Can anyone please explain how the bump-and-go mechanism works in "old-school" toy cars?

It's the one that uses a single swivelling wheel in the front, and when resistance is encountered (due to the car bumping against an obstacle) the wheel turns and reverses the toy car in a different direction. This is achieved with nothing but a motor and some clever gear work, the specifics of which eludes me.

An image of the mechanism:

And a video of such a car in motion (pardon the music):

I have exhausted my googling skills on this. Perhaps I'm missing the right keywords for this mechanism.

closed as off-topic by Brandon Enright, Neuneck, Jim, Kyle Kanos, Prahar Nov 11 '14 at 18:15

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    This question appears to be off-topic because it is about the specific mechanism of a toy car and not the physics behind it – Jim Nov 11 '14 at 14:39
  • @Jim as stated in my second comment to akhmeteli's reply, I'm interested in the actual physics that are responsible for what can otherwise only be achieved through electronics, controllers and sensors. It is a very good example of applied physics and something that should be included in every introduction to physics class. – ktorn Nov 12 '14 at 1:23
up vote 1 down vote accepted

According to :

"These kind of toys usually have a simple mechanism driven by a single motor, it's not easy to describe tho. Usually, a vertical motor drives a pair of wheels in a free turning cage. It's a kind of differential - imagine the solid back axle of an old car with the prop shaft vertical.

The whole toy balances on these wheels, stabilised by a couple of protrusions on the underside of the toy. Because of the design of the bottom of the toy, when there`s little resistance, the robot moves forwards, but if it is stopped by an obstacle, the resistance on the wheels causes the the cage to be spun by the motor, thus steering the toy in a different direction - Imagine how a dodgem (bumper) car works except the steering wheel is turned by the motor when the driving wheels are obstructed."

EDIT (11/11/2014): OP added some information in the comments, so, not being able to inspect the toy, let me hypothesize. Probably, the toy's behavior can be implemented in several ways. For example, the following is possible. For example, the motor can drive (rotate) both the wheels and the cage via gears, but not simultaneously (for example, if the gears have shallow teeth, and when the gear driving the wheels is engaged, the gear driving the cage is not, and vice versa). Far from obstacles, wheel rotation is unimpeded, so they rotate, and the cage does not (as the relevant gear is disengaged). Furthermore, cage is impeded by friction between the wheels and the floor. When the toy bumps into an obstacle, the wheels are impeded (the resistance to wheel rotation is greater than resistance to cage rotation), so the wheel driving gear disengages and the cage driving gear engages. This mechanism also explains why "when the toy is lifted off the ground it's the cage that spins, not the wheels" - the cage rotation is not impeded by friction between the wheels and the floor.

  • I'd seen this page already, and it does try to explain it, but the key is "the resistance on the wheels causes the the cage to be spun by the motor". I'm still not clear how the resistance on the wheels shifts the force from turning the wheels to turning the cage. – ktorn Nov 11 '14 at 7:03
  • In other words, in terms of physics, what exactly happens in the differential that makes one wheel turn in one condition, and then the cage turn in another condition. Seems like when the toy is lifted off the ground it's the cage that spins, not the wheels. That's also puzzling. – ktorn Nov 11 '14 at 8:39

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