An RC car steers with a servo: a motor that, instead of spinning, turns to a position and holds it. The remote sends a command by invisible light, the receiver passes it to the servo, the servo turns a small gear, and that gear slides a toothed rack that swings both front wheels. The servo turns about three times as far as the wheels do, and it moves in steps, not a smooth sweep.
The First RC Car (Servo) (level 4, about 50 minutes) has every link in that chain, so this guide follows a single steering command through it, one part at a time.
1. The remote: a dial and a beam of light
The car has two motors with two jobs, and the remote has a dial for each. The build's wiring step puts the M motor on the receiver's red socket (channel A) and the Servo on the blue one (channel B): one dial drives, the other steers.
The remote sends blinking infrared light, too red for your eyes to see. The receiver has to "see" it, so point the remote at the car, the way you would with a TV. The Remote Control idea page has a quick test: hide the receiver behind your hand and see if the car still listens.
2. The servo: turn to an angle, then hold
A drive motor spins as long as it has power. A servo is different. Its output turns to an angle and stays there, like your hands holding a steering wheel. The build's own note on the pinion step says it can only turn a quarter turn each way, which is perfect for steering. Turn the Servo by hand while you build and you will feel it spring back to the middle.
The servo also moves in steps. Philo's tests of the Power Functions servo found seven positions on each side of the middle, up to 90°, so each step is about 13°. A compatible servo may behave a little differently, so check yours by turning the dial one click at a time and watching the wheels.
3. The pinion and rack: turning becomes sliding
On the servo's output sits an 8-tooth gear, the pinion, resting on a gear rack fixed to the red tie rod. Turn the pinion and the rack slides sideways. This is rack and pinion, the same idea as in a real car's steering.
How far does the rack slide? A gear rolls its own radius for every radian it turns. An 8-tooth pinion has a radius of half a hole, so a quarter turn (1.57 radians) slides the rack about 0.79 hole.
4. The steering arms: sliding becomes steering
The tie rod is pinned to the end of a steering arm on each front wheel, 1.5 holes from the pin the wheel turns on. As the rack slides, both arms swing and both wheels turn together.
The arm swings until its tip has moved sideways by the rack's slide: 0.79 of its 1.5 holes. That is an angle of about 32°. So 90° at the servo becomes about 32° at the wheels, roughly a third. The build's animation steers the wheels 25° each way, which needs about 73° of servo turn.
Try every link in the lab. Click through the steps, then swap the pinion for a bigger one and see what you gain and what you lose.
Interactive: Steer with a servo. The First RC Car’s front axle from above: click the remote dial through the servo’s 7 steps, swap the pinion on the servo, and predict how far the rack slides, how far the wheels turn and when the steering jams.
The trade-off: a bigger pinion
A 16-tooth pinion has twice the radius, so every step slides the rack twice as far and the car turns more sharply for a small click of the dial. But at full lock it would slide the rack 1.57 holes, more than the 1.5-hole arm can follow, so the steering jams before the servo reaches the end of its travel. In the lab you lose the last step with a 16-tooth pinion and three steps with a 24-tooth one.
The small pinion is the sensible choice: every one of the servo's steps is usable, and each step is a gentle change.
Where this build fits
- Start with steering you turn yourself. The Steering Go-Kart (level 3, about 30 minutes) turns its front wheels from a steering wheel, and One Motor, Knob Steering (level 3, about 40 minutes) adds a drive motor while you steer with a knob as you walk beside it. The RC car's build page lists both as builds to do first.
- Then go remote. The First RC Car's play notes suggest a figure of eight round two cups, then a very tight circle. Listen in the tight circle: the solid back axle makes one tyre scrub. The build points to the Diff-Lock Mud Truck (level 4, about 60 minutes), whose differential lets the back wheels turn at different speeds, with a lock for mud.
- For how tight your car can turn, and why the inner wheel should steer more than the outer one, see steering and suspension tests.
Check it on the real car
- Lift the front wheels off the table. Click the steering dial one step at a time and count the steps to full lock.
- At full lock, measure how far the tie rod has moved with a ruler or by counting holes.
- Drive a figure of eight. Does a single step steer enough, or do you need full lock?
Check yourself
Interactive: Check yourself. A few quick questions on the ideas in this guide, each with an explanation.
- What makes a servo different from a normal motor?
- In the First RC Car, which remote channel steers?
- The servo turns 90°. Why do the front wheels only turn about 32°?
- You swap in a bigger pinion on the servo. What happens?
