Be extremely careful about motorbike rentals. Avoid it if you can.

No matter what, they will ask you to deposit your passport. Avoid that if you can.

In case you can't do without a bike, testride it and check everything maticuously. Make them write down the smallest scratch or damage to the bike. If you're renting a dirtbike, avoid taking it to the east of the island. Roads are extremely challenging even to the experienced biker.

Make them remove the mirrors, if you don't have your own plastic parts, try to choose one that has already scratches, this way an additional scratch won't matter that much. Usually the optics are in a great condition (as they charge 3x normal price for fixing after a mishap, they can afford it) but the mechanics are not. 

Even brake failure (due to overheating) can occur. You will be held responsible for ANY damage occuring, while the bike is in your hands. Even if you have not caused the damage. Any part braking will be on your bill (x3 the usual value). Meanwhile some operators have started to prevent you from undertaking indendent repairs by making you sign an additional contract, that threatens you qith a 10.000 Baht fine if you do. On the same contract is a pricelist for each part, signing you completely surrender to their goodwill.

Being stranded with a defunct bike will also mean that you have to arrange the truck to bring it back - and pay for it, they will not help you and the charges will be extortionist.

Apart from the North-West, do not rent a bike if you can avoid it. You are homing into an experience, likely to ruin your holiday.

Police will not help you in case of dispute. 

All petrol left in the bike will be taken by the renter. All petrol gauges are manipulated, making you fill in more petrol than you need, most new bikes do not have a petrol/ reserve switch.