I believe it is the same physics engine as WRC 5/6/7 but I assume with modifications for bikes. I can't get a feeling of the front sliding if you push it and as you say over the bumps the front grip is unpredictable. One thing I have noticed watching onboard videos is the revs of the bike increase at the end of Sulby straight when the bike is lent over because the diameter of the tyre is less on the edge than in the center. This doesn't happen in the game and looking at the physics I think the tyre has a constant diameter all the way across. Another giveaway this is an engine for cars adapted to bikes.