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How Power Flows through the Drive Train

Most vehicles come with two basic types of drive trains: front-wheel drive and rear-wheel drive. The names are self-explanatory: With front-wheel drive, the power flows from the engine to the front wheels, which drive the car. With rear-wheel drive, the power flows to the rear wheels, and they’re in charge.

Surprise! (There are all-wheel drive, four-wheel drive, and other variations, but I keep it simple for now.) On almost all vehicles, the front wheels respond to the steering system, which directs the vehicle around curves and turns. On rear-wheel-drive vehicles, all the parts involved run like the cars on a train, from the engine in the front to the wheels at the rear.

(That’s probably why they called it the “drive train.”) Although today more vehicles have front-wheel drive — which combines the transmission and other parts into one neat package called a transaxle — in order to get a good look at every part involved in a drive train, it’s easier to follow the power through a rear-wheel-drive vehicle with a manual transmission. When you know what each part does and whether or not it’s done manually, automatically, or combined into some exotic new mechanism, you’ll stay.

track through the more detailed descriptions of each type of vehicle found in separate sections of this chapter. To make your ride smoother show all the components as they’re located on a vehicle with rear-wheel drive and a manual transmission. If your vehicle has front-wheel drive, shows you the way the power flows through a transaxle. The principles and parts are pretty much the same in each type of drive train.

side of the axle to rotate at a different speed. This ability is necessary because, when you go around a sharp curve, the outside wheel travels farther than the inside wheel and has to move more quickly than the inside wheel, just like the ice skater at the end of a snap-the-whip line.

The differential also provides the drive wheels with extra power by using its gears to convert varying numbers of revolutions of the driveshaft into one revolution of the drive wheels. This is called the gear ratio. I get into details about gear ratios in the later section “Manual Transmissions.” On a front-wheel-drive vehicle, the transaxle is in line with the center hubs of the front wheels.

axles (sometimes called half shafts) emerge from each end and run straight to the wheels. The differential changes the direction of the power coming out of the engine and into the transaxle.

Four-wheel drive

If you own a sport-utility vehicle (SUV) that features four-wheel drive your drive train includes a transfer case. The transfer case controls the power to the front and rear drive axles. When you switch from two-to four-wheel drive with either a dash-mounted switch or a floor-mounted shifter, a gear in the transfer case engages the front driveshaft along with the rear driveshaft so that all four wheels get power from the engine. In two-wheel drive, only the driveshaft connected to the rear wheels gets power from the engine, and the front wheels are disengaged.

Last word

The transfer case also usually has settings for low and high gears. Low gear delivers a lot of torque (pulling and pushing power) to the wheels for traveling in very rugged, muddy, snowy, or slippery terrain at slow speeds. High gear is for traveling at moderate speeds on normal roads in wet or moderately slippery conditions.

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