There's a fair bit of altitude gain between the lower valleys and the upper reaches of the Himalyas as you can imagine. Get's pretty cold and snowy in the high Himalaya's in December, you'll be limited to the lower elevations for sure.
As Niello points out, the fall trekking season is now (October/November) but last time I was there, we were happy to get out of the mountains by the end of November as the snow started to accumulate up high. Passes get snowed in and the seasonal herders and settlements retreat back down the valley for the winter.
That mostly impacts the trekkers and climbers rather than riders though the rideable roads into the mountains don't get up that high. The end of the road takes you only to Jiri or somewhere around there if you are looking at heading up toward the park.