Thailand’s climate is broadly the same all year round: tropical, humid and hot with temperatures almost always at least 30C. At certain times of the year you can find extremes within the general theme.
From Bangkok northwards, the wet season runs roughly from July to October. During this time you’ll have to deal with rain storms most afternoons but usually these don’t last long. Since the roads are in good shape in Thailand, the rain normally doesn’t disrupt travel if you’re happy to pedal while getting wet. Sometimes the rain is quite welcome and refreshing after a hot and sweaty day! The rainy period coincides with low tourist season so the trade-off for braving a few thundershowers is less crowded accommodation and tourist attractions.
From November to February is the cooler season in the north of Thailand. Days will be sunny and tourists will be flocking in from northern climates. March to June brings prohibitively hot temperatures. Take great care to avoid cycling in the heat of the day and always carry plenty of water.
Things are slightly different in the south, where variations can cause quite different weather from one side of the penninsula to the other. On the west coast, most rain tends to fall between April and October but on the eastern side it’s wettest from September to December.
Travelfish have an excellent guide to weather in Thailand.