Bike displacement/ combined rider bike weight > #. Maybe a couple different classes. Yes you can add riders (2up) to "make weight". Knobbies are allowed to cut the grass between T1 & T4.
Pit bike race was excellent. I think there were over 20-25 bikes, if not more. And a small bike actually won it. I need to start short-cutting the course or something!
Small bike did win it, with some excessive track shortening - I protested the validity of that win but was promptly denied.