Lesson 6: Drivetrains

In this lesson we will cover: 1. How Drivetrains Work Layouts Transmissions Disconnecting devices Driveshafts Differentials Axles Transfer Cases 2. Drivetrain Maintenance Checking/changing/adding Fluids Inspecting CV boots Recognizing clutch wear Driving habits that minimize clutch wear

What is the drivetrain? The drivetrain includes all of the components that are involved in transferring the power from the engine to the wheels. This picture of a four-wheel-drive vehicle shows these components. The transmission is connected to the engine and receives power from it. The transfer case, driveshafts, and differentials carry the power from the transmission to the wheels.

Rear-Wheel-Drive Layout This picture shows the drivetrain layout of a typical rear-wheel-drive vehicle. It is made up of the following parts: Transmission Driveshaft Differential Axles Each component is necessary to complete the link between the engine and the wheels. Engine

Four-Wheel-Drive Layout In addition to the components that are on a rear-wheel-drive vehicle, four-wheel-drive vehicles have the following components that allow the power to be transferred to all four wheels: Transmission Driveshaft Differential Axles Transfer Case Front Driveshaft Front Differential Front Axles

3-Dimensional views of RWD and 4WD drivetrains.

Front-Wheel-Drive Layout Most of the small cars on the road today are front-wheel-drive (FWD). The layout is very simple, reducing the number of components required and decreasing the weight of the vehicle. Because of these advantages these cars typically get better fuel economy and require less maintenance. This design also concentrates all of the weight of the drivetrain over the drive wheels, giving the car better traction on slippery surfaces. The transmission and differential are combined into one component called a TRANSAXLE The AXLES connect the transaxles directly to the front wheels

3-dimensional view of a FWD drivetrain

Transmissions All cars have transmissions. The transmission provides different gear ratios for the drivetrain. When a car is starting from a stop, a lot of torque (force) is required, but very little speed is needed. In 1st gear, the transmission uses a gear ratio that causes the wheels to turn slower, but with more torque. As the car gets going faster, less torque is required, but more speed is needed. Therefore the transmission could be shifted into 2nd gear and so on. If, once the vehicle is moving down the highway in 5th gear, it comes to a hill, more torque will again be required to climb the hill. The driver must shift down to a lower gear. A car has many different gear ratios to provide the right ratio of torque to speed that is required for any given condition.

Transmissions—How Gears Work A gear set consists of a driver gear (the one doing the turning) and a driven gear (the one being turned). In a transmission, the driver gear is connected to the engine, while the driven gear is connected to the shaft that goes to the wheels. In the gear set on the left, the driven gear is larger than the driver gear. Therefore the driven gear will turn only one-third as fast as the driver gear. However, it will have three times more torque (turning force). This would make a good 1st gear. In the gear set on the right, the driven gear is smaller than the driver gear. Therefore the driven gear will turn three times faster than the driver gear. However, it will have only one third of the torque. This might make a good 5th gear.

Transmissions Another important function of a transmission is to provide a reverse gear. By adding an idler gear to the gear set, the driven gear will run in the opposite direction than it would otherwise. This manual transmission allows the driver to choose between five different gear ratios and a reverse gear.

Automatic vs. Manual Automatic and manual transmissions perform the same functions. They both have 1st, 2nd, 3rd gears, and so on. The biggest difference is that the manual transmission must be “manually” shifted by the driver, while the automatic transmission “automatically” shifts gears. Optional Links: How manual transmissions work How automatic transmissions work How CVT’s work How DCT’s work Did you know? Today some transmissions are being used which blur the lines between “automatic” and “manual.” These include Continuously Variable Transmissions (CVT’s) and Dual Clutch Transmissions (DCT’s).

Disconnecting Device The drivetrain must include some way of disconnecting the engine from the drivetrain. If not, there would be no way to stop the wheels from turning without stopping the engine also. In cars that have manual transmissions, a clutch performs this job. The clutch disconnects the engine from the transmission so that you can stop the car, start the car, and shift gears. The clutch is located between the engine and the transmission. When the driver presses the clutch pedal, the engine is disconnected from the rest of the drivetrain. To understand how important its role is, imagine trying to stop a car with a manual transmission without pressing in the clutch pedal! Optional Link: How clutches work

Disconnecting Device On a car with an automatic transmission, there is obviously no clutch. The job of disconnecting the drivetrain from the engine is done by a torque converter. The torque converter works similarly to two fans placed next to each other. With one fan turned on and the other one left off, soon the air movement of the first fan will cause the second fan to begin to move. The second fan could easily be stopped and held still, but as soon as it is released again, the airflow would cause it to turn again. Now, imagine that the first fan is connected to the engine and the second fan is connected to the transmission. Then, imagine both fans very close to each other inside of a housing filled with fluid. This is how a torque converter works, and this is why you can stop a car with an automatic transmission without stopping the engine. Optional Link: How torque converters work

Driveshafts The Driveshaft (also known as a driveline) is simply a long tube that transfers the rotational power from the transmission to the differential. Typically these are only used on rear-wheel-drive and four-wheel-drive vehicles. Driveshafts often include universal joints that allow the shaft to change angles when the car’s suspension system goes up and down in response to bumps or dips in the road. Driveshaft Universal Joints

Differentials The differential splits the torque and sends it to the left and right wheels. When a car is going straight down the road, the wheels are both turning at the same speed and the torque is split equally. However, when cornering, the outside wheel must turn faster in order to cover a greater distance than the inside wheel. The differential allows the wheels to turn at different speeds while still applying some torque to each wheel.

Differential The differential gears are what allow the wheels to turn at different speeds. Click on the picture to see an animation showing how the differential gears work. One drawback of a regular differential is that if the car gets stuck, it will send all of the power to the wheel with the least traction, causing only the one tire to spin. Because people have seen this, many people have mistakenly assumed that only one wheel drives the car. Optional link: How differentials work To right wheel Differential gears To left wheel From transmission/driveshaft

Axles Axles transfer the torque from the differential to the wheels. They can either be solid shafts, or CV axles, which have flexible joints at each end. The flexible joints, known as CV joints (constant velocity joints) are designed to compensate for the up-and-down movement of the suspension as well as the turning of the wheels during steering (front wheels only). CV axles may be found on FWD, RWD, and 4WD vehicles. Axles

Four-Wheel Drive vs. All-Wheel Drive Four Wheel Drive (4WD) and All Wheel Drive (AWD) vehicles are similar. Both use a transfer case to transfer the power to all four wheels. However, there is one very BIG difference: 4WD: Drivers of 4WD vehicles select 4WD, locking the transfer case, forcing the front and rear wheels to turn at the same speed. This works fine on snow, ice, dirt & gravel, but does not work well on pavement. When cornering, all four wheels must turn at different speeds; if the vehicle is on dry pavement and wheels are locked into 4WD some part of the drivetrain will likely break, resulting in expensive repairs. Transfer Case AWD: Vehicles with AWD transfer power to all four wheels, but also allow the front and rear wheels to turn at different speeds. Therefore, AWD vehicles can be driven on all types of road surfaces without damaging the drivetrain, and do not need to be shifted in and out of AWD. AWD vehicles are becoming more common as the technologies that make them possible are advancing rapidly.

Inspecting and Servicing your Drivetrain The next few slides will guide you through the routine checks and services that you, as an informed owner, can perform on your vehicle’s drivetrain.

Inspecting CV boots CV boots are typically located on each end of the CV axles on your car (each axle has two boots). Front-wheel-drive cars have CV axles on the front wheels. 4WD and AWD cars may have them on all four wheels and sometimes on the driveline(s). The boots keep the special grease inside the joint and keep dirt and water out. If the boots crack or tear, the CV joint will soon be destroyed because of lack of lubrication. If you have a cracked boot, the boot (and probably the joint) must be replaced.

Inspecting CV boots A good time to inspect for cracked CV boots is at every oil change. The boots that fail most often are the front outer boots near the front wheels. They are under the most strain because the front wheels are constantly moving in every direction. Besides the cracks, the tell-tale symptom of a CV joint that must be replaced is a repetitive clicking noise heard during sharp turns. A worn CV joint can sometimes be ignored for short periods of time but, if not repaired, it can eventually break off completely, leaving you stranded. Most repair shops today will recommend that you replace the entire axle when the boot or joint is damaged. This is because it is often less expensive to replace the axle than to pay the labor cost of having someone repair the joint or boot.

Checking Automatic Transmission Fluid (ATF) Because procedures vary from car to car, refer to your owner’s manual for specific instructions for checking your automatic transmission fluid. Checking the ATF level typically requires that you start the car, let it warm up, then leave it running in park or neutral while you remove the dipstick. Make sure the fluid level is between the “full” and “add” marks. Unlike engine oil dipsticks, one pint is typically required in order to bring the level from the “add” line to the “full” line. Note: Don’t confuse the engine oil dipstick with the ATF dipstick. The ATF dipstick is often red, but not always. Check in your owner’s manual to make sure. If transmission fluid is low, there may be a leak. If you have to add fluid, use a funnel and add it through the dipstick tube. There are many types of ATF, and they are not all the same! To prevent damage to your transmission, only use the exact type of fluid that is called for in your owner’s manual. Return to I-learn now and watch the video entitled, “Checking ATF.”

Checking Manual Transmission, Transfer Case, and Differential Fluid Although some manual transmissions have dipsticks for checking the oil level, most do not. When checking the fluid level on a manual transmission, transfer case, or differential, you usually have to locate the fill plug, remove it, then stick your finger in the hole to determine the level. The fluid should be right to the bottom of the fill plug hole. Transfer case Differential If the fluid level is low, check for leaks. Add fluid through the fill plug hole. Again, make sure you use only the right types of fluid for each application. Some manual transmissions use motor oil, ATF, or a special synthetic fluid with required additives. Using the wrong fluids can cause thousands of dollars in damage to your drivetrain. Return to I-learn now and watch the video entitled, “Checking Manual Transmission and Other Drivetrain Fluids.”

Changing Drivetrain Fluids: Just like your engine oil, your drivetrain fluids must be checked and serviced regularly! Check the maintenance schedule in the owner’s manual of your vehicle to find out which fluids and filters need to be changed and how often. For best results, stick as closely to this schedule as possible, because it will be much less expensive in the long run than neglecting it would be. For example, having your transmission serviced could cost between $75-$150 (and because you know when it should be done, you can schedule and budget for it), but having the transmission replaced or repaired could cost $2000-$4000.

Recognizing a clutch that is wearing out: The final point in this lesson deals with knowing how to recognize a clutch that is wearing out. If you are shopping for a used car or if you just want to get a problem taken care of before it leaves you stranded, you should know the symptoms of a clutch that is on its way out! When you let your foot off of the clutch pedal, the car usually starts to move when the pedal is about half-way up. If you start to notice that the pedal has to come up farther and farther before the car moves, you likely have a worn out clutch. If you don’t get it fixed, eventually the pedal will come all the way up and the car just won’t go! How long should a clutch last? Some people only get a few hundred miles out of a clutch while others get close to 200,000 miles. What is the difference? It all depends on the way you drive your car. Here are some tips to get the most out of your clutch: Start out from stops gently but quickly (more than 90% of clutch wear happens when beginning from a stop) Don’t rest your foot on the clutch pedal while you drive Make shifts as smooth as possible, trying to match engine speed to vehicle speed Don’t use your clutch to hold your car on a hill; that’s what the brakes are for (any time the clutch pedal is partially depressed, the clutch is wearing, otherwise no wear ever occurs)

Drivetrains Summary: In this lesson we have covered: What the drivetrain is Different layouts The purpose of the transmission Disconnecting devices (clutches & torque converters) Driveshafts and differentials Axles Transfer cases (all-wheel drive & four-wheel-drive) How to check CV boots Checking and changing drivetrain fluids Recognizing and preventing clutch wear Feel free to go back and review any part(s) of this lesson before taking the quiz.