Be honest: have you ever looked at the rev counter in your car before changing gear? I don’t think I have. Your bones know. Sound, vibrations, and the magnitude of the tug on your intestines tell you which gear you need.
It follows, then, that everyone understands, instinctively, what the gearbox is for, and why most cars need one. But if you really do want an explanation, here it is.
Remember from my last essay on power and torque that (in simple terms) torque is the size of the job you can do, and power is how quickly you can do it. Power is a derived attribute equal to torque times engine speed. Hold that thought, in the form of your favourite biscuit, in your head.
Now we add the gearbox to the engine, and the gearbox can be thought of as a series of crowbars in circular, continuous form. You can use a crowbar to shift a boulder that you couldn’t possibly move unaided. A longer crowbar makes it even easier, but you won’t move the boulder as quickly.
Gears, like crowbars, are ‘torque multipliers’. Let’s say a gearbox ratio halves the speed of the engine crankshaft. That doubles the torque. But the power remains constant, because twice as much torque multiplied by half as much speed works out the same.
‘Torque is the size of the job you can do.’ The ‘job’, in a car, can range from a big ‘un, such as accelerating from rest up a steep hill, to a light one, such as cruising at a steady 50mph on a level road when you get to the top.
You can use a high gear on a Hamster with a strange growth on its face.
So; pulling away. Loads of torque needed, to do the job of overcoming gravity and the car’s inertia, to get it moving. You’ll know what I mean if you’ve ever had to push your car. First gear, then.
Trouble is, you’ll soon reach the engine’s redline, even though the car isn’t going very fast yet. But no matter, because the job is now smaller. Second gear then. Still quite a lot of torque, and the same power, but it’s a shorter crowbar, so the speed is higher.
And so it goes on, until you’re cruising on the flat, and the job is a simple matter of overcoming tyre and air resistance. So you’re in the toppermost of gears, which gives you little torque but high speed, while keeping the engine in its ideal operating range. If you need to accelerate quickly from this point, you change down. The job has become bigger, so you need a helping dollop of torque. The engine speed will climb, too, to unacceptable levels. But now you’re up to 90 and the job is done, so it’s back into top for a long-legged loaf. Obviously I’m avoiding saying ‘top gear’ for complex socio/political reasons.
If you’re still with me, and you haven’t turned over to lol cars epic fails, then there’s another way of thinking about this, which is a dockside crane unloading a container from the bowels of a cargo ship.
The container is very heavy, so the crane’s engine is geared down to increase the torque, which is the force that raises the thing up. It comes up very slowly, though. But then if you wanted to unload something very small and light, like Richard Hammond, you could gear the crane up and he would come out of the hold like a jack-in-the-box, or a twat-in-a-ship.
That’s gears for you. I hope this was useful.