Ever wonder how the 400 horsepower and 360 lb/ft of torque under your hood seem to magically make their way from the engine to the wheels? Through mechanical advantage and torque multiplication, transmissions allow a fast-spinning engine to efficiently transfer power to a slower moving drive shaft, and ultimately make you accelerate. Torque is "multiplied" because the output shaft of the transmission revolves slower than the incoming engine speed, causing an increase in rotational force. Most modern automobiles fea
An external view of a transmission
An external view of a transmission
ture transmissions with a variety of different gears, which allows the engine to perform at the optimal RPM range (for maximization of torque) while still maintaining fuel economy.
Understanding gear ratios is the basis to understanding why different gears are needed. As a car goes faster, the engine spins faster, and it has a maximum RPM which it cannot exceed, known as redline. This is about 6.5k RPM on most cars, although shifting is normally done at about 3k RPM to save gas (most power is generated in the high RPM range). When an up shift occurs, the next gear is a larger gear that has a lower gear ratio. Each successive gear change will lower the engine’s RPM, because the engine will not rotate as fast for a given rotation in the transmission output shaft. The faster the engine is rotating in relation to the transmission output shaft, the more torque that is generated. A fast spinning engine also increases wear and gas consumption, however. At a given speed, a car has several possible gears it can be in, with the higher gears providing for a lower RPM, higher gas mileage, and less available power.

A representation of how gear ratios work
A representation of how gear ratios work


Ex: A 2004 Corvette has the following gear ratios
1st gear 2.97:1 2nd gear 2.07:1 3rd gear 1.43:1 4th gear 1.00:1 5th gear 0.84:1 6th gear 0.56:1
Reverse 3.28:1

The first number is how many times the engine has to rotate for one (the 2nd number) rotation in the transmission output shaft. The last two gears have the transmission shaft spinning faster than the engine, which saves gas at the expense of acceleration (these are called overdrive gears). Having high ratios that are close together means that you have to shift more often (or the computer does). Knowing this information allows you to…

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Velocity of car = Circumference of Tire x Engine Speed / Gear Ratio of Transmission X “” of Differential
The differential is a final gear that the other planetary gears connect to.

Since gears involve revolving parts, acceleration, velocity, and distance revolved can be determined using kinematic equations.
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These equations are similar to linear kinematic equations, except for changes in the variables.
external image 0314baf5055d6f9fd53f7b9112d78c36.pngis angular acceleration, external image baa1f2342cca9315b434fa72c2e340d6.pngis initial velocity, and external image 7137e9503f4c7489d0c6d696a48f2b07.pngis initial angular position

Gasoline powered cars need transmissions in order to stay in an optical torque band, while also going at high speeds. The first cars ever produced featured manual transmissions, though they were fairly different in structure than the stick shifts of today. In 1937, GM introduced a conventional manual transmission that featured a frictional clutch. Unlike in an automatic transmission, this driver operated clutch directly connects the drive shaft to the engine with minimal loss in power. Depressing the clutch pedal disconnects the
A BMW 6-speed shifter
A BMW 6-speed shifter
engine from the spinning wheels and allows for a gear change (most cars have 5 gears, though performance cars have 6). The benefits of this transmission include more efficient power transfer (~15% total loss from the crank to the wheels compared to ~20% in automatics), the option for the driver to select the optimal power output from the engine given a certain driving situation, better gas mileage due to an extra gear, faster shifts, and of course, its more fun! Drawbacks include not being able to apply makeup, talk on the cell phone, and drink coffee at the same time.

How different gears are selected in a stick shift
How different gears are selected in a stick shift

The first true automatic transmissions were introduced by GM in 1948. In these transmissions, a fluid coupling device known as a torque converter takes the place of a mechanical clutch. Although there is still torque multiplication due to the difference in speeds between the input and output shaft, there is more frictional loss than in a clutch. The torque converter also allows the vehicle to not stall when in gear and stationary, by prohibiting its turbine from turning. Hydraulic pressure is used to determine optimal shifting patterns (in older cars, now, it is much more computerized), and to conduct gear changes, which adds to its inefficiency. As of late, the quality of automatic transmissions has increased greatly due to American demand for them. Traditionally they were only 4 speed, but now they have expanded to as many as 7-8 gears (Mercedes and Lexus, respectively).

The makeup of a torque converter
The makeup of a torque converter

The newest introduction into the automotive world is the twin clutch automatic transmission. High performance cars have begun to incorporate this transmission, because it provides the best of both worlds (an automatic mode, and a clutch less manual mode). They feature two different automated clutches, each of which is responsible for alternating gear changes (one clutch does 1st, 3rd, 5th, while the other one does 2nd, 4th, 6th). This means that there is always a clutch ready for gear engagement, which allows for instantaneous gear changes (faster than a human can ever shift, while maintaining the direct control of a manual). Many enthusiasts believe that this system is the future of all transmissions, and with the incredible benefits that it can offer, it is easy to see why.
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The new Lexus IS-F features a dual-clutch 8 speed transmission
The new Lexus IS-F features a dual-clutch 8 speed transmission

www.startribune.com/cars/11357916.html -

Student Comments

Sonia Bansal - Why do some cars (Mercedes and Lexus) have 7-8 gears? What is the benefit of this?
naveen - Well sonia, those cars can benefit from having a lower RPM at highway cruising, therefore better gas mileage and less engine noise

Sam Edwards - Why does shifting at about 3k RPM to save gas?
naveen - Good question Sam. Shifting at a higher RPM would give you more power but use more gas because the engine is spinning faster. If you shift at a lower RPm you risk lugging the engine (not enough power to get you going up a hill for example!)

William Chan - How do paddle shifters work?
naveen - You would ask this question. Paddle shifters in automatics tell the commputer to go up or down a gear. IN a double clutch car it changes the gears without you having to use a clutch.

Greg Sturm - What would be the units for a gear ratio? From the equation you gave I got revolutions, is this right?
naveen - Yes..ex: 3 to 2 revolutions

Robert Lopez - Are the gears in the same layout for a car with automatic transmission and a car with a manual transmission? How does the lever work differently (if there is a difference)?
naveen - Thank you Robert for that question. Manual transmissions ususally have more gears than an automatic in a specific car. They are also operated with a frictional device known as a clutch, while an automatic transmissions makes use of a fluid device called a torque converter. There is no lever in an auto transmission, when the car is in park, there computer does the shifting for you.

Brandon Siegenfeld-How do hybrid cars switch there gears so that they are connected to either the gas or electric engine?
naveen - Good question Brandon, most hybrids use a CVT, or contiuously variable transmission. There are no set gear ratios like in regular cars, and the transmission can adjust its gear ratio by sliding accross a belt..its hard to explain.

Kevin Norris - Why does a torque converter have more frictional loss than a clutch?
naveen - A torque converter consists of a fluid that the parts move through...in a clutch the engine is directly connected to the wheels, metal on metal

James Song- What is the difference in ideal efficiency between a torque converter and a clutch?
naveen - 5-15%

Douglas Chin - What would happen if both clutches in a dual-clutch setup were activated? If the car were in 3rd, then suddenly 4th and 5th were pressed, then would the car stutter between them?
naveen - dog wat are u saying man.. in a double clutch car the clutches are computer operated so that could never happen... and anyway it is hypothetically impossible to have 2 gears connected at the same time..u wanna play some halo tomorrow?