# How does a two-gear pull-back car toy work?

This is not a robotics question, but this Stack Exchange is the closest I could find to mechanical engineering. Please refer me to a better place to ask this, if one exists. Hopefully someone might just know this.

I got a pull-back car for my boy at McDonalds, and it has two gears. It starts slow, then speeds up after about two seconds. It's impressive to me, especially given the inherent cheapness of toys sold by McDonalds. It feels solidly built as well.

I couldn't find anything related to this concept. The wiki on pullback motors does not include any information on multiple gears.

Any ideas on how this works?

• Hi @aliteralmind and welcome to robotics SE. This question is totally on topic here. I just wanted to let you know that a new, general purpose Engineering SE site just opened up.
– Paul
Jan 24, 2015 at 6:00
• Can I have a picture please ? Thanks.
– user11204
Nov 30, 2015 at 20:58

These types of low cost 2 speed transmission almost always are based on some gyroscopic effect on a simple gear train.

You are right, even knowing how they work, I could not find any described on the internet. Very surprising.

Look at this image;

Imagine as the speed increases, the lower block will rise.

So now you have to imagine it attached to a shifting lever (or gears themselves).

Combine the 2 concepts and you have automatic 2 speed gear shifting.

In practicality, it is usually less costly to use infinitely variable transmissions for the same effect on anything of decent size / value.

• Not totally sure if gyroscopic effect is entirely accurate, Conservation of Angular Momentum may be a more appropriate description. But I think you followed what I was saying. The faster it spins, the larger the diameter becomes, mechanically lifting the bottom ring in the process. Jan 24, 2015 at 8:17
• Thank you. I now understand how the gyroscope moves the gear selector, and the concept in general. But only the first picture is clear to me. It's hard to see in the second picture how, when the shifter is moved (shifted), how a new gear is selected. Perhaps it's my inexperience with these kinds of diagrams, but I would think there should be a second location (a different configuration of gear sizes) that is sitting there unused, waiting for the gear to be shifted to. It seems like threre is only a single gear. Jan 24, 2015 at 16:31
• You are right, that is not the best diagram in the world to show the idea. Indeed one gear on either shaft would be offset so that only one pair at a time is engaged. Jan 24, 2015 at 18:41