I get that low gears > more power but low speed. I get that high gears >low power but high speed.
But can someone give me the brain dead intuition of why you need to change gear “sizes”? A single sentence if possible.
I’ve tried Googling it but they always use a bike example. I’ve never ridden a bike. Or they start talking about ratios and it just goes over my head.
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Your engine has a narrow range of RPMs where it works. Too slow and it stalls out because friction slows it down before the next piston can fire, and too fast and it tears itself apart.
If you need to be able to go at a crawl in a drive thru or pulling into a parking spot (maybe 5km/h), and you set that for your lowest RPM (about 1000 RPM), you would top out at 5000 RPM being 25km/h. That’s not very useful.
So you need to add gearing in. Each gear in the car is a different ratio of engine turns to tire turns, letting you cover the entire range of speeds that your car needs to go.
As an added benefit, combustion engines are most efficient at lower RPMs, so you can also typically get to your cruising speed in an appropriate gear to minimize your RPM, which decreases fuel consumption.
You might notice that electric cars don’t have gears. Because their engines don’t have the same moving parts with pistons firing (only the actual driveshaft rotation), they don’t need gears because they can just directly drive the tires at different RPMs.
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