CarTutorialSection_2.pdf

“The Car Tutorial” Part 2

Creating a Racing Game

for Unity

unity

“The Car Tutorial”― Creating a Racing Game for Unity, Part 2

Part 2: Tweaking the Car

Suspension damper

Center of Mass

Drag Multiplier

Suspension

Speed, turning and gears

Suspension range

Exporting the car as a package

Suspension spring

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“The Car Tutorial”― Creating a Racing Game for Unity, Part 2

Part 2: Tweaking the Car

If you’ve followed along part 1 of this tutorial and assembled the car, you are already at a point

where the car is pretty awesome. But if you have driven it around a bit, you will probably have

noticed that there is after all room for some improvement.

This is where tweaking enters the picture. In game development tweaking is a crucial part of making

your game fun, challenging, awesome or whatever goal you have for your speciﬁ c game. The idea is

that when you have setup the nuts and bolts that makes your game run, there might be something

that doesn’t feel quite right - maybe you want the car handling to be a bit different, maybe you want

the top speed to be slightly different, or maybe you want to change the scene lights.

A major strength of Unity is it’s tweakability - as you have seen, all the public variables in your scripts

are shown in the inspector, so you can change values without going into the code. And even more

powerful: When you’ve made a change you just hit play, and you will instantly see the result of that

change. You never have to wait for the scene to be rebuilt or for a major recompile of the code.

Center of Mass

Now the most obvious thing that needs to be tweaked is probably that the car can very easily be

ﬂ ipped around when turning (if you haven’t tried yet, then play again and speed up a bit and

then turn from side to side while speeding - the car will will ﬂ ip over pretty easily). The reason for

this ﬂ ipping is that we haven’t yet deﬁ ned the car’s center of mass. All the forces that are applied

to the Rigidbody of the car, are all applied at the Rigidbody’s center of mass. The center of mass

of the Rigidbody will be calculated by Unity according to the Colliders attached to the Rigidbody,

either on the same GameObject or on child objects. Since the center of mass of a car is typically not

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“The Car Tutorial”― Creating a Racing Game for Unity, Part 2

the center of the car (and probably not the center of mass that Unity calculates), we want to set the

center of mass ourselves.

The position of the center of mass for a car de-

pends on the placement of the engine and other

factors, and it can vary a lot from one car model

to another. For the car in this tutorial project the

center of mass could be a little behind the engine,

slightly above ﬂ oor height of the car. See it’s posi-

tion in this image:

•

Create a new GameObject and drag it to the

Car game object.

•

Rename it to

CenterOfMass

•

Reset the

CenterOfMass’s Transform (click the little cog wheel to the right of the Transform in

the Inspector and select ‘ Reset ’. This will give it the same position as its parent, the car.

•

Adjust its position to somewhere you like. Either do it by dragging, or type in the position. A

good position for this car’s center of mass could be (0, 0.1, 0.65).

•

In the Inspector assign the

CenterOfMass to the slot for it in the Car script Component.

In general, it is a bad idea to have the center of mass be positioned to either side of the center in

the x-axis, because this will make the steering behave oddly, and thus we have also set the x vari-

able of the position to 0.

You can also change the Camera’s target to be the CenterOfMass Game Object instead of the car

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“The Car Tutorial”― Creating a Racing Game for Unity, Part 2

itself. This will give a slightly different feeling - play around with it and decide which setting you

like the most.

Suspension

Another factor that can heavily change the behavior of the car is the properties of its suspension.

The job of a car suspension is to maximize the friction between the tires and the road surface.

When you are driving the car over a bump, all of the wheel’s vertical energy gets transfered to the

frame. If we did not have an intervening structure, this could easily result in the wheel loosing con-

tact with the road completely, and afterwards slamming down into the road because of the force

of gravity. The suspension is that intervening structure.

We have three different variables to tweak from the Inspector - the range, the damper and the

spring. All are part of the WheelCollider class that we use on the car’s wheels.

To the left we see the car with the standard settings, and to the right we see it with a much larger

suspension range. Combined with the spring and damper properties, you can make it behave like

everything from a formula one car to a huge monster truck. Of course the graphics need to match

the settings to make it believable though!

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