Thanks for your reply. I hadn't tried it, I assumed it was for 4 wheeled vehicles only. I've just done some experiments and the bike holds together and suspension seems to work but the tyres don't have enough friction and there's none of the self stabilising effects of the steering angle and gyroscopic forces so it's impossible (for me at least) to control.
Here's the file for anyone interested: https://drive.google.com/open?id=1hy7hHxJgM4O82_4g5dz2xi1pabmpXNx3
I guess I should take a look at how the vehiclebody and wheels are implemented and see if I can do something similar for a motorcycle but with more friction and including the self stabilising effects.
I'm still curious why the joints don't work though.