When it is said that servos are used in RC cars, this means they power the forward motion. But when it is said they also turn the car, is that due to making 1 wheel turn differentialy from the other such as to make the car turn?
A basic servo only rotates around 90deg either way from a zero point. The output shaft has a cross piece that pushes or pulls a shaft to the steering.
The drive is usually via a DC motor or stepper motor.
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I googled for a picture, guessing that will shed some light.
This was the first result that came up:
1 picture saves a thousand words!
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Marciokoko:
When it is said that servos are used in RC cars, this means they power the forward motion.
No. Most RC cars use powerful DC (or brushless DC) motors for driving the wheels. The servo is only used for steering control. The drive motors are controlled by an Electronic Speed Control (ESC) and the ESC is usually controlled in the same way as a servo - using the Arduino Servo library.
You could use a continuous rotation servo for driving the wheels but, strictly speaking, a continuous rotation servo is not a servo because it has no position control - only speed and direction.
...R
Got it! I was thinking it was the IC controlled differential rotation of one servo on the left vs the one on the right, would make the car turn by applying more force to the right motor, making the car turn right.
But now from what i see, the servos are actually used to turn a steering shaft much like turning a wheel in a real car.
Thanks guys
So I wouldnt buy something like this:
Emgreat 4-wheel robot smart car chassis because it doesnt turn. It just runs straight.
I should instead get some servos, horns and make a more controllable robot!
Actually, Im thinking ill start smaller and just hack my kids RC car 
and just add some range finders and be done with it.
Of course that will turn.
It just does that in another way than the car you might be driving yourself (assuming you're not driving yourself to work in a tank).
If you drive the left wheels forward, and the right wheels back, you will do a right hand turn.
You can even spin the car without moving forward, depending on the grip those wheels have on the surface they're on.
By the way, those motors aren't servos, they're not even close.
And don't be a bully to your kid.
Get your own toys.