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When you’re driving a manual transmission vehicle you don’t need to know a lot about what’s happening under the car. If you learn and follow some basic rules about how to use the clutch pedal and the gear shifter, you’ll be fine. You know that when the manual transmission car is in gear and your foot is off the clutch pedal, the clutch is engaged and the engine is transmitting power to the wheels. When you press the clutch pedal, the clutch is disengaged and no power goes to the wheels.
You know you have to disengage the clutch when you’re shifting gear. You know that if your speed is too low for the gear, the car will struggle and may stall. If your speed in the gear is so high that the tachometer reaches the redline, the engine loudly protests and can even be damaged.
It’s OK to simply accept these things as being true without worrying about why that’s so. However, it’s still worth knowing a little about what’s going on in the background. There’s some satisfaction in understanding what happens when you push the clutch pedal and shift gears. Also, people are more inclined to do things properly, such as shifting smoothly and pushing the clutch pedal all the way to the floorboard, when they understand what’s going on in the transmission.
What follows is a simple overview of the parts and functions of a manual transmission. It’s at a level to help the average driver understand how the movements of their hands and feet translate into moving the manual car steadily forward.
Why Do Cars Have Different Gears?
Your self-propelled lawnmower doesn’t have multiple gears. You only vary the forward speed by controlling the throttle. The earliest cars operated that way, too. The trouble was, they jerked forward as they started and you couldn’t make them go too fast without damaging the engine. With small engines and slow cars, that worked.
Engines are effective at only a certain range of RPM, called the power range. Trouble is, the range isn’t wide enough to provide the different levels of force, or torque, required for different driving conditions. For example, it requires more torque to get a car moving from a standstill than it does to keep moving. You need more torque to climb a hill than to drive on the flats. You increase torque by turning the wheels at a lower speed relative to the speed of the engine.
The problem is to provide various levels of torque with a relatively narrow band of engine RPM. The solution is to change the speed of the wheels relative to engine speed with gearing. Even if the engine is at the same RPM, the wheels turn slower in a lower gear than in a higher gear, and the engine provides more torque.
(The term “gear” can be a bit confusing, because sometimes it refers to the gear you’ve selected on the gear shift, and sometimes it refers to the little wheels with teeth that mesh together within the transmission.)
A transmission meshes gears to achieve different gear ratios. When you’re in first gear, the engine drives a gear with fewer teeth, and it’s meshed with a larger gear with more teeth. That larger gear drives the wheels. Therefore the wheels spin more slowly than the engine but with more power. This is your lowest gear ratio. As you go up through the gears, the gear ratios get higher. In the highest gear there are fewer teeth on the output side and the wheels spin faster than the engine. You don’t need as much torque to maintain cruising speed once you’re there.
What Are the Parts of a Manual Transmission and How Do They Work Together?
The transmission in your stick shift car is sometimes called a gearbox because that’s what it is: a box with a whole bunch of those little gears inside of it.
The transmission contains three shafts. The input shaft is attached to the engine, and the output shaft attaches to the differential and drives the wheels. They’re connected by the countershaft, also called the layshaft, which has gears that mesh with the other two shafts. There’s one countershaft gear that connects to the input gear and many countershaft gears paired with output shaft gears.
The input shaft isn’t always hooked up to the engine. It’s only attached when the clutch is engaged. When you press the clutch pedal, the connection is broken. The engine continues to spin and the wheels continue to turn, but they’re independent of each other.
When the clutch is engaged, how is it determined what gear you’re in? It’s in the connection between the countershaft and the output shaft. Assuming you’re driving a 5-speed, there are six pairs of meshing gears (one is for reverse) on the countershaft and output shaft. All of these pairs are always meshed, so that all of the output shaft gears are always turning when the clutch is engaged. However, all but one are turning independently of the output shaft itself. The output shaft runs through the gears but it spins independently. Only one output gear at a time is connected to the shaft and actually moving the vehicle. It’s connected to the output shaft with a collar.
Collars lie between the gears on the output shaft. A 5-speed has three collars, each one between the two gears it controls. When you put the shifter into a gear, it moves a gear selector fork that’s attached to the collars and engages the right collar with the right gear. The collar is a dog clutch that has dog teeth which make the actual connection with the gear.
There can be a synchronization problem here. The collar and the new gear are spinning at different speeds and there’s a potential to grind when the dog clutch is forced into the gear. To alleviate this, different manufacturers provide different synchronizer mechanisms that match the speed of the collar and the gear so they can engage smoothly.
When the manual vehicle is in neutral gear, none of the individual gears is connected to the output shaft. All of the gears spin merrily away but the output shaft doesn’t turn.
Reverse gear is a little different. There’s an idler gear between the countershaft gear and the output shaft gear that makes the output shaft change direction. Reverse gears are not synchronized. You have to stop to shift into reverse. If you’ve accidentally tried to force the gear knob into reverse while moving forward, you probably didn’t like the sound you heard. Your gearbox didn’t like it, either.
How Does This Relate To My Driving?
Here’s a summary of how your transmission responds when you get into your manual car and head down the road. As you start your car and shift into first gear, the gear selector fork moves the appropriate collar onto first gear on the output shaft. As long as you have the clutch depressed, there’s no connection between the engine and the manual gearbox, so the shafts in the transmission aren’t moving. If you “blip” the gas pedal, the engine RPM increases but the transmission is unaware that you’ve done anything.
When you raise the clutch pedal toward the bite point, each clutch disc moves toward the other. When you feel the bite point, the clutch is partially engaged to the point when the transmission’s input shaft has begun to spin. This turns the countershaft, and all the countershaft and output shaft gears begin to spin. First gear on the output shaft is engaged with the shaft, and the shaft will turn according to the gear ratio of first gear. Because that’s a low gear, the output shaft turns more slowly than the countershaft but with greater power.
As you move past the bite point, the connection from the engine through the clutch becomes more engaged, and the input shaft and countershaft move faster. More power is delivered to the output shaft, and the car begins to creep forward.
When you take your foot completely off the clutch pedal and press the accelerator pedal, the engine RPM increases and the input shaft turns faster. The clutch is fully engaged and the car is moving forward in first gear.
Now it’s time for changing gear into second. You press the clutch and the engine stops delivering power to the transmission. The countershaft and output shaft are still turning (as long as the wheels are turning) but are no longer being driven. As you move the gearshift to neutral, the gear selector fork moves the collar away from first gear on the output shaft. As you continue the motion of the gear lever into second, the selector fork engages the dog teeth of the collar into second gear.
As you release the clutch pedal, the engine reengages with the input shaft and drives the vehicle forward in second gear. You now have a higher gear ratio, and the same engine RPM will produce less torque. That’s good. The car is moving and you don’t need as much torque. The same thing happens as you shift into third and all the way to your highest gear.
You can see how smooth shifting and clutching is important in driving stick. If you “pop” the clutch in your new gear by releasing the pedal too quickly, the input shaft will suddenly start turning at high speed and will want to lurch the car forward. It may even stall. If you ram the gear stick into the new gear, you can defeat the transmission’s synchronization mechanism. The grinding that you hear is the dog teeth scraping against the new gear rather than smoothly sliding in.
Now That I Know This, So What?
You can safely and successfully drive a manual transmission vehicle for a lifetime without knowing any of this. It’s not going to be on any test, and it’s unlikely that anyone’s going to quiz you on it. However, there’s a certain level of satisfaction in knowing what’s going on when you press the clutch pedal and move the shift lever.
A manual transmission isn’t magic. What the car does is technologically inevitable based on the action you take. Now that you how much your transmission does for you, be kind to it. Avoid transmission repair by practicing good shift stick driving. Treat your transmission to all its scheduled maintenance such as transmission fluid changes.