Looks like I quoted the above reply incorrectly
I thought I would copy and paste the information that one of the tour companies gives about bikes. Here is their website:
https://www.redspokes.co.uk/cycling-holidays/essential-information.php?c=23&t=152"Equipment and bike stuff
Bikes for this trip need to be:
Tough because bikes can take a hammering on Tibet's roads. Low geared too, a 22-tooth granny cog on the front crank driving an 11-32/34 block on the back will do the trick. Comfortable as there are some long days so it is essential your saddle and riding position are right.
A mountain bike is a good option. Front suspension will help with the washboard tracks and potholes, but it is not essential. Full-suspension is OK, but may be hard work on the tarmac stretches. If you have flat bars, fitting bar ends will give you a different hand position and help with the climbs. Touring bikes. The trip can be done on a traditional touring, but it is important to ensure it has got strong enough wheels, sufficiently wide tyres (37mm is about the minimum) and low enough gearing. It's worth investing in a pair of 36 spoke, hand-built wheels, using a good quality rim (e.g. Sun Rhyno, Mavic or Rigida).
Tyres
The route is a mixture of poorly maintained tarmac and very rough gravel tracks. The best tyre for the job is probably the Schwalbe Marathon XR. It's very tough and hardwearing, and has a decent tread for the gravel without being too knobbly for tarmac.
Spares
We would advise the following: multi tool, tyre levers, pump, 2 spare inner tube, spare spokes of the correct length to fit your wheels - check with a bike shop, brake pads, a few spare links for your chain, spares for any unique or high-tech items on your bike, e.g. fluid and bleed kit for hydraulic brakes.
Please ensure that you bike is in good working order before you leave on tour. If you are not the most mechanically minded, we suggest that you get your bike looked at by a professional bike mechanic. In particularly we would suggest looking and adjusting all wheel spokes, greasing all your bearings, checking your brakes/gears cable, tighten all nuts and bolts, check chain, quick release clamps, tires and put in new inner tubes and check wheel rims are not worn. In addition, make sure the bike is well serviced and ride it with as much care and attention as possible."
Here is what they say under Bike Hires:
" Bike Hires
For this tour, we have Trek 4300 bikes (or similar) in a range of sizes. They are mostly new for this season and all are well maintained bikes with Shimano components and suspension forks. Spare parts (tubes, tyre, chains etc.) are included in the price of the bike rental and will be carried in the support vehicle with a tool kit and track pump. The bikes are equipped with flat pedals. Those who prefer to use spd or clip-in pedals, are welcome to bring these with you, along with your own cycling shoes and cleats. You are also welcome to bring your own saddle. The guide will carry some spares and tools for fixing punctures etc. but we suggest bringing a couple of spare inner tubes (26 x 1.75), a pump and a puncture repair kit. If you are able to do a simple repair yourself this will avoid the need to wait for assistance should the team or support vehicle be any distance away. Helmets are not included in the bike hire, you will need to bring your own helmet."