FWIW I don't find the effect unpleasant and it does give an impression of movement. Unlike motor sport where the whole subject is travelling across your field of view, a biker shows movement both horizontally and vertically, and you can't pan for the latter.
The only solution would be a faster shutter speed.
Edit
If you'd have used 2nd curtain sync then the blurring in front of the front forks would have been behind the forks and looked more realistic. If you're going to do a lot of this in the dark or low light I'd say use the fastest sync speed you can and 2nd curtain sync.