(Easy to Understand) Car Transmission FAQ
Last updated: 02/19/08

OUTLINE
	Basics
	1.) What is a transmission?
	2.) What is a clutch?
	3.) What is a gearbox?
	4.) What is a gear ratio?
	5.) What is a differential?
	6.) What is engine braking?

	Differences between Automatic and Manual transmissions
	1.) What is the difference between an Automatic and a Manual transmission?
	2.) What about the cars with no clutch pedal but has controls for you to shift manually?
	3.) What are the advantages and disadvantages of Automatic, Manual, and Manumatic transmissions? 

	More advanced topics
	1.) What do the different positions on an Automatic gearshift do, such as "P", "N", "D", "D4", "D3", "D2", "D1", "L", "L2", "2", "Overdrive",  etc.?
	2.) What is an H-pattern gearbox?
	3.) What is a sequential gearbox?  What is a sequential manual gearbox?
	4.) What is a semi-automatic gearbox?
	5.) What is heel-toe?
	6.) What is double-clutching?
	7.) Can you shift without the clutch in a manual transmission?
	8.) What is a synchronized gearbox?
	9.) What is a dogbox?
END OUTLINE	

BASICS

1.) What is a transmission?

A transmission does two main things:

	1.) It proportions the power generated by the car's engine into suitable amounts of acceleration and speed before delivering the power to the car's wheels.  Without this, the car would not accelerate nicely or it would not go as fast as you want it to.

	2.) It connects and disconnects the engine from the wheels as necessary.  Disconnection is necessary to keep the engine running while the car is stopped (otherwise you'll have to restart it everytime you stop, which will eventually drain the battery, not to mention being time consuming plus having some other problems) and also to reduce shock while changing gears.

A transmission is made up of a few components, namely: clutch, gearbox, differential.  The clutch does the connection/disconnection.  The gearbox and differential does the power proportioning.  There are also some controls for the driver to use, including gearshift lever, clutch pedal, buttons, etc.

Transmissions have been historically categorized as "automatic" or "manual" (a manual transmission is also known as a "stick shift") although recent developments have started to blur the line between the two.  For example, most automatics now come with a manual sequential shift function.  Likewise, some manual transmissions have machinery to allow for automatic shifting and clutch action.  This has led to the use of other terms to categorize transmissions, such as "manumatic" or "semi-automatic".

Note that the word "transmission" can mean several different things depending on the context.  It can mean the entire power delivery system (combination of clutch, gearbox, and differential), which is the meaning used in this answer, or it can mean just the gearbox and differential, or it can mean just the gearbox itself.  It is not always clear, and even experts will often ask when in doubt.

2.) What is a clutch?

A clutch is a device that can connect or disconnect the engine from the wheels.  Disconnection is necessary to keep the engine running while the car is stopped (otherwise you'll have to restart it everytime you stop, which will eventually drain the battery, not to mention being time consuming plus having other problems) and also to reduce shock while changing gears.

3.) What is a gearbox?

A gearbox converts the power of the engine into suitable components of acceleration and speed as needed.

A gearbox is a unit with typically 4 or more sets of gears (sometimes as many as an "infinite" number of gears in the case of a Continuouly Variable Transmission or CVT).  Each gearset allows you to have a different balance between acceleration and speed.  The engine produces power.  Without going into detailed physics, just think of power as something that can be converted to either acceleration (aka force) or speed.  Depending on the situation, you might want to convert all of the power to acceleration, or all of it to speed, or some to acceleration and some to speed.  When the car is stopped, you need acceleration but no speed.  When the car is going fast, you need speed but not much acceleration.  Each gear in the gearbox gives you a different proportion to suit different needs.

4.) What is a "gear ratio"?

A gear ratio is the ratio of input-force to output-force for a gearset.  A gearset is just like a lever.  You push down on one side of the lever and the other side pushes up on something else, but with a different force and travelling at a different speed.  In cars, the input is not a "push", but a rotational force from the output shaft of the engine, and the output is likewise a rotational force going to the wheels.

In a car, you would typically have 4 or more different set of gears, each providing a different gear ratio.  (Very often, the word "gear" is used as a synonym for "gear ratio").  Each ratio provides a different balance between acceleration and speed.

5.) What is a differential?

A differential is a device that can deliver power to two different wheels while the wheels are spinning at different rates.  This is necessary in a car because many situations can occur that will make the wheels spin at different rates compared to each other (such as when you are cornering).  Depending on the design of the differential, the car's stability can be improved, sometimes significantly especially in slippery conditions such as rain, dirt, mud, or snow, and of course, in high speed racing applications.

Another function that is almost always built into the differential is an extra gear ratio which multiplies with the gear ratio coming out of the gearbox.  It may sound complex, but it's simply like having another gearbox after the gearbox, except this one almost always has only one fixed gear ratio, unlike the gearbox which has usually 4 or more selectable gear ratios.  The reason for this extra gear ratio is to keep the gearbox components within reasonable physical dimensions, and to have an easy way for manufacturers to produce two cars with different gear ratios but sharing the same gearbox.

6.) What is engine braking?

Engine braking is a braking force provided by the engine.

When you give an engine gas, it wants to spin up, simple.  But if don't give an engine gas and try to somehow spin it up (either by hand or by some other means), you'll find that it's hard to do.  This is because, due to the way the engine works,  there is vaccum as well as air compression going on inside which, due to air pressure, makes it hard to spin.  When you give the engine gas (and electrical spark), the gas works with the air to create power which spins the engine, but without gas, the air works against the engine and forces it to stop spinning.  Now, imagine that you are coasting (off the gas pedal), and the engine is still connected to the wheels, then the engine would provide a braking force that cause the wheels to slow down.  That's engine braking.  It's the same with electric cars, except in this case the magnets in the motor cause the engine to not want to spin unless there is current.

Engine braking adds to the stopping force from the brakes, which reduces brake wear.  This is especially effective when going downhill for long distances, as brakes can overheat fairly easily if used nonstop for a long time, whereas engine braking stays effective indefinitely.

Engine braking is a major difference between automatic and manual transmissions.  While all engines provide braking, the effect is only felt when the engine is connected to the wheels.  In a manual, the driver has full control over the connection of the engine to the wheels, so engine braking can be used or not used as desired.  But in an automatic, this may not be true as most transmissions are designed to disconnect the engine from the wheels during coasting.  This is done to allow for smooth downshifting but at the cost of not taking advatange of engine braking.  Manumatic transmissions, on the other hand, do allow the driver to take advantage of engine braking.

DIFFERENCES BETWEEN AUTOMATIC AND MANUAL TRANSMISSIONS

1.) What is the difference between an Automatic and a Manual transmission?

The most obvious difference to most people who have not driven a manual is that an automatic transmission shifts for you (the user never has to do any work shifting), whereas a manual transmission requires the driver to shift manually by way of operating a gearshift lever as well as a clutch pedal in order to shift.  However, there is another major difference which becomes obvious the first time one tries to drive a manual, and that is a manual transmission displays engine braking due to the way it works.  The engine braking can also be felt in an automatic by putting the gear selector into certain positions other than the normal "Drive" or "D" position.

Mechanically, a major difference is the type of clutch deviced used.  As of yet, virtually all cars advertised as "automatic" uses a torque converter for the clutch, whereas virtually all cars advertised as "manual" uses a plate-type clutch (usually just called a "clutch") for the clutch device.  While both devices serve to connect or disconnect the engine from the wheels, the torque converter is an automatic device with no action required from the driver.  It, however, suffers from power loss and slower response times.  The plate-type clutch, on the other hand, is a solid connection device with no power loss and faster response times.  However, it requires the driver to skillfully operate it.

Another major mechanical difference is the type of gears used in the gearbox.  As of yet, virtually all cars advertised as "automatic" use what are known as planetary gearsets in the gearbox.  And virtually all cars advertised as "manual" use helical gears in the gearbox.  The two types are vastly different in looks and function.  Planetary gearsets allow for automatic shifting because the shift action can be easily performed by using electro/mechanical actuators.  However, they have more wear items and more delicate parts, which leads to lower reliability.  Also, because numerous actuators are required for planetary gearsets, it is not feasible to produce a manually shifted planetary gearbox with a direct shift linkage; it works better with remote actuation.  Helical gearsets have the benefit of using less parts and less wear items, which means lower cost and better reliability.  But they are not as simple to shift in a mechanical sense, and thus are almost never used for automatic transmissions.  

Another mechanical difference is the type of controls given to the driver.  For an automatic, the user manipulates a shift lever which currently is usually nothing more than an electrical switch which is made to look and feel like a mechanical lever.  The signal from the lever is sent to a computer which, after processing, then sends the actual control signals to the gearbox which activates the shifting.  The manual transmission, on the other hand, has a shift lever and a clutch pedal, both of which are directly connected to the actual machinery.

2.) What about the cars with no clutch pedal but has controls for you to shift manually?

(I will call this type of transmission "manumatic" from the rest of this question.)

Manumatics are like automatics in that you can choose to have the transmission do all the gear shifting for you.  But in addition, they also offer a "manual mode" where the driver can select gears manually in a sequential fashion.  The clutch actuation is completely automatic for both automatics and manumatics.  The main differences between manumatics and non-manumatics are the following:

	1.) In manumatics, when you put it in manual mode, the car will no longer do any automatic shifting, that is, it will stay in the gear you put it in (although there are some exceptions to this).

	2.) Engine braking is active for all manual gear positions in manumatics.

Note that manual gear selection alone is not new.  For decades, most automatics have controls to manually select gears.  The extra positions are usually labelled as Overdrive, D3, D2, L2, L, and the like.  However, most of those extra positions still have some automatic shifting going on and there is also no certainty of engine braking.  It's the ability to completely disable automatic shifting and the implementation of constant engine braking that is new to manumatics.

As of now, almost all manumatics use a torque converter and a planetary gearset gearbox, so mechanically they are very similar to non-manumatics.  However, one would hope that the manumatic gearbox is more durable to withstand extra shifting by the user since the manual shift function is usually advertised as a "feature".

While upshifts are always relatively smooth, the smoothness of downshifts in manumatics varies greatly among different implementations.  Some can be so rough as to not be usable in a lot of cases.

Some cars offer controls similar to manumatics, but instead use a plate-type clutch and a helical gearbox, along with hydraulic/electrical actuators to do the shifting and clutch action.  They look very similar to a typical manumatic but are very different due to the presence of the plate-type clutch and helical gearbox.  Mainly, they do not suffer from any of the disadvantages of the torque converter or planetary gearsets.  These transmissions are not called manumatics but rather semi-automatic.  They are usually very expensive.  Other combinations between dfferent types of clutches and gearboxes may exist.

3.) What are the advantages and disadvantages of Automatic, Manual, and Manumatic, transmissions?

AUTOMATIC
Advantages:
-Easier to learn
-Easier to operate, especially durring emergency situations
-Easier to operate for physically  handicapped
-Can create a calmer demeanor whille driving
-Faster acceleration from a stop ((at the risk of reliability)
-Allows for left foot braking

Disadvantages:
-Gear selection may not be fully ccontrollable by driver
-Engine braking behavior may not bbe fully controllable by driver
-Ease of use can lead to driver beeing distracted by other things
-Fully automatic action creates noo rhythm for the driver, which may not be good in some cases
-Some power loss
-Slower response time to throttle  input
-Less reliability due to more moviing parts and more wear items
-More easy to overheat
-More expensive (although not alwaays true due to supply and demand)
-Heavier

MANUAL
Advantages:
-Fully controllable gear selectionn
-Fully controllable engine brakingg behavior
-Difficulty of use forces the drivver to be more concentrated on driving
-No power loss
-Faster response time to throttle  input
-Creates a rhythm while driving whhich can be helpful in cetain situations
-More reliable due to less moving  parts and less wear items
-Less likely to overheat
-Cheaper (although not always truee due to supply and demand)
-Lighter

Disadvantages:
-Harder to learn
-Harder to operate, especially durring emergency situations
-Harder or impossible to operate ffor physically handicapped
-Improper use can lead to easy breeakeage
-Can create an unpeaceful demeanorr while driving
-Slower acceleration from a stop ddue to clutch engagement time
-Slower shift time (in most cases  for a street car; not true for race cars)
-Does not allow for easy left foott braking (in most cases for a street car; not true for race cars)

MANUMATIC
Advantages:
-Easier to learn
-Easier to operate, especially durring emergency situations
-Easier to operate for physically  handicapped
-Can create a calmer demeanor whille driving
-Faster acceleration from a stop ((at the risk of reliability)
-Allows for left foot braking

Disadvantages:
-There is often a delay between geear selection and the actual shift
-Not all the gears may be user conntrollable
-Computer can intervene unexpecteddly with the shifting
-Downshifts can be extremely roughh
-Ease of use can lead to driver beeing distracted by other things
-Some power loss
-Slower response time to throttle  input
-Less reliability due to more moviing parts and more wear items
-More easy to overheat
-More expensive (although not alwaays true due to supply and demand)
-Heavier

MORE ADVANCED TOPICS

1.) What do the different positions on an Automatic or Manumatic gearshift do, such as "P", "N", "D", "D4", "D3", "D2", "D1", "L", "L2", "2", "Overdrive", "+", "-",  etc.?

Particular implementations can of course vary, but typically they work as follows:

D (with Overdrive ON), D4 (or whatever number that corresponds to the highest gear), D with a circle, and similar:
-Automatic shifting.
-Use all gears.
-Free coast (no engine braking), oor maybe have some engine braking depending on implementation.

D (with Overdrive OFF), D3, D with no circle, and similar:
-Automatic shifting.
-Use all gears except the highest  one.
-Good chance of engine braking whiile in highest gear, probably no engine braking in lower gears.

D2, L2, 2, and similar:
-Automatic shifting, or may be loccked in 2nd gear.
-Use only 1st and 2nd gear (unlesss locked in 2nd gear).
-Engine brake while in 2nd gear, mmay not have engine braking while in 1st gear.

D1, L1, L, and similar:
-Use only 1st gear.
-Engine brake always on.

M (or whatever symbol they use to mean manumatic mode):
-No automatic shifting (although mmachine can downshift to 1st or 2nd automatically when stopped; sometimes 1st gear is not selectable).
-Engine brake always on.

+, Up, and similar:
-Go to the next higher gear (althoough upshifting past 2nd gear while stopped is often disallowed).
-No automatic shifting.
-Engine brake on.

-, Down, and similar:
-Go to the next lower gear (althouugh sometimes 1st gear is not selectable).
-No automatic shifting.
-Engine brake on.

P
-Put transmission in neutral (no cconnection between input and output of gearbox).
-Insert parking paw (a piece of meetal) into a spot which locks the gearbox from spinning.

N
-Put transmission in neutral (no cconnection between input and output of gearbox).

R
-Reverse gear.
-Usually has engine braking.

2.) What is an H-pattern gearbox?

An H-pattern gearbox is a manual (i.e. non-planetary) gearbox in which the gear lever directly operates the shift rails which selects the gears.  Due to the way the shift rails are usually designed, such a gear lever always move in an H shaped pattern, thus the name.

The advantage of an H-pattern gearbox is that any gear can be selected at any time with a single motion.  The disadvantage is that it can be possible for the driver to accidentally select the wrong gear since there is a slight bit of precision involved when operating the lever.  In racing situations, due to the g-forces involved and the speed in which shifts are made, it can become somewhat difficult for the driver to move the lever precisely everytime.

3.) What is a sequential gearbox?  What is a sequential manual gearbox?

A sequential gearbox is a manual (i.e. non-planetary) gearbox in which there exists an intermediate device which sequentially moves the shift rails on the gearbox to select the gears.  The intermediate device is typically a cylindrical looking unit with grooves cut in it which fit over the shift rails.  This device is sometimes called a "shift cone".  The shift cone is designed to select each gear sequentially as it is turned.  The shift lever(s) has only two positions, one to upshift and one to downshift.

This kind of gearbox is found in nearly all motorcycles and many race cars.  Its advantage is that there is virtually no chance of the driver making a shift error since only two positions exist.  The disadvantage is that you cannot select an arbitrary gear with one single motion.  Also, it requires some kind of visual display in order to show the driver what gear the gearbox is in, since the gear lever(s) always stays in the same spot no matter what gear it's in.

A sequential manual gearbox is a sequential gearbox operated manually by the driver via direct linkage.  It also means that the driver has to operate the clutch manually.  A sequential gearbox where the shifting is done by the driver via remote linkage (i.e. electronics and hydraulics) is called a semi-automatic.

4.) What is a semi-automatic gearbox?

A semi-automatic gearbox is a manual (i.e. non-planetary) gearbox in which the gear changing is done by remote actuators rather than by a direct mechanical linkage between driver and gearbox, and in addition, the transmission uses a plate-type clutch, which may or may not be remotely actuated, although in most cases it is.  Note the differences between this and a manumatic; while both typically offer automatic shifting and automatic clutch action, a manumatic uses a torque converter and a planetary gearbox instead.

A semi-automatic gearbox usually uses a sequential gearbox, and provides only up and down shift controls to the driver.  Neutral, reverse, and park are implemented differently depending on the vehicle.  Most race cars with semi-automatics still rely on the driver to operate the clutch while the car is stopped (albeit with remote control).  Most street cars with semi-automatics do not provide a clutch pedal; the clutch is always automatic.

Since the user controls are only electronic mechanisms, a small lightweight lever can suffice, even for the clutch which would otherwise be a heavy foot pedal if it was a mechanical linkage.

5.) What is heel-toe?

Heel-toe is a driving technique which allows for quick yet smooth downshifts while braking.  It is usually done while driving manual transmissions, although the technique can be used for automatics and especially manumatics, but to a lesser extent.

The procedure is (remember this is only done when under braking):

-Brake with right foot.
-While still braking, push in the  clutch with left foot and soon as possible...
-hit the right amout of gas with tthe right foot without letting go of the brake (or clutch) while at the same time shift to a lower gear.
-Let go of the gas and clutch (braakes should still be on).

The difficult parts are:

-Reaching both the brake and the ggas with the same foot.  Thus the term heel-toe.  Typically, the toe is used to brake, it then pivots on the brake pedal as the heel swings to the right to hit the gas.  Depending on the car's pedal positioning, the heel may have to be on the brake and the toe on the gas in order to be able to reach both pedals.  In such cases, the procedure is much more difficult because it's necessary to lift the entire right leg in order to hit the brakes initially.  Some cars might have the pedals positioned in way to make heel-toe impossible.
-Not varying the brake pressure ass you hit the gas
-Gasing the right amount to bring  the engine rpm to the correct level for a smooth shift
-Timing everything and doing it faast.

Why do you have to do it?
There is no other reliable way to produce a quick yet smooth downshift while braking.  It is a given that if one tries to downshift without reving up the engine, the rpms will not match the drivetrain, leading to clutch wear, excessive engine braking and possibly loss of control as a result.  Typically, without heel-toe, you would have to wait until the braking is finished before engaging the lower gear.  The disadvantage of this is it takes a little extra time to engage the gears and clutch compared to heel-toe where the gear changing is already complete before the end of braking.  The advantage of heel-toe become even more apparent when one is braking for a corner.  By completing the gear change before the end of braking, one is free to gas the car through and out of the corner.  Without heel-toe, it can become awkward to try to steer the car around the corner while depressing on the clutch or trying to re-engage the clutch.  Needless to say, heel-toe is a fundamental technique to race car driving.

6.) What is double-clutching?

Double-clutching is a technique used to match the rpms between the two mating members (namely, input and output shafts) in a manual transmission during a shift.  It is used to reduce wear on the gearbox and even to allow for any shifting at all when one is skipping certain gears downwards, and it is also necessary in order to allow for any shifting at all when driving a gearbox without synchronizers.

The procedure:

-Push in clutch.
-Put gear lever in neutral.
-Release clutch.
-Blip the throttle just the right  amount, and while the rpm is still up...
-push in clutch then put the gear  lever in desired gear and optionally release clutch (all while the rpm is still up) or...
-if you didn't let go of the clutcch before the rpm drops, optionally do another throttle clip to match rpms and while the rpm is still up then...
-release clutch.

The difficult parts are:

-Blipping the throttle the right aamount (the first time) to match rpms between gearbox members
-Putting in gear lever at the righht moment while the rpms are "correct"...
-While still having enough time too let go of the clutch before the rpm drops.

Note that the first blip (with the gear in neutral) is to match rpms between the two mating members in the gearbox while the second blip (if done) is to match rpms between the engine and the gearbox input.  If the car is slowing down (like while braking), the two blips would have to be different amounts.  To do it in a single blip requires very fast action between putting it in gear and letting the clutch out.  But notice that it is not necessary to try to do it in one blip if one's purpose is just to protect the gearbox or to facilitate shifting.  While using the procedure, it's easy to tell whether one did it right or not.  When done right, the gear lever will go into the desired gear effortlessly, sometimes it even feels like it gets "sucked" into gear, like it "wants" to go in.  When done wrong (the wrong amout of blip), the gear lever will still take effort to push in gear, or you might even feel some metal clanking inside as you push it in gear.

Double clutching can be combined with heel-toe to skip gears while braking.

Why do you have to do it?
In many if not most manuals, if you try to skip gears from anything above 2nd gear down to 1st, or try to go from 5th to 2nd or 6th to 3rd, you'll find that it does not want to go in gear.  It takes considerable pressure and time for the gear lever to go in gear.  This is because there is a large difference in ratio when you try to skip gears.  With modern gearboxes, there are synchronizers (kind of like little metal pressure plates) which will match the rpms between the two gearbox mating members before actually engaging.  But synchronizers do wear out over time, and the most wear occurs when you try to skip gears.  It's possible to hear the synchronizers rubbing when skipping gears if you listen for it.  So if you want to skip gears quickly without wearing the synchronizers, you have to manually match the rpms between the two mating members, which is what the double-clutching procedure does.  When done properly, the gear lever will move into gear effortlessly and quickly.  Some very old cars, certain specialty cars, and some race cars do not have synchronizers at all.  In those cases, double clutching is required to prevent serious grinding.

7.)  Can you shift without the clutch in a manual transmission?  How do you do it?

To shift without the clutch reliably without causing wear or damage requires the gearbox to be of a type called a dogbox as well as proper technique from the driver to match rpms.  While it is theoretically possible to have smooth clutchless shifts with a synchronized gearbox (which is what all stick shift street cars are sold with), it requires way too much precision to be done quickly and consistently.  Generally, the advantage of not using the clutch while shifting is to have quicker shifts.

Procedure for clutchless shifts, preferably with a dogbox:

For upshifts (assuming on the throttle before shift):
-Release enough of the throttle too put drivetrain in zero-torque condition (i.e. no acceleration and no engine brake).
-Shift into neutral (or false neuttral for sequential gearbox).
-Release more throttle to allow rppm to drop down.
-At the moment when the rpm matchees the speed of the vehicle for the desired gear, shift into desired gear.

For downshifts (assuming off the throttle before shift):
-Apply just enough throttle to achieve zero-torque condition (i.e. no engine brake and no acceleration).
-Shift to neutral (or false neutraal for sequential gearbox).
-Blip the throttle to raise the rppms to the proper amount for the next gear and...
-Shift into desired gear.

The difficult parts are:

-Applying just enough throttle to  achieve zero-torque condition before moving lever to neutral (although with a dogbox this is not so critical).  This reduces wear on the gearbox and makes it easier to move out of gear.
-Finish shifting into gear at the  right moment when rpms are matched between the engine and the wheels.  This prevents grinding and wear.

For fast and smooth upshifts, it's preferable for the car to run a light flywheel and/or have an ignition cut-off mechanism during upshifts so as to make the rpms drop sooner during the false neutral.  Also beneficial would be to have engine management controlling the idle torque and idle rpm appropriately during an upshift.  Also note that the above describes an optimum way to do clutchless shifting which results in minimal wear to the machinery.  With a dogbox, it's possible to achieve long term reliability even if the above steps aren't performed totally accurately.  When everything is done correctly on a car with optimum machinery, both upshfits and downshifts as described above happen in just a blink of an eye with one smooth motion.

For a sequential gearbox, the rpm matching occurs during the false neutral which exists between gears.  The difficulty is to time the blip or lift at the right time so that most of the rpm change occurs during the false neutral when the drivetrain is disconnected.  Note that this false neutral only lasts a split second during a shift, so practice is needed to get the timing right.  There is no doubt that in racing where manual gearboxes are used, much of the reliability of the gearbox depends on driver skill.  Many racecar drivers shift with the clutch even with a dogbox in order to preserve reliability.  The drawback of shifting with the clutch is that you cannot brake with the left foot and take advantage of the increased precision and reaction time that comes with it.

8.) What is a synchronized gearbox?

A synchronized gearbox is a gearbox where there are devices which force the two mating members of a gearset to rotate at similar speeds before mating.  Basically, whenever you try to mate two members of a gearset together (like when you are changing gears), the gears will grind unless they are rotating at close to the same speed.  In a synchronized gearbox, there are things called synchronizers, which are basically like small metallic plates, which force the two members to similar speeds before allowing the gears to mate.

In a manual transmission, each gear ratio is created by having two gears meshed together.  One gear takes input from the engine and spins the other gear, which in term is connected to the wheels.  When you change gears, you are basically replacing the input gear with another gear of a different size.  (This is not how it works in a typical gearbox, but the concept is similar.)  Note that the clutch must be disengage during the gear change to prevent massive grinding.  The problem is that the new gear, due to it's different size, will always want to spin at a different rate than the original gear.  Even though the clutch disconnects the engine from the gearbox during a gear change, there is still considerable mass from the things that are attached to the gears.  Thus to prevent grinding, synchronizers are used to bring the input member to a similar rotational rate as the output member before engaging the two gears.

All manual transmission street cars for many decades now use synchronized gearboxes.  Double clutching is a technique that enables one to change gears without grinding even with a non-synchronized gearbox.  Synchronizers are a wear item.  They are probably the weakest members in a manual gearbox and are the cause of most failures.  Proper technique from the driver is required to lengthen the life of the synchronizers.

9.) What is a dogbox?

(Again, the actual workings are different, but here is the general idea.)

A dogbox is a manual gearbox in which the gears engage using large blocks formed into the flat side of one gear and large holes formed into the flat side of the other gear.  The locations of the blocks correspond to the location of the holes.  The large blocks are called dogs.  A gear with the blocks is called a dogring because there is a ring of blocks around the gear.  These type of gears are also known as face gears because they are in essence large gears cut into the face side of the gear.  There are no synchronizing elements to make two mating members rotate at the same speed before mating.  The dogs will simply jam into the holes and force the two gears to mesh together.

The advantage of a dogbox is that the large dogs and holes which form the mating elements are very strong compared to the much smaller elements found in synchronizers.  This makes dogboxes almost ubiquitous in race cars.  The strength of dogboxes allow them to be shifted without using a clutch without causing catastrophic failure for relatively long times.  However, the actual reliability depends heavily on the technique of the driver, as even dogboxes are not indestructible.  Basically, it's critical that the driver match the rpms during clutchless shifts to minimize wear on the gearbox.

The main disadvantage of dogboxes is that they are extremely loud.  This is why they are never used in street cars.  If fitted onto a typical street car, the noise from the gearbox can easily be louder than the engine.  Even on a race car they are often the most noticeable noise source to the driver from inside the cockpit.  The reason they are so loud is because they usually use straight cut gears, also known as spur gears.  This is in contrast to street cars which use slant cut gears, also known as helical gears.  Due to the way they work, straight cut gears are much louder than slant cut gears.  There are certain technical reasons why straight cut gears are preferrable in dogboxes.  To put it simply, they are used to maximize strength and efficiency. Another loud element is the way the gears mesh together in a dogbox.  The metal elements literally clash into each other, causing a loud "clang" when a shift is done.  In contrast, a synchronized gearbox mostly has elements sliding across one another during shifts without any components actually hitting each other that hard.