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

None of these answers are ELI5 enough for me.

#ELI5

Pretend you’re on a bicycle. It’s a simple bicycle, just one gear.

Now pretend that you can’t coast! So…

* If the pedals move, the wheels will move.
* If the wheels are moving, the pedals are moving.

Got it so far? The wheels are **fixed** to the pedals, there’s no coasting at all!

Okay. So you start pedaling, and you go maybe 3mph while you’re pedaling. Neat.

Now you go over the top of a hill, and you start going downhill. And you gain a lot of speed! Too much speed! What do you do?

“Well for one, I’ll stop pedaling”.

Nope, can’t coast. You can’t stop pedaling. This bike is fixed so that when the wheels move, the pedals move. So now the pedals are moving a LOT faster than you really want to go!

So now what?

Well, you put your feet on the pedals and you TRY YOUR HARDEST to **slow the pedals down.** You could call it “trying to pedal backward” but you get the idea. Your feet are on the pedals, and you do your very best to slow the pedals down!

You can’t stop the pedals, because you’re going downhill too fast. So you do your best to just slow the pedals down a little. This is hard work! So you’re grunting and groaning and making all kinds of noises!

**Some trucks** are set up kind of like that. Some trucks, if you put the transmission into a gear that is “slower” than the speed you’re actually going, then the truck will try to “stop the pedals” by using the ENGINE.

Instead of the engine making the wheels go, they’re trying to make the wheels stop. Just like your feet on the pedals going downhill.

What you hear is the engine working really hard to do that.

##THIS IS ELI5.

This is far from complete, and very far from totally accurate. So go ahead and add corrections, but realize I left out a lot ON PURPOSE.

I know. I didn’t explain HOW the engine/gears do this. This is ELI5.

I know. I didn’t explain the difference between engine braking and actual Jake brakes. This is ELI5.