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 is to slow down the truck without using the brakes. The reason you don’t want to use the brakes is because after a while they get hot and risk catching on fire.

When you let off the accelerator, the wheels turn the motor more than the motor turns the wheels… Which causes the wheels to slow down (like a brake) because turning the motor is difficult.

Down shifting makes the wheels try to spin the engine even faster, which is harder to do, so it slows the wheels down even more.

This works with fixed-gear bike as well…. Except instead of a motor, the wheels cause your peddles to move, which causes your legs to move. The more you try to resist your legs from going up and down on a down-hill, the slower the bike would go.