From a weather point, the peak seasons of northern Vietnam are mid-September till mid-November and March till mid-May. These are also the seasons when we strongly recommend early bookings of bikes and tours to ensure availability.
There is no best season for a motorbike ride. Each season has its unique characteristics and advantages, as well as, a range of local festivals. Summer runs from mid-May into late August with the months of June and July being the hottest and most humid. The cool and dry season starts from the end of November to early March. Expect December into end-February to offer fairly chilly rides up on the mountains so bring warm clothes in that period.
At QT Motorbikes and Tours we often get the question if it is likely to rain. Yes, it is likely to rain on any day in any place in Vietnam – that is why this country is so green and lush. However, weather up in the mountains is very changeable and it, like the views, can shift from valley to valley. Like anywhere one needs to adapt the speed to weather, but there is no real problem or extra risk with some rain.
The so-called "rainy season" of June to mid-August has earned its name simply from the fact that that it is the time we see the heaviest downpour in northern Vietnam. However, it might not rain at all on the days you are out. This season is characterized by thundery rains (often late afternoon/night). It might be really heavy rain when it comes down, but it is normally short-lived and after the rain, you may have a blue sky for the rest of the day.
Remember that the pilot that took you to Vietnam did not Google weather sites weeks in advance to determine the flight route. S/he got the latest predictions an hour before your flight and adapted to it. So should you, when going out on a bike! No matter whether you will enjoy your ride!