Quoted from toyotaboy:Funny how much designs change over time. The original playfield art didn't have any wheels for the motorcycles because there were going to be pop bumpers there. Apparently they were too obtrusive for the ramps
http://www.pinballnews.com/games/fullthrottle/index4.html
The pop-up bumpers/spinners are still there.
From the article linked in the OP: "A number of the final game's features were missing, including the pop-up bumpers"