Motor & power system

Your target speed and tire size set how fast the wheels have to turn. Multiply by the final drive and you have the motor RPM. Divide that by the pack voltage, 3.7 V per cell, and you have the KV. None of this depends on the motor’s power: it’s the speed the motor has to spin to, before load slows it down. That’s also why the RPM limit doesn’t change with the pack: only the gearing can bring it down. The limit is checked at a full charge, 4.2 V per cell.

To reach a speed in a time, your car has to pull at least that speed divided by the time at some point, however the run is shaped. That pull, times the car’s weight, is the push the tires must give. Through the tire’s radius and the final drive it becomes a torque at the motor, and a brushless motor makes about 9.55 divided by its KV newton-metres per amp, so the torque becomes a current. Lower KV means more torque for each amp, which is why the same launch draws less current from a lower-KV motor on a bigger pack.

The power figure is the least the car can put down at its peak and still make it: the power of a car that holds the same power from a standing start to the end of the run. The two are separate floors, reached at different moments of different runs, and real runs sit above both. Neither is an ESC or battery rating: those depend on how long the current flows and how well the parts cool.

Assumptions, not measurements of your car: 10% lost in the drivetrain, 15% in the ESC and motor, 60% of a 2WD car’s weight on its driven wheels, and the grip range for each surface. Pack current uses 3.7 V per cell; the motor’s RPM limit is checked at 4.2 V, a full charge.

A car can’t pull harder than its tires grip. On packed dirt a 4WD car holds roughly 0.6 to 0.8 g; a 2WD car only has the weight on its rear wheels to push with (we take 60%), so it breaks traction sooner. Past that point, more power only means more wheelspin. Grippier tires, more weight over the driven wheels, or a smoother trigger get you further than a bigger motor. The grip figures here are typical values, not measurements of your car.

Whether a particular motor, ESC or pack can deliver the launch, and how hot it will run. That depends on its internal resistance, cooling and how you drive, and few makers publish enough to work it out. It also ignores air drag, which is small off the line but decides top speed at the other end. Use it to narrow your choices, then check with a GPS pass and a temperature gun.