Making the wheels of a car roll right

[Guest]

I have a photo of a car with the wheels separated, and I want to find an expression or something that will help make the wheels look like they are rotating at the right speed.  Any suggestions?

I have used AE for a while but expressions are still pretty foreign to me.

Thanks for any help!

[Rick Gerard]

Simple geometry should do the trick. First calculate the circumference of your circle. C = 2πR. Then you take the circumference and divide that into 360 parts to create the first part of a ratio between movement in x and the circumference. Why 360? Because there are 360º in a circle. Now movement in X is multiplied by the ratio.

Here's an example that work well when the wheel moves in a straight line along the x axis:

r = width/2;
c = 2*Math.PI*r;
R = 360/c * position[0];\

Give this a try. I hope you can understand the math. It's not much more than 7th grade math.

[Guest]

It's been a long time since I was in 7th grade, and frankly I am not that bright. Can you help just a little more?

Do I put this into an expression?  My wheels are 140 pixels high by 140 pixels, if that helps.

r = width/2;
c = 2*Math.PI*r;

R = 360/c * position[0];\

[Rick Gerard]

Add that expression to the rotation property.

[Guest]

I found a great tutorial on this:
http://www.cgarena.com/forum/viewtopic.php?p=66547

You simply add an expression to the rotation of the wheel. Then connect the rotation of the wheel to the x position of the car. It worked great.