Five motor myths: is the strongest Technic-style motor always best?

No. The strongest motor is best only when the job is hard work, like lifting or climbing steeply. On a flat floor the XL motor usually makes a *slower* car than the little M motor, because the XL turns slowly. The right motor is the one whose speed and strength match the load, and gears let you shift that match either way.

This guide tests five common myths with real measurements of the M, L and XL motors at 9 V (from Philippe "Philo" Hurbain's well-known motor comparison). For the gear side of the story, see What is torque?.

Before you start

You'll get more out of this after one motor build. The Motor Buggy (level 3, about 30 minutes) is a good first one: an M motor drives the back axle through an 8-tooth gear and a 24-tooth gear, so the wheels turn three times slower than the motor but with more force. The Mounting a Motor page shows how motors are fixed into a model.

The one idea behind every myth

Every electric motor has two numbers that matter. Its no-load speed is how fast it spins with nothing to push. Its stall torque is the twist at which it stops dead. In between, the harder it has to push, the slower it turns, along a nearly straight line. The measured values:

• M motor: about 405 rpm with no load, 11 N·cm at stall.
• L motor: about 390 rpm, 18 N·cm.
• XL motor: about 220 rpm, 40 N·cm.

Change the motor, gearing and hill below and watch where the dot lands on each motor's line. Then test the myths.

Myth 1: the strongest motor makes the fastest car

On a flat floor a small car needs only a little twist to keep rolling, so each motor runs close to its no-load speed. That makes the M motor the faster choice. In the lab's 500 g car with no gears, the M drives at about 104 cm/s and the XL at about 62 cm/s. The XL is mostly carrying spare strength it never uses.

Myth 2: for hills, always pick the XL

The XL does keep its speed best as the hill gets steeper. On a 25° ramp it is working at only about a fifth of its stall torque. But in the lab the L motor still climbs faster, at about 70 cm/s against 53 cm/s, because the ramp asks the L for under half its stall torque and its line starts much higher. The XL wins when the load is truly heavy. That is why the XL Power Crane (level 4, about 35 minutes) pairs it with a worm gear to lift loads.

Myth 3: gearing up always makes a car faster

Gearing up makes the wheels turn faster than the motor, but the motor has to push three times as hard to do it. On the flat that can work. On the 25° ramp, the M motor geared up 3× needs about twice its stall torque, so it stops. If a geared-up car stalls on a slope, gear it back down.

Myth 4: two motors make it twice as fast

Two motors on one axle share the work, so each pushes half as hard. But neither can spin faster than its no-load speed, so on the flat the gain is small: about 104 to 111 cm/s in the lab. A second motor is a strength upgrade, not a speed upgrade. It pays off on ramps, rough ground and heavy loads. The Skid-Steer Loader uses two motors differently, one per side, so it can steer by itself.

Myth 5: gearing down is always better for climbing

Gearing down makes climbing easy for the motor, but the wheels turn slower. If the motor wasn't close to stalling anyway, the car just goes slower. In the lab on the 25° ramp, the M motor direct climbs at about 43 cm/s, and geared down 3× like the Motor Buggy it climbs at about 31 cm/s. Gear down when the motor is struggling, not by default.

Test it on a real build

1. Build a motor car and mark a start line and a 2 metre finish line on the floor.
2. Time three runs and take the middle time.
3. Prop a board up as a ramp and time three climbs.
4. Change one thing only: the gearing, the motor or the car's weight (tape a few spare parts on). Predict first, then time it again.

Real cars are slower than the lab, because batteries sag under load and axles rub. Compare your setups with each other rather than with the lab's numbers. The RC Racer (level 5, about 75 minutes) is a good next step: its L motor drives the back wheels through a corner gear and a differential.

Check yourself