Category Archives: Transmissions

Understanding Your Transmission, Part 2: Automatic Transmissions

If you haven’t read my entry on manual transmissions, you may want to give that a brief glance, as it provides some useful background information on the basic ideas behind an automotive transmission. Some of these basic principles are shared by both automatic and manual transmissions. For example, both use different gear ratios to keep the engine’s output within its ideal RPM range as the car accelerates and decelerates.

Unlike a manual transmission, in which the driver selects gears himself, the automatic transmission has only one “drive” setting. As the driver accelerates, the transmission shifts automatically through the different gears.

In a manual transmission, a driver selects different gear ratios, i.e. there is a first gear, a second gear, etc. In an automatic transmission, however, all of the gear ratios are produced by an ingenious device called a planetary gearset.

However, an automatic transmission uses the same basic gear ratios as a manual transmission. You have first, second, third, and overdrive gears, neutral (engine idles but is disengaged from transmission) and reverse. On rear wheel drives, the transmission is usually mounted at the back of the engine and connected to the wheels by a long driveshaft. In a front wheel drive, the transmission is combined with the final drive to form something called a “transaxle,” mounted under and to the side of the engine. These are two of the most common arrangements, but there are others as well.

On newer cars, gear shifts are determined and controlled by a computer. However, the earliest known version of an automatic transmission was developed in 1904—long before the digital age—while the basis of our modern automatic transmission was full developed by the 1960s. On older automatic transmissions, the process of determining and activating gear shifts is a purely mechanical process. The automatic transmission is a beautiful and complex system, with a number of components:

Planetary Gearset– a collection of gears that can produce a wide array of gear ratios.

Torque Converter– acts like a clutch, allows engine and transmission to be disengaged from one another

Governor and Modulator or Throttle Cable– monitor speed and throttle to determine when to shift

Valves– use input from the Governor, Modulator, and stick shift to control gear shifts

Clutches and Bands– change gear ratios in the planetary gear set

Seals and Gaskets– keep oil pressurized and contained in the system

Hydraulic System and Pump– provide necessary lubrication; activate valves, torque converter, clutches and bands, and other key parts.

Computer (newer cars)- takes the place of a range of devices, including valves, governor, modulator, etc.

I’ll explain how each of these parts works in greater detail below.

The Planetary Gearset

This is a beautiful and elegant device; it operates on fairly basic principles but achieves very complex results!

 

Figure 1: Planetary Gearset, cross-section

The sun gear sits at the center, which meshes with two or more planet gears that are all attached to the same planet carrier. These gears then meshed with the outer ring gear. All of these gears remain meshed constantly.

Locking these gears together in different combinations produces different gear ratios, i.e. different relationships between input speed and output speed. Let’s look at a few examples.

1.      Lets’ say that the ring gear = input and the planet carrier = output. We’ll then lock sun gear so that it remains stationary. As the ring gear turns, it causes the planets to “walk” along sun gear. This produces an output that is slower than the input, as you have in first gear.

2.      However, say we unlock sun gear and lock it to the ring gear instead. With these two elements locked together, all of the gears will turn at the same speed, so that input speed and output speed are the same. This 1:1 ratio usually occurs in third gear.

3.      What about reverse gear? First, lock the planet carrier in place. Then, use the ring gear as input and the sun gear as output. The planet gears will act like the idler gear on a manual transmission, causing input and output gears to spin in different directions.

These are the basic principles behind a planetary gear set. As you can see, it’s important that the different parts can be locked and unlocked, joined to one another, etc. How is this accomplished? Take a look at the diagram below:

 

Figure 2: Planetary Gearset, side view

As you can see, the ring gear is used as the input, while the planet carrier is directly connected to the output shaft. However, notice that there are clutch packs connecting the planet carrier to the sun gear, which is connected to a drum containing the pistons that activate these clutches. These clutch packs can be used to lock the planet carrier and sun gear together, so that both spin together and the sun gear becomes, in effect, the output. Next, notice the bands on either side of the sun gear’s drum. These can be used to lock the sun gear in place.

These bands are usually made of steel and are activated by a remarkable hydraulic system, which I’ll discuss a bit later. The clutches are activated by pistons, as indicated on the diagram. Hydraulic fluid enters these pistons and activates the clutch; springs cause the clutch to release when pressure is reduced.

A real transmission will use two or more planetary gearsets in combination to produce up to eight different speeds. For example, one kind of compound planetary gearset contains one ring gear, which is always the output, but has two sun gears and two sets of planet gears. The input is transferred between the small and large sun gears, while in second gear, for example, the compound gear set behaves as two planetary gear sets, essentially, with the larger sun gear acting as a sort of second ring-gear. The mechanics of it are awesomely complex!

Other Gears

All of these gear shifts occur when the car is placed in “drive” or “reverse.” As you know if you drive an automatic transmission, however, there are other settings that one can select with the gear shift lever.

Usually, an automatic transmission will have two “low gears.” There is a second gear option, usually marked “2” or “S,” that limits the transmission to the first two gear ratios (or, in some cars, locks it in second gear.) This can be useful when driving on ice or hilly terrain; however, remember that you can’t go too fast in these gears!

There is a also a first or “low” gear option, marked “1” or “L.” Like the second gear option, this can be used in difficult driving conditions or when towing a heavy load.

Parking

Unlike a manual transmission, automatic transmissions also have a “park (P)” gear, in which a small pin or bolt is used to lock the drive wheels in place, preventing the car from moving. When the lever is used to select park, a spring pushes this bolt through a notch on the transmission housing, thereby keeping the transmission—and therefore the wheels—from moving. If the bolt isn’t lined up with a notch when “P” is selected, the transmission turns slightly, until the bolt can fit through a notch. This is why automatic transmissions sometimes roll slightly when the brake is released after parking.

Essentially, a very small mechanism is used to lock up the transmission. For this reason, drivers of automatic transmissions should always use the emergency brake (usually a foot brake) in addition to placing the car in park, to avoid placing undue strain on this mechanism, particularly when parked on hills.

Torque Converter

Like a manual transmission, automatic transmissions also have a “neutral (N)” gear. In this gear, the engine will keep idling, but the wheels won’t turn. This gear isn’t used as frequently on an automatic transmission as on a manual transmission, as in an automatic one can come to a stop in drive without stalling. Instead of a clutch, however, an automatic transmission uses something called a “torque converter” to connect (and disconnect) the engine and the transmission.

A torque converter looks like a large donut. It’s usually around a foot in diameter and is attached to the engine’s flywheel. The torque converter is a fluid coupling, meaning fluid (oil) is used to transmit the circular motion produced by the engine to the transmission. Imagine that you have two fans: one is plugged in, while the other isn’t. You place the fans so that they’re facing one another and turn on one of them. If you hold the blades of the fan that isn’t on, it won’t turn. However, as soon as you let go, these blades will start to move, until they come close to the same speed. This is the basic principle behind a torque converter, which uses oil instead of air. A torque converter has three parts: pump, turbine, and stator.  (See figure below)

 

Figure 3: Torque Converter, side view

The turbine provides input to the transmission, while the pump is directly connected to the converter housing, which is in turn fixed to the flywheel, so that housing and pump spin at the speed of the engine’s crankshaft. Both pump and turbine have blades or fins attached, like a fan. As the pump spins, it throws fluid outwards. This fluid, moving in a circular motion, begins to turn the turbine. Because of the configuration of blades within the turbine, the fluid changes its direction of rotation inside the turbine. Once the fluid exits the turbine, it is sucked into the stator. The stator reverses the direction of the fluid and returns it to the pump. This prevents the reversed fluid from slowing down the pump, which would make the torque converter very inefficient.

The amount of power, or torque, transferred from engine to transmission depends on how fast the pump is turning. When the engine is turning very slowly, i.e. when the car is idling at a light or stop sign, the turbine will barely turn at all. Because so little power is being transferred to the transmission, it’s easy to keep the car still by keeping one’s foot on the brake pedal. As the pump speeds up, the turbine will slowly start to accelerate, although it will lag behind the pump for awhile. When the turbine’s speed begins to approximate the speed of the pump, then a maximum amount of torque is being transferred.

Shift Circuits

If you think the torque converter is clever, you should see the system devised for activating gear shifts. On newer cars, a computer is used to activate gear shifts. However, automatic transmissions evolved long before the digital age, and older automatics are entirely mechanical. So, how does a mechanical system “know” when to shift gears?

This problem is more complex than simply judging the speed of the car. If you read the entry on manual transmissions, you’ll know that lower gears give you more power, allowing you to accelerate faster and to climb steep hills. If you step on the brake pedal hard, the car will stay in a lower gear for longer, in order to provide faster acceleration. However, if you accelerate slowly, the car will shift gears sooner. When more power is required, as on a hill, the transmission will downshift automatically.

The “brain” of the transmission is a hydraulic system in which oil is routed through a complex series of metal passageways (the device looks a bit like a computer circuit.) In order to shift correctly, the transmission needs input both on how fast the car is going AND how hard the car is working. The first piece of information comes from the governor.

The governor is connected to the transmission output, which in turn determines the speed of the car. As the transmission turns, so does the governor. The governor contains a spring loaded valve and is connected to the hydraulic system. As the governor spins faster, the valve opens more, allowing a greater amount of oil through.

The second piece of information—how hard the engine is working—comes from either a throttle valve or a vacuum modulator. In cars with a throttle valve, a cable connects the valve to the accelerator; the more the accelerator is depressed, the more the valve opens. A vacuum modulator achieves a similar effect.

Both of these elements then connect to a shift circuit (See diagram below.)

 

Figure 4: A basic shift circuit

Shift valves provide pressurized oil to the clutches and bands that activate different gears by locking and unlocking components of the planetary gear set. Each shift valve controls one particular shift, i.e. from first to second or second to third. Oil enters each shift valve in three directions: from the governor, from the throttle valve, and from the pump. When open, oil flows from the pump to the clutches and bands, causing them to activate.

As the car speeds up, the amount of pressure on the right side of the valve builds, as the valve in the governor opens further. When the car is moving fast enough, the shift valve will move to the left, causing a shift to the next higher gear.

However, the throttle valve also provides input into this system. If the car is accelerating quickly, the throttle pressure will be higher, counteracting the pressure from the governor. This means that the car has to be moving faster in order for the shift to occur. The reverse occurs when you accelerate slowly.

The operation of each shift valve is triggered by the amount of pressure coming from the governor, so that particular ranges of pressure correspond to the operation of the first-second valve, second-third valve, etc. These complex interchanges are controlled by the valve body, a piece of metal with passageways molded into it, like a computer circuit. These passageways channel fluid to the appropriate valves. The manual valve is a kind of “master valve” connected to the shift lever. When different gears are engaged, the manual valve feeds the appropriate circuits. For example, if you shift into “2,” the manual valve will feed shift circuits for the first two gears but inhibit the others. On computer-controlled transmissions, electric solenoids are used to direct fluid to the appropriate valves.

Hydraulics

As I’ve mentioned above, an automatic transmission relies on a complex and extensive hydraulic system. In fact, an average automatic transmission will contain up to ten quarts of oil! Pressurized oil is used to lubricate the moving parts in the transmission, power the bands and clutches, activate shifts, power the torque converter, and cool the entire system. For this last purpose, the oil cycles through a chamber submerged in the antifreeze, in order to remove excess heat. The automatic transmission’s pump also plays a crucial role in making sure that all parts are supplied with the necessary pressurized fluid.

All in all, automatic transmissions are a mechanical marvel!

To read more on a broad range of subjects from “How To Change A Tire” to “How To Jumpstart Your Car”, visit DefensiveDriving.com’s Safe Driver Resources website!

Check out these sites for more information about online defensive driving in Texas, online defensive driving in Florida, and business driver safety.

Advertisements

Driving Manual Transmission

I was, I’ll confess, a reluctant driver. The process of learning to drive seemed like a lot of stress for not that much reward (especially as I was pretty convinced that I’d never have a car of my own.) However, my mother, sick of driving me around, dragged me to the DMV on the day I turned 16.

Having barely mastered driving automatic in time to scrape through my driver’s test (I didn’t learn to parallel park or reverse in a straight line until years later), I never thought I would learn to drive standard. Why bother making driving even more annoying? It seemed like a huge hassle for not too much reward.

Then, miraculously, I had the opportunity to own my first car. The only catch was that it was standard shift. I had to learn quickly. So, I found a large parking lot and slowly and painfully began to practice with my new car.

Years later, I am immensely glad that I learned to drive standard, and I strongly suggest that everyone—whether or not you think you will ever drive a standard shift car—should cultivate this skill. Find a friend with a manual transmission and bribe him or her into taking you for a few practice sessions. You won’t regret it, and you may end up becoming a standard shift convert like me.

So, why bother?

First of all, driving standard is more fun. Driving automatic is pretty much a mindless exercise: you push a pedal and you move. A monkey could do it. Driving standard, however, requires more skill; as a result, driving becomes more of a challenge or game than a mindless grind. Plus, you’re sure to impress all of your automatic-driving friends with your new skills.

Second, driving standard is safer. With a manual transmission, you have greater control over the vehicle. Once you’ve learned to operate your clutch effectively, you’ll be able to dictate how much torque you send to your wheels in different situations. For example, you’ll be able to control exactly how much power you exert on the road if, say, you are trying to climb a slightly icy hill. Additionally, you have to pay a lot more attention to what you’re doing when you’re driving standard; it’s hard to “drift off” in the way that you can when driving automatic. This increased attentiveness to your car, the road, and driving conditions around you will make you overall a safer driver. Plus, should your battery fail, you can also push-start a manual transmission car—no need for jumper cables.

This brings me to my next point. Driving standard is cheaper. First, cars with standard shift are usually less expensive than their automatic counterparts. With fewer and simpler moving parts, they are also easier to maintain and last longer. Additionally, standard shift cars tend to get better fuel economy; on average, they are 5% to 15% more efficient. This is because these transmissions weigh less, have fewer electrical parts (like the automatic transmission’s hydraulic pump) to power, and lose less power in the transfer from engine to wheels. It’s also easier—and, in fact, quite natural—to use gas-saving driving techniques on a standard shift vehicle. See my previous entries on saving gas and hypermiling for more information about this.

Finally, standard shift cars are more efficient because they often have more gear ratios than automatic cars (5-6 as opposed to 4, although this is changing.) This, as well as the greater degree of driver control, makes it easier to keep the car within its power band: the RPM range at which the engine is most efficient. (To understand why this is the case, you may want to check out my recent entry on how a manual transmission works. This is good background information for anyone who wants to learn to drive standard.)

Last but not least, knowing how to drive standard is a crucial life skill, like learning to swim; even if you don’t use it on a daily basis, you never know when you may be called on to do so. For example, while most cars in the US are sold with automatic transmissions (and almost all rental cars are automatic), much of the world doesn’t use automatic transmissions. In India, Europe, Latin America, and Africa, standard transmission are far more common. For example, in 2008, 75% of vehicles made in Europe had standard transmissions. This means that if you ever have the opportunity or desire to travel or live abroad, you will likely need to know how to drive standard in order to get around. I, for example, ended up living in Southern Africa—something I never foresaw when I was 16—and needed to drive standard in order to get around. If I hadn’t had this skill, I would have been in a very difficult position.

There’s really no reason NOT to learn how to drive standard, aside from the fact that it takes a little bit of time and effort to master. Furthermore, there are lots of reasons why one would consider owning a standard shift rather than an automatic. The only significant drawback is that it takes a more effort to drive a standard shift in stop and go traffic, although even this isn’t so bad if you have mastered the use of your clutch and practice gas saving driving techniques. Some automatics can shift faster than the driver of a standard shift can, but the advantage gained by this is small.

So, it’s decided: you’re going to learn to drive standard. Now, where to begin? First, you’ll need a car. Find a friend or family member who owes you a favor and convince them to take you for a practice session. On your first time out, you’ll want to begin practicing in an open and empty space; an unused parking lot is ideal. Avoid places with too many obstacles, like lamp posts or trees. On your next time out, you’ll also want to practice on a stretch of straight, empty roadway. Have a good location or two in mind before you set out.

The Basics

Once you’ve reached your practice space, have your friend park the car and trade places with you. Once you’re in the driver’s seat, take a moment to familiarize yourself with the lay of the land.

First, a standard transmission car has three pedals. (See figure 1) Your accelerator is on the far right, and your brake is in the center, just as on an automatic car. However, you have a third pedal on the left, the clutch pedal. You’ll operate brake and accelerator with your right foot, as you’d normally do; you’ll use your left foot to operate the clutch.

 

Figure 1: typical standard shift car

Now, look over to your right. The stick shift itself is in the center of the car. In most cars, the gear pattern will be outlined on top of the stick shift. (See figure 2) It’s usually a variation on this H shape, although the placement of reverse varies. Note that the center position is neutral, i.e. no gears are engaged. The shift will feel a bit loose when in neutral; testing the feel of the shifter is one way to make sure that you are, in fact, in neutral. (The gear shift on each car will look and feel a bit different; sometimes the difference between neutral and “in gear” will be seem pretty slight.)

 

Figure 2: gear shift close-up

Before you turn the car on, take some time to familiarize yourself with the gears. Begin by shifting into first gear. In order to shift, you will first have to depress the clutch. With the clutch down, you will then be able to push the gear stick into gear. In order to get into first gear, push the stick gently to the left and then away from you. The shifter should slide easily into the right position. Remember that you should never force a shift; it should always be a smooth and easy sliding motion.

Once you’ve got it into first, then try pulling it straight towards you, past the center point, and into second. Practice the neutral-first and first-second shifts a few times.

Next comes the second-third shift. This one can be a bit trickier. First, put the stick back into neutral. Then, push it more or less straight “up” (away from you.) Now, go back to second and try going straight from second to third. You may also want to try shifting from first to third; you’ll never actually do this while driving, BUT it can help you to get a feel for how the placement of these gears differs, as they can sometimes be mistaken for one another.

Fourth gear will be down and to the right; fifth gear will be up and to the right. Sometimes fifth gear is a bit further right than fourth; ask the car’s owner about the placement of fifth gear. Now, practice shifting 1-2-3-4-5 several times. Then, try reverse. This gear will be a bit different on each car. Sometimes you will have to pull up or push down on the gear stick or push a button. Again, ask how to shift into reverse, and then practice this shift as well.

When you actually start driving, you will only use two gears to start from a standstill: first and reverse. Also note that you should never shift into either of these gears while the car is moving.

Getting the Car to Move

Depending on the car and your own foot-eye coordination, this could be a lot more difficult than you’d think (then again, it could also be easier.) In general, new cars have “stiffer” clutches, which makes them a lot harder for a beginner to operate. If possible, try to find a car with an easy clutch to practice on.

Now, it’s time to start the car. On some cars, you will have to have the clutch fully depressed in order to start the car. On other cars, particularly older ones, you should start the car in neutral, with the clutch released. Ask the owner of your car what to do. Note, however, that you should always park a car in first gear. Thus, it’s important to remember to either shift into neutral OR depress the clutch before starting the car. If you start the car in gear, it will immediately lurch forward and stall, which can be a big problem if you’re in a cramped parking space.

Once the car is started, take a moment to listen to the sound of the engine. You may want to roll your windows down while practicing, as hearing the sound of the engine is an important cue for gear shifts. Next, take note of the RPM dial on your car and note the speed at which the engine is idling.

Now, shift into first gear, keeping the clutch depressed. Then take off the parking brake; unlike many automatics, manual transmissions use hand brakes. Begin to let the clutch out, as slowly and smoothly as possible. At some point, you’ll hear the engine speed beginning to drop (the pitch will become lower.) You’ll also notice the RPM dial beginning to drop. Once you reach this point, push the pedal back in again. This point is the “friction point.” This is going to be different for every car, so you’ll have to experiment a bit to get the “feel” of it. If you let the clutch too far out, the car will stall; once you’ve experienced this (which will likely happen more than once on your first day out), you’ll know what sounds to look for when you are approaching the stalling point. When you do hit the friction point correctly, however, the car will likely start to move forward very slowly.

Once you’ve identified the friction point, you’re ready to add the accelerator to the mix. As you approach the friction point, slowly start to depress the accelerator. You release the clutch as you feed the accelerator; you are aiming for a smooth exchange between pedals. You don’t want to depress the accelerator too much and rev the engine while the clutch pedal is in, as this will put a lot of stress on the clutch. At the same time, you’ll stall if you don’t depress the accelerator enough. You’re aiming for a smooth and slow motion, so that the car “glides” forward.  The goal is to keep the RPMs constant, so using this dial could help you to identify the correct accelerator-clutch ratio.

It may take a fair amount of practice to get this shift down. Personally, it took me about ten tries to just get the car to move. Other people do it on their first go. A lot will depend on the car you’re driving. Take heart, though, and hold out for that “aha!” moment when the feel of the process “clicks” with you. Also, starting in first is the most difficult shift, so once you’ve mastered this, the rest will be easier. At first, you’ll be pretty slow with this shift. Keep working at it until it only takes you a couple of seconds.

Moving Faster

Once you’re comfortable shifting into first, keep the car in first and start to accelerate. Notice that the RPM climb quickly (and listen to the whine of the engine as well.) Push the clutch in and brake. Before you go any further, note that you should always depress the clutch before braking.

Now, shift into first again and accelerate. Once you get to around 3-4000 RPM, which should happen pretty quickly, depress the clutch, shift into second, and engage the second gear, using the same clutch-accelerator exchange that you did for first gear. This shift should be a lot easier.

Drive slowly around the parking lot in second gear. Practice pushing in the clutch, coasting, and then releasing the clutch to put the car back into gear. Once you’ve mastered first and second gear, you’ll probably need to find a different practice space in order to try out the other gears.

Out on the Road

As you accelerate, you will continue to shift into higher gears. How do you know when to shift gears? Well, once you’ve been driving for a while, you’ll know from the sound of the engine. Basically, you don’t want the RPM to go too high (a screaming sound from the engine) or too low (a low-pitched spluttering sound, indicating you’re going to stall soon.) You’ll quickly get a feel for when you should shift.

Each car is a bit different, though. However, the following speed ranges should help you get a general idea of when to use each gear:

First gear: from 0 to 15 MPH

Second gear: 5 to 25 MPH

Third gear: 15 to 45 MPH

Fourth gear: 30 to 65 MPH

Fifth gear: 45+ MPH

First gear is really only used for starting the car. You’ll shift out of first almost immediately, unless, say, you’re maneuvering into a parking space. Once you get more experience, you’ll also be able to start the car in second gear; this can be useful if you’re in a situation, say on an icy slope, where you need more torque as you start. Most city driving is done in third gear; fifth gear is primarily for highway cruising. (This is the overdrive gear, which improves fuel mileage; for more on how this works, see my previous entry.)

On your first day out, you probably won’t go above third gear. As you get more practiced, you’ll be able to head out on the highway to try out fourth and fifth gears as well.

Downshifting

Of course, sometimes you’ll need to slow down. In these cases, you’ll need to downshift. First, notice that when you release the accelerator but don’t depress the clutch, the car slows down significantly, unlike on an automatic, in which the engine “coasts” more when the accelerator is released. If you want to coast downhill, you’ll need to depress the clutch and/or shift into neutral.

As a result, if you need to slow down, you don’t necessarily need to brake; you can also do so by taking your foot off the accelerator. This is called “engine braking,” a technique which is particularly useful when you need to adjust your speed by a small amount. If you slow down too much, you will need to downshift.

Smooth downshifts can be difficult; the trick is to make sure that you have slowed down to the speed at which the two gears overlap.  Push in the clutch and shift down to the lower gear. Then, let out the clutch and push in the accelerator. The “feel” of downshifting may be slightly different. The goal, however, is to get as smooth a shift as possible. If you feel a lurch when you re-engage the gears, you are probably going too fast for that gear.

Downshifting can also be helpful when you want to slow down gradually. Say, for example, that you’re approaching a red light. You can downshift and then putter along in a lower gear in order to avoid having to come to a full stop before the stoplight, thereby saving gas.

Two warnings about engine braking: first, remember that people behind you may not be driving standard (or may not be familiar with how a standard shift car works.) They won’t be expecting you to slow down without braking. If you are slowing down by downshifting, make sure that you tap your brakes once or twice to alert drivers behind you.

Second, don’t use engine braking to slow down ALL the time; some people recommend this as a way of saving your brakes. However, unless you are a very skilled driver, downshifting will put stress on your transmission. Transmissions are usually much more expensive and difficult to repair than brake pads, which are easily replaced.

Parking

You’ll have noticed by now that, unlike an automatic transmission, standard shift cars don’t have a “P” or “parking” gear. Neutral is NOT a parking gear! Your car will roll easily when in neutral. For this reason, make sure that you get used to ALWAYS setting the parking brake when you park, if this isn’t already a habit for you. You should also shift the car into first gear after you have turned it off.

Be careful, however, that you shift back into neutral OR depress the clutch before starting the car. On some cars, you will have to depress the clutch in order to start. If this isn’t the case for your car, then start in neutral instead, as starting with the clutch pedal in can place extra strain on the clutch.

Hill Starts

Unlike an automatic transmission, manual transmission vehicles roll on hills. On an automatic, you can stop on a hill, sit for a moment, and then depress the accelerator and start forward again, without sliding back. However, in a manual transmission, you’ll start to roll—even in places where you didn’t know there were hills.

Once you’ve gotten comfortable with shifting in and out of gears one-four, you’ll need to practice hill starts. Begin on a gentle slope, in an area free from traffic. First, come to a complete stop on the hill. Release the brake slightly and notice how you start to roll. Hit the brake again. Now, put the car in neutral and set the handbrake; don’t pull the handbrake up all the way, just far enough so that you won’t slip. Put the car into first gear. Begin to release the clutch and depress the accelerator, while keeping one hand on the handbrake. As soon as you feel the gear “catch,” release the handbrake. The goal is to roll forward without slipping at all, but also without grinding the gears or revving the engine too high.

Dos and Don’ts

DO always set the parking brake.

DO shift into neutral at red lights and during other long stops.

DON’T “ride the clutch,” i.e. drive with your foot resting on the clutch.

DON’T brake without depressing the clutch.

DO downshift on hills if you need greater acceleration.

DON’T downshift into first gear.

DO spend a lot of time practicing in safe locations.

DON’T give up if you find your first day on the road difficult; you’ll improve more quickly than you think!

DO take the time to learn to drive standard. It’s fun and useful, and will make you a better driver!

To learn more about this topic, or a broad range of subjects from “How To Change A Tire” to “How To Jumpstart Your Car”, visit DefensiveDriving.com’s Safe Driver Resources website!

Check out these sites for more information about online defensive driving in Texas, online defensive driving in Florida, and business driver safety.

Understanding Your Transmission, Part 1: Manual or Standard Transmissions

What is a transmission?

The transmission transfers power from the engine to the wheels. Basically, by using gears and a device called a clutch, the transmission converts the spinning energy of the engine into torque, expressed as  the force exerted on the road by the tires.

You need a transmission in part because every engine has a redline point—it can only spin so fast (so many RPMs) before it explodes. You’ll see the redline point marked on the RPM gauge on your dashboard. At the same time, there is a fairly narrow range of RPMs at which the engine is producing the maximum amount of power. A transmission consists of primarily gears, which can be used to convert a single input speed into an array of output speeds. As a result, your engine can continue operating near its optimal RPM while the car moves at different speeds. Your transmission also makes it possible for you to move in reverse as well as in forward without actually having to change the direction that the engine is spinning.

There are several kinds of transmissions: manual, automatic, and continuously variable transmissions (CVTs), as well as variations on these like the manumatic and semi-automatic transmissions. Most cars in the US these days have automatic transmissions, although many people still choose to drive a manual transmission. In many other countries, including much of Europe, Africa, and South America, manual transmissions are far more common, however.

In a manual transmission, the driver manually selects different gear ratios as he or she speeds up and slows down. Manual transmission vehicles have a third pedal, called the clutch pedal, which is used to activate the clutch (more on this later) in order to change gears. The average car will have five different gear ratios, as well as a reverse gear, although some older cars have fewer gear ratios and some high performance cars have more.

An automatic transmission uses a device called a planetary gear set, combined with a torque converter, to shift gears as the driver accelerates and decelerates, without any input from the driver herself. (There is also such a thing as a manual-style transmission that is controlled by a computer, rather than by the driver; this is known as an automated transmission.)

A continuously variable transmission has an almost endless array of gear ratios. CVTs used to be expensive and unreliable and so weren’t used often. However, the technology has improved, and now CVTs are appearing in more and more cars. For example, the popular hybrid Toyota Prius uses a CVT.

Manual and automatic transmissions each have their pros and cons; I’ll discuss these in the following post, which will deal with learning to drive a manual shift car. For now, I’m going to focus on the workings of a manual transmission. First, this is a simpler system to understand, as the same basic principles are used in an automatic transmission. Second, understanding how this system works will help you to learn to drive a manual transmission, if you don’t already know how to. If you do drive “stick” or “standard,” learning about the inner workings of your transmission could help you to be a more efficient driver and increase the life of your transmission. 

Manual transmission—the basics

A transmission has input and output shafts. The input shaft is connected directly to the engine by the clutch; this shaft thus turns at the same RPM as the engine. The clutch, however, allows the driver to disconnect the engine from the transmission, which is necessary in order to change gears.

I’m going to start by describing a very basic two-speed transmission. Once you’ve grasped the basic principles through this simple model, it will be easier to understand the more complex workings of the typical five-speed transmission.

First, the transmission is connected to the engine by the clutch. A clutch is a device that lets you disconnect the engine from the transmission. When you push in the clutch pedal (the third pedal on a manual transmission vehicle), the engine will be running but the car won’t be moving. In the diagram below, the green shaft and gear are connected to the clutch. When the clutch pedal is released, this gear will spin at the same speed as the engine.

 

Figure 1: two-speed transmission diagram

The red shaft is called the layshaft. The red gears are connected to the shaft, so that the whole unit spins together. The green gear engages with the red gear on the layshaft; as a result, when the clutch is released, the layshaft spins together with the engine.

The blue shaft is connected directly to the drive shaft (through a device called a differential, which I’ll discuss in a different entry.) If the wheels are spinning, then the blue shaft will be spinning. Now, here is a key point. The blue gears—unlike the red and green gears—aren’t locked onto the blue shaft. Instead, they spin on bearings. For this reason, the blue shaft can turn inside these gears without actually turning the blue gears.

This is important because, as you’ll notice, the blue gears are meshed together with the red gears. If the blue gears were locked onto the blue shaft, then you wouldn’t be able to coast with the engine off or roll your car when it breaks down, because the drive shaft would turn the engine crankshaft when it moved.

There is, however, another component: the collar. This is the purple device on the blue shaft. The collar IS fixed to the blue shaft; you’ll also notice that the collar has teeth, as do the blue gears. When you move the gear selector fork, you slide the collar right or left, until it meshes with one of the blue gears. Once collar and gear mesh, then that gear starts to turn the blue shaft, and, by extension, the drive shaft.

So how does this allow you to get different output speeds from the same input? You’ll notice that the gears on the red and blue shafts are different sizes. When the smallest red gear turns once, it will only turn the bigger blue gear a fraction of a rotation. Thus, you’re turning higher input RPM into a lower output RPM when the collar engages the larger blue gear. When the bigger red gear engages the smaller blue gear, you’re producing more than one rotation in the blue gear for every rotation of the red gear. This results in a higher output speed. (This is called overdrive, which I’ll explain when I discuss a standard five-speed transmission.)

 As you can see from this diagram, a small motion of the stick shift inside the car will shift the collar left or right, engaging a different gear. This is the basic principle behind an automatic transmission.

The Five Speed Transmission

Most cars, however, need more than just two gear ratios. Here is an updated version of the diagram above, with more gears added, to reflect a typical five-speed transmission:

 

Figure 2: five speed transmission

The basic principles and parts— gear ratios, collar, layshaft, gear selector fork, etc.—are the same. Here, however, you have three collars instead of one and six red and blue gears, instead of two. These correspond to the five gears labeled on a stick shift (first, second, third, etc.) and a reverse gear.

When you shift from first to second, you move the first collar from left to right. When you shift from second to third, however, you are pulling the first collar to the left and sliding the second collar to the right—so a single move of the stick shift actually produces two motions in the gear selector forks.

This is possible because, in a five speed transmission, there are THREE rods, each connected to one of the collars, that are ALL engaged by the stick shift lever. If you’ve driven a manual transmission, you’ll be familiar with the different positions of the shift lever, each of which corresponds to a different gear ratio:

 

Figure 3: shift lever positions

 

The red circle marks the shift lever. When it is in the central “pivot” point, as shown here, then none of the blue gears are engaged, and the car is in neutral. To shift into first gear, the lever is moved to the left and up, along the lines shown.

As you can see from this diagram, there are three vertical axes: first-second, third-fourth, and fifth-reverse. Each of the axes corresponds to a different shift rod, which is in turn connected to one of the three collars. Do you see how this makes sense? You use the same rod to shift between first and second (i.e. same collar), so you move the shift lever from “top” to “bottom” on the diagram, which in turn moves this collar from left to right.

When you shift from second to third, you move the shift lever from bottom to top and from left to right, through the pivot point in the center. This allows you to shift one collar to the right and the other to the left. Notice that the shift rods move in the direction opposite to the motion of the shit lever itself. See Figure 4 below.

 

Figure 4: shifting rods

Reverse Gear

What about reverse gear? As you can see in Figure 2, the reverse gear is slightly different from the other gears. Instead of the blue gear meshing directly with the red gear, these two gears are separated by a third gear, called the idler, which reverses the direction of rotation.

When a gear turns, its teeth push against the teeth of the meshed gear, turning this gear in the opposite direction. If the red gears are turning counter-clockwise, then the blue gears will be turning clockwise. However, in the case of the reverse gear, the idler gear will be turning clockwise; the blue gear will then be turning counter-clockwise, in the opposite direction from the other blue gears (remember this is possible because none of the blue gears are fixed to the blue shaft.)

Overdrive

Take a look at figure 2 again. As you’ll see, in a five-speed transmission like this one, only one of the red gears is larger than the corresponding blue gear. This gear ratio corresponds to fifth gear, which is also known as an “overdrive” gear. When selected, the output speed will be higher than the input speed. As a result, this gear is generally used for highway cruising at high speeds. Under these conditions, this gear ratio will improve fuel efficiency and often allow for quieter engine operation. The fifth gear is generally best engaged at speeds over 45 mph.

 

Synchronizers

Now, you may have already noticed a potential problem with this system. The blue shaft (and the collars) spin at one speed, while the blue gears spin at different speeds. Thus, the speed of the collar and the speed of a blue gear may not match. How, then, can you get the “teeth” on the collar to fit into the teeth on the gear? When these don’t fit together properly, you’ll hear a nasty grinding noise (which you’ll likely be familiar with if you drive a standard transmission.)

In older cars (and in some modern race cars), drivers had to use a technique called “double clutching” to avoid this problem. When a driver wanted to change gears, he would first engage the clutch and shift into neutral, so that the collars weren’t engaging any gears. He would then rev the engine until the RPM of the engine reached the right speed: the speed at which the desired output (blue) gear would be spinning at the same speed as the wheels—and hence the same speed as the collar. He could then lock the collar and blue gear together.

This of course makes driving more of a fine art than a basic skill. I’ve been driving standard for years, and the thought of double clutching still makes me quail. Luckily, in 1952, Porsche developed a simple yet elegant solution for this problem: the synchronizer (see Figure 5).

 

Figure 5: synchronizer

The synchronizer (or “synchro”) is a small cone on the outside of each blue gear. The collars have a corresponding hollow—a negative version of the synchro. As the collar is brought towards the blue gear, the cone and the hollow make contact first. Since these parts just fit together, the friction between them causes the collar and the blue gear to spin at the same speed BEFORE the teeth actually lock the two together. Shifting suddenly becomes much, much simpler!

Most cars have synchronizers on all gears except for the reverse gear. For this reason, you generally have to bring the car to a full stop before shifting into reverse. (Otherwise, you’ll hear that dreaded teeth-grinding noise!) However, some manufacturers—Lamborgini and BMW, among others—do use synchros on the reverse gear.

The Clutch

The clutch is the devise used to connect and disconnect the engine from the transmission. On manual transmission cars, the clutch is operated by a pedal. When the clutch pedal is fully depressed, the engine is disengaged from the transmission. When the clutch pedal is fully released, the engine is fully engaged, i.e. is transferring all of its torque (power) to the transmission.

The clutch has three main components: the flywheel, the clutch disc, and the pressure plate.

The flywheel is a large steel or aluminum disc connected to the crankshaft. It helps to dampen engine vibrations, as well as serving as the base for the clutch.

The clutch plate is a second disc covered with a material that will create friction when in contact with the flywheel. The clutch plate is connected to the transmission input shaft. When the clutch pedal is released, the clutch disc is pressed against the flywheel, so that flywheel and clutch plate spin together.

The pressure plate sits on the other side of the clutch plate; it’s basically a spring-loaded friction surface.  These springs push the pressure plate into the clutch plate. When the clutch pedal is released, the clamping force of the springs is activated; the opposite happens when the pedal is engaged.

The whole system is contained in the clutch housing.

Because the plates engage one another by friction, this isn’t a simple “on/off” system; as the pedal is being depressed or released, the clutch plate engages partially and slips partially. This “slipping” is what allows the driver to start the car from a standstill or change gears while in motion, as it allows the rotation speeds of engine and transmission to gradually come into alignment.  Learning to properly deploy the clutch in order to accomplish this, however, can be a bit difficult for new drivers to master.

Now that you know the theory behind how a manual transmission works, I’ll discuss the practice of actually driving a standard shift car in my next entry!

 To learn more about this topic, or a broad range of subjects from “How To Change A Tire” to “How To Jumpstart Your Car”, visit DefensiveDriving.com’s Safe Driver Resources website!

Check out these sites for more information about online defensive driving in Texas, online defensive driving in Florida, and business driver safety.