Consider two "friction" wheels - a Pinion driving a Gear through contact of their outer diameters. The speed ratio is
![design - Involute gear curve when root diameter falls below base diameter - Engineering Stack Exchange design - Involute gear curve when root diameter falls below base diameter - Engineering Stack Exchange](https://i.stack.imgur.com/YafhH.png)
design - Involute gear curve when root diameter falls below base diameter - Engineering Stack Exchange
Consider two "friction" wheels - a Pinion driving a Gear through contact of their outer diameters. The speed ratio is
![design - Involute gear curve when root diameter falls below base diameter - Engineering Stack Exchange design - Involute gear curve when root diameter falls below base diameter - Engineering Stack Exchange](https://i.stack.imgur.com/ueewg.png)