Currently you can't mountain bike in areas designated wilderness. I would like to make clear in the 1984 Wilderness Act that mountain bikes are mot motorized transportation.
In 1984 Congress enacted the US Wilderness Act. It is a critical peace of legislation that has only a minor problem. The forest protection administration is claiming mountain bikes as motorized transportation and preventing mountain bikers from enjoying the trails they have rode on for years. You are probably asking why worry about a law that is over 30 years old. Forest protection administration has moved that ban to include potential wilderness and wilderness study areas. This makes areas like Bitterroot, MT and many more off limits.