You can ride a bike to work. If you're cray (like me)
. I see sometimes mormon missionary kids riding bikes in their white shirts and of course wearing a helmet.
It took me a while to get used to the traffic, but it can be done. Nowadays I always ride a bike if the time & distance allow for it. In any kind of traffic. Up to and including the Sukhumvit rush hour and the Din Daeng intersection. Trips which are generally declined by ex Imperial Kamikaze Pilots, as they consider it much too dangerous...
Remember that driving is done here on the left
side of the road. Thais are generally quite forgiving in traffic. It rarely happens I get into trouble because of cars. Usually you have to be very aware of human torpedo's: a guy on a motorbike is just as dangerous as a WW2 torpedo. Exactly the same mental capabilities and they can only go forward. Fast. What they lack in explosive power they make up with much higher speed.
I've been riding a bike to work since 2001. There are plenty of bicycle shops, especially in the poorer areas of town. You'd be amazed what a humble shopowner of a bicycle repair shop can do for you, and how little they charge for it. A very good area is called 'mahachak', near Klang Hospital in Chinatown. About 40 dedicated bicycle shops. Mainly low cost bikes, but also some reputable dealers for upscale brands, like Trek. Also a lot of repairshops close by.
High end shops you can find all over the city. Not a lot, but Bangkok is quite large. So you find plenty of them. They tend to be rather expensive and not quite as friendly as your local bike repair shop.
Biking community? I don't quite know, as I am more or less a loner. There is some biking going on, in clubs. Maybe if you have some info about them, mail me.
Helmets: I'm Dutch, so I never wear them. Ever seen a Dutchman with a helmet on a bike? (The last who did were taken POW after the German invasion in 1940)
According to Thai law, a bicycle is a pedestrian on wheels. You can ride on the sidewalk, and have to be careful on the road. Not that people will hate you for it, but simply because they aren't used to it.
I never wear a mask, as I think the benefits don't weigh up to the disadvantages. You prevent breathing in dust particles, but that is about it. Dangerous gasses you certainly inhale. Not even a military gasmask will help. A bike is a big advantage in a traffic jam, as you can zig zag very easily though the traffic. Actually, I rather cycle through a almost not moving traffic jam. Almost zero danger. Except from those human torpedo's who do the same thing.