I guess I never really noticed, the whole bottom of the frame is a swing arm. I had a V-RROOM motor back in the day, thought I was cool! 8) Of course I had to take it apart to see how it worked. On the electric motors shaft was a round disc, about 1 1/2", with 3 or 4 loose roller like things, that beat on a plastic Cozy-like cup refill's bottom, anybody remember Cozy Cups?