This tracked chassis from Dagu Electronics makes a great base for building a small, tank-like RC vehicle or autonomous robot. The chassis includes a battery holder and two DC motors, with an independent drive train for each tread, so all you need to add to make a complete robot is a robot controller and the sensors of your choosing. A unique mechanism allows you to configure the clearance of the chassis by adjusting the angle of the gearbox assemblies. This version of the chassis does not have encoders.
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The Rover 5 tracked chassis, made by Dagu Electronics, is a durable robot platform with caterpillar treads that let it drive over many types of surfaces and uneven terrain. All you need to get the chassis moving is a robot controller or motor controller to drive the two built-in DC motors, which powered by the included 6-AA battery holder (AA batteries sold separately). A small Phillips screwdriver and an Allen wrench are included for working with the chassis.
More info at the Pololu Website