# Using pydriver for wheel rotation along path

I did not find any previous examples of doing this so here is my solution:

When the **length of the path curve** (actual length, not path length) and
**radius of the wheel** are known, the **rotation can be calculated based on
Speed-curve** that controls the vehicle motion.

NOTE: You must enable script links in order activate the pydriver.

Here is the expression:

b.Ipo.Get("ppspeed").getCurve("Speed").evaluate(b. Get("curframe")) * (79.7/21.1) * (-36)

- "ppspeed" is the
**name of the speed IPO curve.** -
79.7 is the
**lenght of the path** - 21.1 is the
**perimeter of the wheel**.

The **length of the path** can be found by pressing PrintLen button (make sure that CurvePath is active):

The perimeter of the wheel you have to calculate yourself :)

PS. I did not quite understand the logic of the rotation handling of this driver. Rotations are n * 10 degrees, so if I want to rotate object 90 degrees, the script must output 9. Thas why there is mysterious 36 in the expression above.