How to calculate planetary gear ratios?

[sanddrag]

At one time or another I knew how to do this but it has been a while.

What is the formula for a planeatary gear ratio involving the tooth counts on sun, planet, and ring gears assuming the ring gear is held stationary?

Thanks.

EDIT: I looked a little harder and found it to be (sun+ring)/(sun).

[team222badbrad]

While this was mentioned, does anyone know any good websites that tell you how to do this?

This is one thing I would like to know how to do!

I could probably google it, but there are probably a million websites that are not what I need.

[Doug G]

Have you had a chance to read the white paper by Paul Copioli, here, it covers very well planetary systems and of course dual motor planetaries also. There's also a good pdf written by Martin L. Culpepper, here which helps a lot. Good luck