The first three times I visited Chiang Mai, way back when, I rented a road bike, an old aluminium Trek with Shimano 105 components. In total I rented the bike for just shy of two months. When I eventually did the calculation I realised that the money I’d spent could have bought me a brand new version of the bike I had been renting plus a fortnight in my basic Thai apartment.
Choice is better now than it was then. It is possible to pick up a cheaper deal (with commensurately cheaper components) or a fancy carbon steed (at some considerable cost) but ever since I have been something of a sceptic when it comes to Chiang Mai road bike rentals. If at all feasible, I’d bring my own bike or even consider buying a bike and selling it at the end of the holiday.
However, that isn’t always going to be practicable or cost effective, especially for a short trip. So here is a list of the bike rental options currently available in Chiang Mai. I have no recent personal experience of any of them so there is no endorsement (nor criticism) implied by their inclusion.
One final point. All of these suppliers have a limited stock, especially in the larger sizes. If you are coming and definitely want to rent a bike I’d suggest you make your reservation as early as possible to avoid disappointment.