The Bangkok bike shop L.P.R. has a team of mechanics at each ride ready to fix last minute problems. They don't quote a price for their work, but please give a generous donation if you use their services. They can also take your sick bike away after the ride, restore it to full health and deliver it to your home in Bangkok. For more major repairs they can be contacted at email@example.com or call 08 1815 7948.
If you don't have your own bike:-
L.P.R. rent miuntain bikes for B700 for one day or B1,100 for two days. This includes a helmet. The bike is delivered to the venue, but you must make your own way there. Book in advance by sending an e-mail to firstname.lastname@example.org or call 08 1815 7948.
SpiceRoads Cycle Tours rent mountain bikes for B500 per day, helmets for B50. You will need to pick up and return the bikes from their Bangkok office on Sukhumvit Soi 71. Here's a map. You can book the bike online, or contact them at email@example.com, or call 0 2381 7490.