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

The purpose of downshifting is not only slow down but to have more control of the engine.

To downshift, you:

– lay off the gas
– once it’s at 500rpms, put the clutch in, shift out of gear (it’s in neutral now)
– hit the gas so the engine revs up to about 1500rpms
– push the clutch in
– downshift to the next lowest gear.

Trust me, downshifting is so, so important when it comes to coming up and down steep hills on highways. plus it makes stopping so much easier.