The following is advice on how to cycle across Tibet, based on the experiences of those stupid enough to have a go themselves, and who carry the scars with something like pride.
First Considerations and Regulations
First piece of advice, if you're considering this, is:
Don't be silly. What, are you mad? I mean, it's the Himalayas!
At least, that's the advice you can expect people to give you.
Secondly, we recommend travelling from Lhasa, the capital, to Nepal. For a start the Chinese government will make it difficult to go the opposite way. Also the trip ends with a 3000 metre drop over 30km, which would be astonishingly hard work coming from Nepal, especially as it's all mud, but from Tibet it becomes a psychotic, exhilarating experience which nobody should miss.
As with all things done in Tibet, the major constriction is the Chinese government. You need a series of permits to travel around, and in Lhasa the only way to obtain these is to join a government-approved tour group. This essentially limits you to a trip by LandCruiser, an overblown 4x4, whose longest trip to cover the length of Tibet to Kathmandu is 7 days with stops at major attractions.
Those who like neither the limitations nor the idea of such a trip pretty much have three options: hitching (officially illegal), buses (also officially illegal) or cycling (officially frowned upon).
One happy fact is that the police (PSB) in Shigatse, on the road to the west of Lhasa, issue permits to anyone with a letter of approval from the tourist office. The tourist office issues a letter of approval to anyone who pays the full amount for a cycle permit or who lies through their eye-teeth about having found a LandCruiser and so pays less for a normal permit. In reality the two permits are the same, it's just a cunning ploy to screw more money out of you, and so we whole-heartedly recommend lying on this point any day of the week.
This does mean starting the cycle trip from Shigatse, after having caught a bus from Lhasa, but as this still leaves 800 to 900km over mountain passes to cycle you'd have to be mad to object.
Sorting out the Bikes
The bikes themselves; buy these in Lhasa before going to Shigatse (they can easily be thrown on the roof of the bus). The prices are lower, the selection is wider and the people are a lot more helpful.
There is not really any point paying more than about $30 to $40 for a bike, which is your basic mountain bike. You can easily pay closer to $90 for a nicer looking bike, but beneath the shine and fancy extras it'll be the same piece of rubbish as the cheap ones.
The Chinese cannot make bikes for love nor money and appear to have looked at western ones once, then tried to make something that roughly resembles them. The result is always flimsy and frequently dysfunctional. They make one sturdy bike, the Flying Pigeon, but this has a single gear and is roughly the weight of a Mini1.
Basically pay as little as possible as it's all the same in the end and you'll need the extra money for spare parts.
Strangely you can get quite a good price for them ($10 - $15) if you sell them again in Kathmandu. The reasoning appears to be that they'd made it across the Himalayas so they've got to be good!
Take your new bike to the nearest bicycle repair man and start bargaining. He'll think you're mad but lose the mudguards and stand - although the latter means that every time you pull up to a bicycle repair man he'll run out proudly with a new stand for you.
Get a front rack (a back one hit with a hammer until it fits), the lowest gears you can find and spares of everything you can think of. Remember that the roads are rarely tarmac, mostly rough and everything that can break will, especially as it's Chinese-made. One such trip saw three people, over a three week trip, go through two cranks, three chains, two left-hand pedals and eight right (one ten minutes after it had been put on), two brake handles, one brake cable, 24 brake blocks (two sets each, the second ones went down to the metal over 1km on the descent to Nepal) and one rear wheel. You can also expect to break all the racks and buckle all the wheels, but don't let that worry you.
Needless to say you'll need to do a little maintenance on the road, so bring a little know-how and buy every tool you can lay your hands on, although be warned that these are also Chinese-made and so appear to be made of cheese. However, be aware that the type of bicycle maintenance we're talking of is not of the standard kind.
Common repairs come under three categories:
Replacing the part completely (because it's shattered beyond recognition)
Taking the part to the nearest bloke with a welder (because nothing else will hold it)
Hitting it repeatedly with rocks (in the case of the chain anyway)
Basically you need to have enough know-how to be able to improvise in unusual ways. Do as much swapping and haggling as possible - hopefully the repair man will find the whole thing hysterical. Then ride it round for a few days, go back and replace everything that's already broken.
What to Take
As little as possible. Firstly, the heavier the racks the more the bike frame flexes, which is unnerving in a steel bike frame. Secondly, the more bags you've got, the more there are to break away and jam into the wheel, which is to be avoided when you're having difficulties controlling the bike anyway.
That said, you will need a tent and cooking implements, as it can be three to four days before you reach the next village and there's no vegetation or shelter on the Tibetan Plateau. But all you should really need is those, a sleeping bag, food, spare parts, tools and extra, warm clothing.
The weather varies wildly all day, from baking sunshine to a blizzard and back in half an hour, so you'll need to be ready for all conditions, but this should be additional layers that can be put on hurriedly. There's no real point in having spare clothes as the new ones will just get as dirty and you won't find a shower for three weeks anyway.
On the Road (Dirt Track)
As for the trip itself - enjoy! It has some of the most incredible scenery in the world and the most rewarding experience imaginable.
However, be wary of altitude sickness, which you can expect to succumb to at one point or another to some extent, as the road rises to over 5200 metres at some points. Don't feel guilty about hitching on a lorry over the occasional pass - most people don't hike that high in their lives, let alone cycle it!
Also, unfortunately it has to be said that you will be met with a certain hostility in some villages, especially from the children, who frequently throw stones or try to block the road. The main gripe is against the hordes of tourists who fly past in Chinese-run LandCruisers, stopping in Chinese-owned guest houses and eating in Chinese restaurants. Whenever you do stop to use the village shop, village restaurant or village guest house you'll find their attitude towards you transforms and they become friendly, inquisitive and delightful.
Please don't give to every child that begs, as most of them have homes, meals and families that find it embarrassing. Again the LandCruiser tourists seem to stop and randomly distribute money, sweets or photos of the Dalai Lama, which makes life difficult for those who can't carry enough for the 30 kids in each village or jump back into a LandCruiser and speed away.
You don't really need a map to make it to Nepal, if you reach one of the five junctions on the length of the Friendship Highway just wait for the next LandCruiser to go past and go in the same direction as that.
The Lonely Planet Guide to Tibet is extremely useful for finding approximate locations and the turnoffs for Sakya Monastery and Everest Base Camp, two side trips which are maybe the most strenuous but most rewarding parts of the trip. It should be stressed that the Lonely Plant's information is only approximate and frequently inexcusably inaccurate.
Go On, Do It
In short, all you really need to undertake a trip which, at its first suggestion, sounds preposterous, is a pair of sunglasses (it's very bright whenever the sun does come out), a face mask (the roads are pure dust), a tent, a shed load of spares, not-so-terrible fitness (you'll get fitter very quickly) and a certain level of insanity to consider the whole idea in the first place.
Oh, and a lot of camera film - you wouldn't want anyone to think you were making the whole thing up. Which is the only reasonable explanation for the stories that you'll be telling.