Any tour groups operating in this area who hire motorcycles.
Sounds like a really great trip, and the perfect time of year to go!
Hiring a bike in Lhasa and self touring would be extremely difficult, (you will definitely need a guide a motorbike permit in Tibet). Have you considered doing it in the reverse - ie from Kathmandu to Lhasa along the Friendship highway?
You'll need to arrange your visas and permits before you travel, as well as find a decent guide to travel with you. My husband was looking into this for his birthday, and found a company called Blazing Trails in New Delhi - they do tours across Nepal and Tibet and may be able to help you. http://www.blazingtrailstours.com
Lisa over 6 years ago