An eccentric toy

 When this plastic car is pushed along, the grey bits on the front and back rock from side to side to give the impression of driving over rough ground.

The effect is achieved by spikes on the axle.

When the spike is vertical it pushes up that side of the bonnet. On each end of the axle, there are two spikes set 180 degrees out. But the ones at the bottom of the axle in the photo are shifted by 90 degrees from the ones at the top. So are they out of phase by 90 degrees? Well, when one side of the bonnet is completely up, the other side is completely down. That is 180 degrees out of phase. It is interesting that the spikes need to be 90 degrees out to give 180 degree phase difference. That is probably about projecting from circular motion onto a linear motion.