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Do It Yourself Advice - June 25, 2003

Do It Yourself Advice


How Cars Work: Drive Train
by Kevin Schappell

The drive train serves two functions: it transmits power from the engine to the drive wheels, and it varies the amount of torque. "Power" is the rate or speed at which work is performed. "Torque" is turning or twisting force. Multiple ratio gearboxes are necessary because the engine delivers its maximum power at certain speeds, or RPM (Rotations Per Minute). In order to use the same engine RPM's at different road speeds, it is necessary to change the "Gear Ratio" between the engine and the drive wheels. Just like a bicycle, the car has to switch gears in order to move at a wide range of speeds. Unlike your bicycle, the car's drivetrain also has to allow you to back up. (Well, you could push it backwards if you ate your Wheaties.)

There are actually two sets of gears in the drive train; the transmission and the differential. The transmission allows the gear ratio to be adjusted, and the differential lets the drive wheels turn at different speeds.

Manual transmissions usually have four or five speeds, and often have "overdrive", which means that the output shaft can turn faster than the input shaft for fuel economy on the highway. Some use an electric clutch and a switch that controls whether the overdrive is engaged or not. An interesting development on a few cars is the "clutchless" manual transmission, which uses a stick shift and an automatic electric clutch. Speed and position sensors, mini computers, and throttle controls keep the engine from over-revving when the driver shifts gears. As with many automotive "inventions", this is an old idea which may now reach feasibility due to the computer revolution.

Automatic transmissions commonly use three forward gears to blend speed and torque. In the case of a three-speed transmission, first gear delivers maximum torque and minimum speed for starting. Second gear offers medium torque and speed for acceleration and hill climbing. Third gear allows maximum speed with minimum torque for highway travel. A reverse gear permits backward movement.

A transmission is a speed and power changing device installed at some point between the engine and driving wheels of a vehicle. It provides a means for changing the ratio between engine RPM (Revolutions Per Minute) and driving wheel RPM to best meet each particular driving situation.

Some types of drive train layouts use a "Transaxle", which is simply a combination of the transmission and the differential. These are usually found on front wheel drive cars, but are also used on mid- and rear-engine cars. Some exotic cars have their engine in the front, and a transaxle in the rear of the car for better weight balance.

Torque is derived from power. The amount of torque obtainable from a source of power is proportional to the distance from the center of rotation at which it is applied. It is logical, then, that if we have a shaft (in this case, the crankshaft) rotating at any given speed, we can put gears of different sizes on the shaft and obtain different results. If we put a large gear on the shaft, we will get more speed and less power at the rim than with a small gear. If we place another shaft parallel to our driving shaft and install gears on it in line with those on the driving shaft, we can obtain almost any desired combination of speed or power within the limits of the engine's ability. That is exactly what an automobile transmission does by means of gears and other devices.

There are two types of transmissions; manual and automatic. If you have a manual transmission, you have to shift the gears yourself, usually with a stick located on your console and the clutch pedal. If you have an automatic transmission, the mechanism changes without any help from you. This is accomplished through a system that works by oil pressure. Each shift of the gears is controlled by a shift valve; the gears shift change depending on speed, the road, and load conditions.

Another basic component of all drive trains is some form of a clutch. it allows the engine to continue rotating while the gears and wheels are stationary. Automatic transmission cars use a "torque converter" in lieu of a clutch.

The last component in the drivetrain is the axle. In a rear wheel drive car the axle is in the rear. Engine power is transmitted from the transmission to the axle via the driveshaft. The driveshaft is basically a metal tube with joints on each end called universal joints. These joints allow the tube to move in relation to the suspension and keep power flowing to the rear. In front wheel drive cars the axle is integrated into the transmission thus the term transaxle.

From the back of the engine to where the rubber meets the road, the drivetrain encompasses one of the most complicated systems of your car. Some people say looking at a transmission "makes their brain hurt".

Common Problems:

  • Manual transmission suffer from wear mainly in the synchronizers. The synchronizers make shifting easier and help to prevent gear clash. Over time the synchronizers, which are made of brass, can wear out causing hard shifting and grinding.
  • Automatic transmissions can also wear out, causing slipping and uneven shifting patterns. Universal joints can wear and cause vibrations while driving. Many newer universal joints are sealed and can not be lubricated, leaving replacement as the only option.

Preventive Maintenance:

  • Change the fluid in your transmission at recommended intervals. Your owners manual will give you a time schedule in miles and or months. If you tow a boat or trailer be prepared to change the fluid even sooner. Most owners manuals will give you recommend intervals for severe use like towing or off-road use.
  • Do not "ride" the clutch if you have a manual transmission. Learn to release the clutch in a smooth motion without revving the engine too much. Revving the engine too much while pulling out can cause premature wear on the clutch.
  • If you do tow a boat or trailer, consider getting a transmission cooler for your automatic transmission. Temperatures can approach the boiling point in severe conditions. Most newer trucks come equipped with transmission coolers if sold with a towing package.
  • Make sure your universal joint are lubricated at oil changes if they are the type which can be lubricated. When replacing universal joints try to find replacements with lubrication fittings so you can lubricate in the future.
  • If you have a front wheel drive car, avoid applying the gas to the floor while the wheels are turned at full lock. This puts stress on the universals and can cause premature failure. This can happen when stuck in the snow and trying to get out.

What to discuss with your mechanic:

  • If you are noticing a vibration in the car while driving, make sure to describe when it happens. While accelerating? braking? maintaining speed? when turning?

Good Luck,

Kevin Schappell

Kevin maintains http://www.autoeducation.com where he gives advice on car maintenance, buying, selling, insurance, and financing. A mechanical engineer and car guy, Kevin has decided to spend his online time helping others learn about automobiles.


The above article is provided for the interest and entertainment of our visitors. The views expressed in this article are only those of the author, who is solely responsible for the content. AutoGuide.net does not endorse any of these views, and is not to be held responsible for any of the content provided in the above article.


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