Rama 9
The closest place to Bangkok to do road riding is Rama 9. Directions to get to cafe: Take Chonburi Motorway 7 to Rom Kla exit (just to the left of exit for Suvarnabhumi airport). Turn left onto the service road (back in the direction you were just traveling) and then turn right (approx. 2.5 km back) onto a side road. There’s a huge RK Park billboard on the corner. The cafe is about 200m on the left. A map is here.
Riders meet there randomly on Sat and Sun.
GOTP
If you are interested in road rides in and around Bangkok (including a weekly Sunday ride), then sign up for the GOTP email list: http://groups.google.com/group/gotpbangkok
SpiceRoads Cycle Tours has Trek 2.1 C road bikes for rent for 800 baht per day. Click here for more info on bike rentals.
Bangkok Hash House Bikers
If you would like to go mountain biking once a month somewhere 2-3 hours drive out of Bangkok check out the Bangkok Bike Hash (www.bangkokbikehash.org)