I recently got into cars.
In: Engineering
Most tools take an amount of effort and apply it differently. With gears you can have a gear that has ten teeth turn a gear with a hundred teeth. This would make one turn of the first gear turn ten percent of the second gear. You could also turn a gear with a hundred teeth against a gear with ten teeth. This would make a single turn rotate the smaller gear ten times. The first example had a ratio of 1:10, the second one has a ratio of 10:1. Basically it is a measure of how much effort is multiplied or divided by the machine.
A gear ratio is, essentially, about the number of teeth on each gear.
Imagine you have a gear with 20 teeth connected to a smaller gear with only 10 teeth.
There is a handle attached to the first gear.
If you rotate the first gear once, the second gear will rotate *twice*. So we would say these gears have a 1:2 ratio.
This has all sorts of applications in tons of things, not just cars. But, in cars, a “lower gear” has a lower gear ratio. Let’s just say it’s 1:1. Every turn of the crankshaft turns the axel one time. This means there’s more *torque*. That’s how much rotational force is being applied. High torque is really important to get the car moving from a stop, as it takes a lot of power.
Once you’re moving a bit, you’ll want to go to a higher gear. Let’s say 1:2. Every turn of the crankshaft results in *two* turns of the axel. Now the engine doesn’t have to run at such high rpms to maintain your speed.
As you go faster and faster, you generally want higher gears so you’re not straining the engine.
Gears have teeth. Each tooth on one gear fits into and corresponds to a tooth on the other gear[s] it is connected to along those teeth. So the first thing you do is count how many teeth are on each gear. For the sake of a simple example, we’ll look at where one gear has 10 teeth and the other has 20. We connect the engine to the gear with 10 teeth and the wheels (well, the path towards the wheels) to the gear with 20.
When the engine done one full turn, 10 teeth of the gears have been spun through. For the engine side gear that’s 1 full rotation, but for the wheels side gear that’s only half a rotation. So by the same physics rules as using levers (ie a see-saw) to assist in lifting a heavy object, you have only turned the wheels gear half a rotation, but you have applied double the force (torque, or “muscle” as it were) upon it. Or alternatively, the torque required by the engine was much lower in exchange for the fact that you didn’t travel very far.
So that’s a 20:10 ratio, or rather 2:1. Typically ratios are written as x:1 no matter what, even if it means x is a fractional number.
In a real car, 1st gear is probably something like 3.5:1 ratio, your top gear might be something like a 0.7:1 ratio (the engine gear is bigger than the wheel gear this time!), and the path to the wheels goes through a differential or similar device that likely also provides a permanent gearing ratio of around 3.5:1 again. All numbers are approximate of course, and vary by car. Use these numbers are guides only.
The transmission has a bunch of gears (normally 5+ forward and 1 reverse), and these gears all have different gear ratios. The purpose of these is so the car can move at various different speeds while still being able to keep the engine within its RPM specifications. For automatic transmissions, it’s more complex than just having 5+ different gears that you switch between, but each separate gear ratio it can produce is traditionally called a gear.
Along with preventing the car from revving too high and damaging itself, it also allows you to keep the RPMs low when traveling far distances or efficiency, or high when rapidly accelerating and/or driving fast for fun.
(very) ELI5 is basically that a small gear spinning fast will turn big gear slower (but with more usable force). You’re exchanging more speed to more power. This can be controlled via number of teeth on the gear.
You can take two strings, and tie a weight to each of them; small one to one string, and heavier one to another. Now, spinning the string in your hand as fast as you can, you’re using same amount of energy to spin both, but smaller one will spin much faster than heavy one. The small one will have more speed, but the large one will have more force. So the energy you’re spending is converted differently.
That principle is used in transmission to turn fast-rotating engine crankshaft into slower, but more powerful movement of the wheels.
Gear ratios are adjustment/balancing of the whole works – at which ratio do we convert speed to power?
My car might need to do 3 mph (miles per hour) in heavy traffic, and 120 mph on an autobahn. That is a factor of one to forty.
The engine would stall or get jumpy if I tried to use in at less than 1000 rpm (revolutions per minute), and blow up if I ran it over 6000 rpm. That is a factor of one to six.
What a gearbox does is to use gear wheels in different combinations to adjust parts of those speeds so they can both work in their respective ranges.
As an extra, reverse gear puts an extra cog in the mix. Using an odd number of cogs makes the output run in the opposite direction.
Take a look at a pedal cycle. The cogs are driven by a chain instead of direct contact, but the principle is the same. The rotary speed of a big cog wheel pulls more teeth per revolution that a smaller cog. So the rider’s feet on the pedals turn the wheel by different amounts, and the place where the wheel touches the ground moves faster.
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