As ChinaV says it's very hard to ride TIBET now.
It's my understanding as of last year, western people can't travel by car/Bike from China into Tibet now.
And you have to fly or go by train then rent a bike with a tour company and have all the correct paperwork to ride TIBET.
The border is still open to western travellers coming from Nepal.
It will be interesting to find out what information you come up with.