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Do You Know How Your Transmission Works? Transmission 101

The transmission performs an extremely important duty in the operation of a car. I like to think of the transmission as a funnel in reverse. At the narrow end is the engine, which for all its complexities, has quite narrow outputs. Many mechanics will tell you that in terms of RPM’s, there are no major changes in output until you jump to a two thousand mark.

This means, that even in the lower rpms where incremental gains are more important, your torque power curve won’t turn upward drastically for 2000 rpms. 1000 rpm feels a lot like 2500rpm. If you consider that most cars idle at 1200 rpm and redline at 7000 rpm, then there are only 3 differing power curves per gear on most vehicles.

The transmission takes these very narrow outputs from the engine, and translates it to a large range of speeds, in a variety of ways. For instance, on flat ground, cruising along at 50mph can be done with low RPM in a higher gear than if you were trying to maintain 50mph going up a steep incline.

If it were not for the variable gearing provided by transmissions, you would have cars are stuck in a gear ratio based on top speeds. If you wanted a car to have a top speed of 100 mph, then it would be like driving a car in 4th gear all the time. As you would soon find out, you would have no acceleration starting out with this gearing, and then you would redline almost constantly when you were on the highway. This car would wear out extremely quickly and be almost impossible to drive.

Put another way, a car without a transmission is like a one-speed bicycle. Fun for kids, but not practical for biking to work, mountain biking, racing the Tour De France, etc. Gears = more usefulness and more capability.

Automatic Transmission vs. Manual

The key differences between automatic and manual transmissions are:

  1. Gear changes are accomplished with hydraulic fluid in an automatic, but manuals use a clutch plate
  2. Because manuals use a clutch plate, and because clutch plates wear over time, at some point a manual transmission will need a new clutch (a fairly major expense). Therefore, automatic transmissions are generally less expensive to maintain
  3. Automatics allow for computer control, whereas manual transmissions do not (unless they have an automatic clutch, but this is a pretty rare setup as of 2012)
  4. Because automatics can be controlled by your engine computer, it’s likely a vehicle with an automatic transmission will perform better in terms of emissions and fuel economy because the engine and transmission can by synced very closely
  5. In heavy duty applications, such as big-rig semis, construction equipment, etc., manual transmissions are preferred because they’re inherently stronger and able to take more abuse. This is often the case in racing applications as well.
  6. Race cars often use manual transmissions because they weigh less than automatics
  7. However, many drag racing enthusiasts (especially bracket racers) prefer automatic transmissions because they’re much more consistent than a manual

Mechanically, the main difference between a manual transmission and an automatic are the number of gear cogs. In a manual transmission, there are many gear cogs, usually one per gear, and you manually change to each gear. In an automatic, a single gear is paired with other auxiliary gears to get the same gear-range, its called a planetary gear set.

The Miracle of The Planetary Gear Set

Named after the parts of our solar system, a Planetary Gearset automates the selection of modifying gears (ring gears), to the main gear (planet gears), which is driven by the Sun Gears. If this sounds complicated, don’t fret, its easy to understand. Take a look at the animation below and then read up for more details.

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Ok, lets classify these gears from the most important to the least important. Most important is the drive gear, or Sun Gear. The Sun Gear is called the driver of the transmission because this is the gear that is cranked by the engine itself. It provides the power of the engine to the transmission. It is located in the innermost part of the transmission, and all gears attach to this one gear-set. No Sun Gears = No Motion.

Second, there are the Planetary Gears. These are about the size of a small watermelon, and these create all of the gear ratios that a transmission can achieve. Whilst the sun is the most important in terms of locomotion, the planetary gear set makes the drivetrain unique to the vehicle.

Finally, there are the Ring Gears. These are usually the gears that are stationary to create the different ratios. The really amazing thing about these gears, is that if it weren’t for these stationary gears, then there would have to be more than one system to achieve the same results.

There will be other supporting pieces of equipment in the transmission housing, but of them all, the most important supporting piece is the hydraulic pump. This disperses transmission fluid throughout the whole piece of equipment to reduce friction to maximize efficiency whilst minimizing wear and tear on the gears themselves.

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The entirety of the planetary gear set is the beginning of the process of transferring power to the wheels. It is connected to the engine via the flywheel. The flywheel is a type of clutch that allows gears to turn one way or another. There are usually 2 or 3 more clutches in the transmission, mostly because one of the sun gears turns clockwise, whilst the other turns counterclockwise. (This is so your car can move forward and backwards as well.)

Getting In and Out of Park

All of this information is great for a moving car, but I always enjoy explaining how to stop a car. I mean this in the most generic way possible, parking. You would think that it would be easy to just lock all of the clutches in the transmission to keep the car from moving, but it is actually a more complicated process.

To have a proper park mechanism, it must satisfy a few requirements.

  1. First, the parking level must remain in place when it is engaged.
  2. Second, if you are going to lock gears, you can be sure that you’ll be doing it with another gear, the parking mechanism must engage even if the teeth of the clutch and the gear don’t line up perfectly.
  3. Finally, the whole mechanism must be able to pop out of position if you want to start driving a car when there is a lot of potential energy on the gears themselves. Without that mechanical assistance, taking your car out of park when you are on a hill, and all of the weight of the car is working against you, would be much more difficult.

The parking lock happens on the backside of the automatic transmission. There, on the output piece, there are a series of notches that a parking gear is slid into place and locked, so that the transmission cannot engage. This is all done automatically for you, and sometimes, the teeth line up on the top of the output tooth. This is why your car sometimes rocks back and forth when you try and park it, the transmission had to be moved a little bit so that the tooth can drop into the output valley.

Guest Author Pete Wise is a content developer and white-hat SEO Jedi. This article was written for AAMCO Centers of Colorado, who service Transmissions and are the leading experts in Transmission Repair. Connect with Pete on

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