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?
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I believe you are referring to Jake brakes. That really loud noise they make is engine brakes (jakes) not downshifting (mechanical braking).
A driver can engage his Jakes which will open a valve in the cylinder during the compression stroke so that way gasses will exhaust out of the engine rather than pushing the piston down and creating more power.
Downshifting is using the transmission and engine to slow down. You can only gain so much speed at each gear ratio (unless you have want to turn your engine into a bomb) which is why if you floor it in 1st gear your engine will shoot up to 7,000 rpms but you will only go like 15 mph.
As you increase your gear ratio (the amount of times the wheel will spin for every rotation of the input shaft/engine) you will need less engine rpms to go faster. Thats why when you downshift your engine rpms shoot up. You’re doing 65 mph in 5th gear with 4k rpms, but the lower gear would need you to be going 7k rpms to get the same speed.
Basically when you make the engine work harder to spin the transmission with no gas, it’s going to slow down.
Normal manual vehicles do not have Jake brakes but they can downshift because it is better than hard braking. Semi trucks have Jake brakes but they can also downshift if they have a manual transmission. When you use your engine and transmission to slow down, you’re avoiding the use of your brakes, which keeps them cool. Heat will cause damage and can even cause malfunctions with your brakes.
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