The Automotive Drivetrain
How does my transmission, driveshaft and rear axle work?
From the Transmission, through the drive shaft and ending up in your rear axle, the torque your engine produces reaches the road through your car's drivetrain. Learn the differences between front and rear wheel drive systems, common problems and more...
We need to get this out of the way before we can define parts, and talk about their function. Ever confusing, the arrangement of modern drivetrains is sometimes misunderstood. Let's look at the common arrangements and their strengths and weaknesses...
- Rear Wheel Drive (RWD): The most common layout for older vehicles and most trucks. Power runs from the engine through a transmission and driveshaft to an axle at the rear of the vehicle.
- Front Wheel Drive (FWD): The standard layout for most economy cars and even some compact SUVs since the 1980's. Power from your engine goes through a transaxle (a combination of transmission and axle) and travels to the front wheels through an axle shaft. The main advantage of front wheel drive layouts is that the weight of the engine is over the drive wheels, offering better traction. The downside is that for performance minded owners, weight balance and torque steer is an issue. Torque steer is the natural reaction to torque going to the ground through the front wheels, and shows up as a tug on the steering wheel when you really step on the gas.
- 4-Wheel Drive (4WD): Power goes from your engine to a transmission and then gets split to both a front and rear axle through driveshafts. On trucks and some SUV's the transfer case has several "ranges" to accomodate different driving conditions. The most typical arrangement is 2WD/HI/LO where you would select 2WD for times where you do not need the traction of all 4 wheels and want better gas mileage. HI range is for times when you need all 4 wheels pulling and you will be driving highway speeds. LO is for off-road, low speed driving when you need maximum torque with lower wheel speeds.
- All Wheel Drive (AWD): Very similar to 4WD but all 4 wheels are driven by the engine "ALL" the time. May have a HI/LO option but no 2WD available. Most modern SUVs and cars are AWD. Advantages are sure-footed traction in all conditions, less moving parts than 4WD systems and more compact for installation in cars.
The Transmission: No matter the drivetrain arrangement detailed above, they all have a transmission. The transmission is a device used to multiply torque and allow for higher speeds. Imagine an engine which was directly connected to the rear axle, due to the power curve of the engine, RPM limits and the fixed ratio of the rear axle, the speed range of the car would be severely limited. It would either accelerate very well from a dead-stop but not be able to achieve higher speeds, or it would be slow to accelerate and run well at higher speeds. By varying the ratios in each gear, the modern transmission allows for a car which can accelerate well from a stop and achieve higher speeds. Back in the 50's and 60's transmissions had 3 or 4 gear at most, today new cars can have up to 8 gears and large tractor trailers can have 21 or more gears to move heavy loads. More gears allows for better engine efficiency by keeping the engine RPM's in an optimal range. In the case of diesel engines in large over the road trucks, their operating RPM range is much narrower, so more gears are needed.
Transmissions can come in 3 styles... Manual, Automatic and Constant Velocity (CV) Manual transmissions are connected to the engine with a clutch which allows the driver to engage and disengage the transmission from the engine when the car is stopped or between gear changes. The manual transmission allows for gear changes through a lever, or shifter which manually moves the gear sets into the proper position for the gear desired.
Automatic Transmissions are the most common transmissions found in cars and trucks today. They rely on a fluid filled torque converter which allows an indirect connection between the engine and transmission. When an automatic is shifted in to the Drive position (P-R-N-D-2-1) and the car is at a stop, the torque converter allows some slipping, so the engine does not stall. This also allows the shifts to be smoother than a manual transmission, but does result in a slight loss in inefficiency.
CVT or Constant Variable Transmission is a type of transmission found in a few newer cars (Audi, BMW, GM and Ford). While the CVT transmission has been around for years in snowmobiles and other recreational vehicles, it's only been recently that modern materials has allowed them to take the torque and abuse of being put in a car. The CVT transmission does not have any gears, rather two sets of plates which move and change the ratio between the engine and the wheels. Between these two sets of plates, there is a belt which transmits the torque. The CVT offers no interruptions of gear changes, smooth operation and better fuel economy. Some CVTs do offer "gears" but are really just setpoints in the range of plates which mimics a traditional gear change in a manual or automatic transmission. While a complicated subject, and simplified for this lesson, you can learn more about CVT transmissions by visiting this page.
The Drive Shaft: The driveshaft takes the power from the transmission in a RWD car and transmits it to the rear axle. Since the rear suspension moves up and down, the driveshaft must move as well. Universal joints are used at both ends to allow the driveshaft to move with the suspension and still rotate plus a sliding slip joint must also be employed to extend or contract as the driveshaft moves in and out of the transmission. The driveshaft is most commonly made from steel, but can also be made from aluminum or in exotic cases carbon fiber.
Rear Axle: The rear axle is found on RWD, 4WD and AWD cars, trucks and SUVs. The rear axle takes the rotation of the driveshaft, turns it 90 degrees and distributes it to the two rear wheels. It's job is to make sure that power goes to the rear wheels in the proper proportion. A funny thing happens when turning a corner, the inside wheel (to the turn) turns slower than the outside wheel. The rear axle's differential allows for the inner wheel to slow down while sending power to the outer wheel to complete the turn.
Transfer Case: In AWD and 4WD cars and trucks, the transfer case splits the engines torque to the front and rear axles. Depending on your car or truck, the transfer case may have several gears to allow for different driving conditions. Think of it as an extra gear that lowers or raises the operating range of the transmission. When going slowly off-road over rocks and up steep hills, speed is not an issue, so a lower range of gearing will multiply torque and allow you to get to your destination. For highway driving the higher range of your transfer case will allow for higher speeds, but less torque.
The Transaxle: Simply a combination of the transmission and axle, commonly found in FWD cars although in cases like the Corvette it can be found in RWD cars on occasion. More compact but usually a little more complicated as well, when replacement is needed the costs can be higher than if the components we separate.
Front Axle Shaft: In the case of FWD cars, or most AWD cars, the transaxle or axle will connect to the wheels with a front axle shaft. Similar to a drive shaft, but more complex since the front wheels can also turn with the steering wheel to allow your car to make a turn. The front axle shaft has two CV (constant velocity) joints which allow a great range of motion than a universal joint. They do not typically handle as much torque as a universal joint so in most heavy duty 4WD trucks, a universal joint is used with a solid live axle.
- 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.
- 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?
- Make sure when deciding to get a new or rebuilt transmission, that you have all the details of warranty and prices so you can make an educated decision. Most times you will be better off getting a rebuilt transmission, but if you plan on keeping your car for many more years, and the new transmission is not much more expensive, it could be a good choice.
- When getting an oil change or other regular service, make sure your mechanic lubricates any universal joints, inspects the CV joint boots if equipped and checks fluid level in your transmission. By keeping the items lubricated, you will get the longest possible life out of them.
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