Okay, I got it down that, when you change into a higher gear, it allows the car to go faster. My question is; how?
Each gear combination has a lower numeric ratio of input to output revolutions. The first gear pair may have a ratio of 3 or 4 engine revolutions to one output revolution. This multiplies the torque you need for starting off. All the subsequent gear pairs have lower numeric ratios leading to the top gear which may be 1:1 or lower. Each time you shift to a higher gear the output turns more revolutions for the same number of engine revolutions.
This site on transmission is from HowStuffWorks.
Just read through and keep hitting the “next page” button as required.
Come back if you have further questions.
Let’s try a simple explanation:
Your car is really heavy. And, your little tiny engine has a hard time getting it moving. The most efficient way to get things in motion is with a gear system.
As I’m sure you’re aware, your engine produces power in the form of a crankshaft turning. Imagine that power as a wheel, a gear of fixed size, and the engine is making it turn. That’s how much power you have available. Now, what’s the best way to use that limited power to get your heavy car moving?
Gears are the answer. See, one rotation of what I’ll call our engine-power-gear isn’t enough on its own to get things moving. So, we’ll attach a larger gear to it. That way, when our engine-power-gear (EPG) puts out one rotation’s worth of power, it’ll move the big gear just a little bit. Or, to look at it another way, the engine’s full power is being used to create a small movement, and the power of many rotations of our EPG is put into moving that big gear just once. That adds up to a lot of power (though only one rotation), and this is what you need to break inertia and get your vehicle moving.
But once the vehicle is moving, it doesn’t take that much energy to keep it moving. So, we’ll change from that big gear to a smaller one. Now things are moving more efficiently, so you don’t have to apply as much power to each rotation of the gear attached to your EPG.
In the highest gear, you’re basically just overcoming wind resistance; there’s very little power being exerted by the engine into keeping things moving. But if you try to start from a dead stop in fourth gear, you ain’t going nowhere. The engine just won’t be applying enough power to the gear for each rotation.
Man, I hope that’s understandable. Tell me if I need to clarify anything.
What happens if someone attempts to start in the highest gear? Will something snap? Will you get the “Transmission flying out the side of the car” effect?
In most cases the engine simply won’t have enough torque to move the car, and will chug to a halt.
Starting in a higher gear is not unusual. Well, certainly not fifth gear, but you can usually start in second, and with some really low-ratio off-road vehicles, you can start in third. It all depends on how much torque you have to move the car. Starting in second is usually used when you want to reduce wheel spin, ie on surfaces with dodgy traction. Incidentally, the newer Porsche 911 Turbos with Tiptronic gear shifts automatically come off the line in second gear instead of first.
Gjorp: Also, if you’ve ever ridden a multi-speed bike and changed the gears, you should get a pretty good idea of how gearing works. Try going uphill in a high gear. Pretty tough, eh? That requires a lot of force? Switching to a low gear requires more revolutions, but a lot less force. A similar thing happens with cars.
If you start at too high a gear, the engine will not have enough power to twist the wheels and move the car. Therefore, it will stall. Start at a low gear, and the engine has no problem moving the car.
But also, it is possible to blow a half-shaft or differential in a way not wholly unrelated to this. Of course, you won’t be able to start your car in 5th, but your half-shafts are usually built to be able to withstand only a certain amount of torque. Exceed this “twisting power” and it’ll either snap or bend into a very interesting shape. I’ve never seen this on normal cars, but with off-road vehicles, if you’re bouncing around a lot, sudden changes in grip can have disasterous effects if you’re applying too much power to the wheels. Ah, but that’s another thread, I suppose…
Contrary to what others have said, you can start in 5th (I’ve done it - I was bored), but you have to be very very very slow on letting out the clutch, ie you don’t have the clutch all the way out until you’re well up to a speed that 5th can handle - 50Kph (30Mph) or so. Obviously this is bad for your clutch and serves absolutely no useful purpose what-so-ever. I don’t recommend you try it with you’re own vehicle - try it with a hire car or work vehicle :).
Oh, it helps to have an empty road, this avoids those pesky road rage arguments with people frustrated with your slow progress.
But, for the short answer: Trying to start a car normally in 5th gear will simply result in a stall.
Jman
You’ve got a 5 foot long lever, resting on a fulcrum placed a foot from one end, leaving 4 feet on the other end. Hold the end that’s 4 feet from the fulcrum and move it down 12 inches, the other end moves up 3 inches. Now go the end that’s one foot from the fulcrum and move it down 12 inches, the other end moves up 48 inches.
The first exercise above was a low gear, the second was a high gear. The thing you’re trying to move (the opposite end of the lever) travels farther/faster in high gear.
Transmission gears are specialized levers. As others have mentioned, we use low gear to overcome inertia and high gear to move rapidly.