Why do you need such a strange device between the wheels of a truck

⁣Between the wheels of some trucks, you can see a rather strange metal device that resembles either a small wheel, or a drum, or even some kind of gear. What is it for and what functions does it perform?
as you might have already guessed, the mysterious device is called the "bear". That's just such a nickname he was given by the people in the driver's environment. That's right, the strange contraption is called the "Robson Drive

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