The Unofficial LEGO Technic Builder's Guide, 2nd Edition

20adders and subtractors

Adders and subtractors are mechanisms used to couple two or more motors together. Coupled motors are usually used to control a single function, most often the propulsion of a vehicle. They can work together (in an adder) or against each other (in a subtractor). Both mechanisms make use of differentials, and both are examples of advanced mechanics. The way subtractors work is particularly fascinating.

You’ll find that using adders is a great way to give your motor even more power. Subtractors will be most useful when building tanks and construction vehicles, as these mechanisms have two outputs perfectly suited for controlling two treads.

hard-coupling

But first, let’s consider a simpler way of coupling motors, one that ...