Welcome back to Gearhead 101 — a series on the basics of how cars work for the automotive neophytes out there.
Because you read the Art of Manliness, you know how to drive a stick shift. But do you know what’s going on beneath the hood whenever you shift gears?
Well, today’s your lucky day!
In this edition of Gearhead 101, we take a look at the ins and outs of how a manual transmission works. By the time you finish reading this piece, you should have a basic understanding of this vital part in your vehicle’s drivetrain.
Let’s roll up our sleeves and get started.
What Transmissions Do
Before we get into the specifics of how a manual transmission works, let’s talk about what transmissions do in general.
As discussed in our primer on how a car engine works, the engine of your vehicle creates rotational power. To move the car, we need to transfer that rotational power to the wheels. That’s what the car’s drivetrain — of which the transmission is a part of — does.
But there are a couple problems with power produced by an internal combustion engine. First, it only delivers usable power, or torque, within a certain range of engine speed (this range is called an engine’s power band). Go too slow or too fast, and you don’t get the optimal amount of torque to get the car moving. Second, cars often need more or less torque than what the engine can optimally provide within its power band.
To understand the second problem, you need to understand the first problem. And to understand the first problem, you need to understand the difference between engine speed and engine torque.
Engine speed is the rate at which the engine’s crankshaft spins. This is measured in revolutions per minute (RPMs).
Engine torque is how much twisting force the engine generates at its shaft for a particular speed of rotation.
A car mechanic gave this nice analogy to understand the difference between engine speed and engine torque:
Imagine you were an engine and you’re trying to drive a nail into a wall:
Speed = How many times you hit the nail head in a minute.
Torque = How hard you hit the nail every time.
Think back to the last time you were hammering nails. If you were hammering really fast, you probably noticed that you weren’t striking the nail with much force. What’s more, you probably exhausted yourself from so much frantic swinging.
Conversely, if you took your time between each swing, but made sure that each swing you did make was as hard as possible, you’d drive the nail in with fewer swings, but it might take you a bit longer because you’re not swinging at a steady tempo.
Ideally, you’d find a pace of hammering that allowed you to hit the nail head several times with a good amount of force with each swing without tiring yourself out. Not too fast, not too slow, but just right.
Well, we want our car’s engine to do the same thing. We want it to spin at the speed that allows it to deliver the needed torque without working so hard that it destroys itself. We need the engine to stay within its power band.
If an engine is spinning below its power band, you won’t have the torque you need to move the car forward. If it goes above its power band, torque starts dropping off and your engine starts sounding like it’s about to break due to stress (sort of like what happens when you try hammering too fast — you hit the nail with less power and you get really, really tired). If you’ve revved your engine until the tachometer gets into the red, you understand this concept viscerally. Your engine sounds like it’s about to die, but you’re not moving any faster.
Okay, so you understand the need to keep a vehicle running in its power band so that it’s working effectively.
But that brings us to our second problem: cars need more or less torque in certain situations.
For example, when you’re starting a car at a standstill, you need a lot of power, or torque, to get the vehicle going. If you floor the gas pedal, you’re going to make the engine’s crankshaft spin really fast, causing the engine to go way above its power band, and possibly destroy itself in the process. And the kicker is you won’t even move the car all that much because torque drops off on an engine as it goes above its power band. In this situation, we need a lot more torque, but to get that, we’ve got to sacrifice some speed.
Okay, what if you just press on the gas a wee tiny bit? Well, that’s probably not going to cause the engine to spin fast enough to get into its power band in the first place so that it can deliver the torque to get the car moving.
Let’s take a look at another scenario: Let’s say you’ve got the car moving really fast, like when you’re cruising on the freeway. You don’t need to send as much power from the engine to the wheels, because the car is already moving at a brisk pace. Sheer momentum is doing a lot of the work. So you can let the engine spin at a higher speed without worrying as much about the amount of power being delivered to the wheels. We need more rotational speed going to the wheels, and less rotational power.
What we need is some way to multiply the power produced by the engine when it’s needed (starting from a standstill, going up a hill, etc.), but also decrease the amount of power sent from the engine when it isn’t needed (going downhill or going really fast).
Enter the transmission.
The transmission ensures that your engine spins at an optimal rate (neither too slow or too fast) while simultaneously providing your wheels with the right amount of power they need to move and stop the car, no matter the situation you find yourself in.
It’s able to do this effective transmitting of power through a series of different sized gears that leverage the power of gear ratio.
Inside the transmission are a series of variously sized, toothed gears that produce torque. Because the gears that interact with each other are different sizes, torque can be increased or decreased without changing the speed of the engine’s rotational power all that much. This is thanks to gear ratios.
Gear ratios represent the gears’ relation to each other in size. When different sized gears mesh together, they can spin at different speeds and deliver different amounts of power.
Let’s look at a dumb-downed version of gears in action to explain this. Say you have an input gear with 10 teeth (by input gear, I mean a gear that is generating the power) connected to a larger output with 20 teeth (by output gear, I mean a gear that is receiving the power). To spin that 20-toothed gear once, the 10-toothed gear needs to turn twice because it’s half as big as the 20-toothed gear. This means that even though the 10-toothed gear is spinning fast, the 20-toothed gear is turning slowly. And even though the 20-toothed gear is turning more slowly, it’s delivering more force, or power, because it’s larger. The ratio in this arrangement is 1:2. This is a low gear ratio.
Or let’s say the two gears connected to each other are the same size (10 teeth and 10 teeth). They’d both spin at the same speed, and they’d both deliver the same amount of power. The gear ratio here is 1:1. This is called a “direct drive” ratio because the two gears are transferring the same amount of power.
Or let’s say the input gear was larger (20 teeth) and the output gear was smaller (10 teeth). To spin the 10-toothed gear once, the 20-toothed gear would only need to turn half way. This means that even though the 20-toothed input gear is spinning slowly and with more force, the 10-toothed output gear is spinning fast, and delivering less power. The gear ratio here is 2:1. This is called high gear ratio.
Let’s bring that concept back to the purpose of the transmission.
Below you’ll find a diagram of the power flow when the different gears in a 5-speed manual transmission vehicle are engaged.
First Gear. It’s the largest gear in the transmission and enmeshed with a small gear. A typical gear ratio when a car is in first gear is 3.166:1. When first gear is engaged, low speed, but high power is delivered. This gear ratio is great for starting your car from a standstill.
Second Gear. The second gear is slightly smaller than first gear, but still is enmeshed with a smaller gear. A typical gear ratio is 1.882:1. Speed is increased and power decreased slightly.
Third Gear. Third gear is slightly smaller than the second, but still enmeshed with a smaller gear. A typical gear ratio is 1.296:1.
Fourth Gear. Fourth gear is slightly smaller than the third. In many vehicles, by the time a car is in fourth gear, the output shaft is moving at the same speed as the input shaft. This arrangement is called “direct drive.” A typical gear ratio is 0.972:1
Fifth Gear. In vehicles with a fifth gear (also called “overdrive”), it is connected to a gear that’s significantly larger. This allows the fifth gear to spin much faster than the gear that’s delivering power. A typical gear ratio is 0.78:1.
The concept of gear ratios can be better explained visually rather than through text. This video from the 1940s does a bang-up job explaining gear ratios in a transmission:
Parts of a Manual Transmission
So by now, you should have a basic understanding of a transmission’s purpose: it ensures that your engine spins at an optimal rate (neither too slow nor too fast) while simultaneously providing your wheels with the right amount of power they need to move and stop the car, no matter the situation you find yourself in.
Let’s take a look at the parts of a transmission that allow this to happen:
Input shaft. The input shaft comes from the engine. This spins at the same speed and power of the engine.
Countershaft. The countershaft (aka layshaft) sits just below the output shafts. The countershaft connects directly to the input shaft via a fixed speed gear. Whenever the input shaft spins, so does the countershaft, and at the same speed as the input shaft.
In addition to the gear that takes power from the input shaft, the countershaft also has several gears on it, one for each of the car’s “gears” (1st-5th), including reverse.
Output shaft. The output shaft runs parallel above the countershaft. This is the shaft that delivers power to the rest of the drivetrain. The amount of power the output shaft delivers all depends on which gears are engaged on it. The output shaft has freely rotating gears that are mounted on it by ball bearings. The speed of the output shaft is determined by which of the five gears are in “gear,” or engaged.
1st-5th gears. These are the gears that are mounted on the output shaft by bearings and determine which “gear” your car is in. Each of these gears is constantly enmeshed with one of the gears on the countershaft and are constantly spinning. This constantly enmeshed arrangement is what you see in synchronized transmissions or constant mesh transmissions, which most modern vehicles use. (We’ll go into how all the gears can always be spinning while only one of them is actually delivering power to the drivetrain here in a bit.)
First gear is the largest gear, and the gears get progressively smaller as you get to fifth gear. Remember, gear ratios. Because first gear is bigger than the countershaft gear it’s connected to, it can spin slower than the input shaft (remember, the countershaft moves at the same speed as the input shaft), but deliver more power to the output shaft. As you move up in gears, the gear ratio decreases until you reach the point that the input and output shafts are moving at the same speed and delivering the same amount of power.
Idler gear. The idler gear (sometimes called “reverse idler gear”) sits between the reverse gear on the output shaft and a gear on the countershaft. The idler gear is what allows your car to go in reverse. The reverse gear is the only gear in a synchronized transmission that isn’t always enmeshed or spinning with a countershaft gear. It only moves whenever you actually shift the vehicle into reverse.
Synchronizer collars/sleeves. Most modern vehicles have a synchronized transmission, meaning the gears that deliver power on the output shaft are constantly enmeshed with gears on the countershaft and are constantly spinning. But you might be thinking, “How can all five gears be constantly enmeshed and constantly spinning, but only one of those gears is actually delivering power to the output shaft?”
The other issue that comes up with the gears always spinning is that the drive gear is often rotating at a different speed than the output shaft that the gear is connected to. How do you sync up a gear spinning at a different rate as the output shaft, and in a smooth way that doesn’t cause a lot of grinding?
The answer to both questions: synchronizer collars.
As mentioned above, gears 1-5 are mounted on the output shaft via ball bearings. This allows all of the gears to freely spin at the same time while the engine is running. To engage one of these gears, we need to firmly connect it to the output shaft, so power is delivered to the output shaft and then to the rest of the drivetrain.
Between each of the gears are rings called synchronizer collars. On a five-speed transmission, there’s a collar between the 1st and 2nd gears, between the 3rd and 4th gears, and between the 5th and reverse gear.
Whenever you shift a car into a gear, the synchronizer collar shifts over to the moving gear you’re looking to engage. On the outside of the gear are a series of cone-shaped teeth. The synchronizer collar has grooves to accept those teeth. Thanks to some excellent mechanical engineering, the synchronizer collar can connect to a gear with very little noise or friction even while the gear is moving, and sync the gear’s speed with the input shaft. Once the synchronizer collar is enmeshed with the driving gear, that driving gear is delivering power to the output shaft.
Whenever a car is “neutral” none of the synchronizer collars are enmeshed with a driving gear.
Synchronizer collars are also something that’s easier to understand visually. Here’s a short little clip that does a great job explaining what’s going on (starts at about 1:59 mark):
Gearshift. The gearshift is what you move to put a car into gear.
Shift rod. The shift rods are what move the synchronizer collars towards the gear you want to engage. On most five-speed vehicles, there are three shift rods. One end of a shift rod is connected to the gearshift. At the other end of the shift rod is a shift fork that holds the synchronizer collar.
Shift fork. The shift fork holds the synchronizer collar.
Clutch. The clutch sits between the engine and gearbox of the transmission. When the clutch is disengaged, it disconnects power flow between the engine and transmission gearbox. This disconnection of power allows the engine to continue running even though the rest of the car’s drivetrain isn’t getting any power. With engine power disconnected from the transmission, shifting gears is much easier and prevents damage to the transmission gears. This is why whenever you shift gears, you push the clutch pedal and disengage the clutch.
When the clutch is engaged — your foot comes off the pedal — power between the engine and transmission is restored.
How Manual Transmissions Work
So let’s bring this all together and walk through what happens whenever you shift gears in a vehicle. We’ll begin with starting a car and shifting up to second gear.
When you start a manual transmission car, before you turn the key, you disengage the clutch by pressing down on the clutch pedal. This disconnects power flow between the engine’s input shaft and transmission. This allows your engine to run without delivering power to the rest of the vehicle.
With the clutch disengaged, you move the gearshift into first gear. This causes a shifting rod in your transmission’s gearbox to move the shifting fork towards first gear, which is mounted to the output shaft via ball bearings.
This first gear on the output shaft is enmeshed with a gear that’s connected to a countershaft. The countershaft connects to the engine’s input shaft via a gear and spins at the same speed as the engine’s input shaft.
Attached to the shifting fork is a synchronizer collar. The synchronizer collar does two things: 1) it firmly mounts the driving gear to the output shaft so the gear can deliver power to the output shaft, and 2) it ensures that the gear syncs up with the speed of the output shaft.
Once the synchronizer collar is enmeshed with the first gear, the gear is firmly connected to the output shaft, and the vehicle is now in gear.
To get the car moving, you press down slightly on the gas (which creates more engine power) and slowly take your foot off the clutch (which engages the clutch and reconnects power between the engine and transmission gearbox).
Because the first gear is large, it causes the output shaft to spin more slowly than the engine’s input shaft, but deliver more power to the rest of the drivetrain. This is thanks to the wonders of gear ratios.
If you’ve done everything correctly, the car will slowly begin to move forward.
Once you’ve got the car going, you’ll want to go faster. But with the car in first gear, you’re not going to be able to go very fast because the gear ratio causes the output shaft to turn at a certain speed. If you were to floor the gas pedal with the car in first gear, you’re just going to cause the engine’s input shaft to spin really fast (and possibly damage the motor in the process), but not see an increase in vehicle speed.
To increase the speed of the output shaft, we need to shift up to second gear. So we step on the clutch to disconnect power between the engine and transmission gearbox and shift into second gear. This moves the shifting rod that has a shift fork and synchronizer collar towards second gear. The synchronizer collar syncs up the second gear’s speed with the output shaft and firmly mounts it to the output shaft. The output shaft can now spin faster without the engine’s input shaft spinning furiously to produce the power the car needs.
For the rest of the five gears, it’s rinse, wash, and repeat.
Reverse gear is the exception. Unlike the other driving gears where you can shift up without completely stopping the car, to shift in reverse, you need to be at a standstill. This is because the reverse gear isn’t constantly enmeshed with a gear on the counter shaft. To slide the reverse gear into its corresponding countershaft gear, you need to make sure the countershaft is not moving. To ensure the countershaft isn’t spinning, you need to have the car completely stopped.
Sure, you can force a forward moving car into reverse gear, but it’s not going to sound or feel pretty, and you may cause a lot of damage to the transmission.
Now, whenever you shift your car into gear, you’ll know what’s going on beneath the hood. Next up: automatic transmissions.
Last updated: November 26, 2017