Eli5: What is the purpose of downshifting for a semi-truck and how does it work?

Question

I have always wondered what those loud noises are the semi-trucks make as they appear to be slowing down, but I don’t understand how that helps?

In: 547

Answer 1

I think you’re thinking about the engine brake. It’s using decelerative forces in the engine to slow the speed the wheels are turning at. Shifting down, and slowing down the vehicle with heavy loads with minimal wear on the brakes.