Technical Forum:

Mechanical speed controllers (MSCs) are technologically somewhat interesting, and they're also dirt cheap to make. Which is why you still see the things in practically every electric R/C kit, including this one.

MSCs use a triple-tapped, low value, high wattage wirewound resistor, which a servo-activated wiper-arm arrangement can put in series with the motor. That reduces the voltage the motor sees, and also reduces its speed. But considerable power is wasted as heat by the resistor, and you can't drive at really low speeds - what you get is something like one-third speed, two-thirds speed, and full power.

One-third speed is still faster than many little toy R/C cars can manage, which makes MSC-ed cars unsuitable as indoor toys.

The resistor in the Tamiya kit has a 0.2 ohm resistance between either end terminal and the middle terminal, or 0.4 ohms from one end to the other, which is on par for these things. There's a bit more resistance from the skinny wires to and from the resistor, and from the low-rent spade lug contacts.

With the wiper in the middle position, no power flows to the motor. With the wiper one-third forward or back, the battery's connected to the motor through the whole length of the resistor. With the wiper two-thirds forward or back, the motor's connected to the battery again, but now through one or the other half of the resistor, not the whole thing.

In two-thirds mode, there's less resistance, the motor sees more voltage, more current flows, and the car goes faster - but the resistor dissipates rather more than twice as much heat, and uses only half of its windings to do it. So it's the two-thirds setting that's actually meanest to the resistor.

At the full forward or full reverse settings, the battery's connected directly to the motor, and the resistor gets to cool down.