Manual TransmissionsHow Manual Transmissions Work Transmissions
Every car has a certain rpm range where they will make the most power. This rpm range is known as the power band. A transmissions (when racing) is used to keep a car within that range no matter what speed. At other times a transmission is used to keep a car at a low rpm inorder to reduce fuel consumption. Transmissions must serve another purpose as well. They have to have the ability to take the power created by the engine and be able to connect and disconnect it to the drivetrain.
At times a car requires more power to perform a task, for example when towing something, and all transmissions (including automatic) have lower grears there so you can drop a gear. By dropping a gear you increase your engine rpm at a given speed. More engine revolutions means that for every revolution of the tires you get more power stokes.
The transmission is located between your clutch and drive shaft.
The transmission uses gears to allow the power from the engine to be used in different ways. For example, while traveling up a hill you want more power to go to your wheels for every revolution. By putting it in a low gear it will allow your car to climb the hill easier. To be in a low gear means that the transmission uses a small gear on the engine out put shaft and large one on the drive shaft. This will allow the engine to apply more power per revolution of the drivetrain.
When traveling on a highway, you wouldn't need as much power and you also want the wheels to move faster then the engine. That means that you would want a larger gear on the engine output shaft and a smaller gear on your drive shaft. That would allow the tires to move faster then the engine. Usually the ratio of the 6th gear in a 6-speed is around 1.0, so for every one revolution of the crank shaft there is one revolution of the driveshaft.
Parts
A basic transmission somewhat looks like this.
The crankshaft is always spinning when the car is moving unless of course you decide to stall your car. The shaft coming from the clutch spins the layshaft. The layshaft then spins the two blue gears, which are on bearings. The bearings allows the blue gears to spin on their own when they are not engaged, this way they will not affect anything. The collar is connected to the gear fork which is overly simplified in this picture, however, in a car that is how u would change gears. The collar is directly connected to the drive shaft, that means which ever gear it connects to it directly links the two together, that is the gear and the driveshaft. When the teeth on the collar, known as dog teeth, move into one of the blue gears by the gear fork, it will make the drive shaft turn. When the drive shaft turns the tires will turn. Just so you know I used the colors only to make it easier to explain they are, however, not as colorful in a real car.
Double-Clutching
A problem with this configuration is that it does not have "synchros." Inorder to make the collar spin at the right speed you must do a manuver called the double-clutch. In many older cars you must double-clutch. The first thing you do is push the clutch pedal down in order to disengage the engine from the transmission. This will allow you to put the car in neutral. The next step is to release the clutch and rev the engine till the rpm value of the engine is were it should be in the next gear. This will get the next gear and the collar rotating at the same speed. Finally, you push the clutch in a second time and put the car in its next gear. It is neccesary to get the collar and the next gear at the same speed or you will grind the teeth on the collar. Everytime you need to change gears you will need to use the clutch twice, that is why it is known as "double-clutching." Double-clutching was necessary in many trucks. I specifically remember talking to my grandfather about the army vehicles during world war 2.
Below is a regular five speed transmission. Although it has no layshaft.
If you turn the gear selector on the picture to the right 90 degrees you will see how using a shift knob can change the gears. You must also consider that moving the shift knob backwards pushes the fork forwards because of the pivot point. Also moving the shift knob left and right changes the fork that you move. These motions can be seen in the following image.
Reverse.
To allow your car to go in reverse and extra gear is needed, called the idler gear. This gear reverses the motion of the drive shaft, which in turn will reverse the motion of the tires.
In the image below you will see that the blue gear connected with the idler gear will be moving in the opposite direction.
This gear would always be moving in the opposite motion as the other gears, therefore if you wanted to try and push the shift knob into reverse while moving forward it would not work. The dog teeth would not engage and they would just grind up and make a lot of noise.
Synchronizers
In modern cars double-clutching is replaced by a synchronizer. The synchronizer's purpose is to connect the collar to the gear before the teeth touch the gear. This will allow the collar to synchronize its speed with the gear so that the dog teeth will not grind. This is the basic image of a synchronizer.
The cone protruding from the blue gear is what makes contact with the cone shaped part of the synchronizer. Friction between these two parts is what synchronizes the speed of the collar and the gear. The cone shaped part of the synchronizer slides in so that the teeth can connect with the gear.
Many companies and manufacturers use synchros in there own ways and so this might not be the most acurate but it is the general idea.
