OK, before everyone else starts flaming this guy I'll try to be nice as I am in a good mood today. First Steve,you need to define what you consider a "road trip". If you read this site at all you will see that it encompasses many things and many possibilities. Then you will see the North is probably where you want to be. You might try "Bike hire in Chiang Mai, Update" at Link removed.
If that doesn't do it for you, go to the "Search" in the right hand corner of the page as I did, type in "Bike Hire"; now check the five pages of results that turn up. If that doesn't help THEN ask your questions. Give you a quick tip, I have NEVER seen a 500 enduro rental.
Ever notice that "What the Heck!" is usually the right answer?
Steve, aprt from a few "chicken chasers" there is almost no bike hire in BKK. One guy used to hire out some very old road bikes but Enduro's, no chance. If you go to Pattaya there is plenty of choice although not in Enduros. You will get AT's (The old codgers weapon of choice) and Transalps BUT they normally don't let them out of the district.