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What is a Differential?

 

D

id you know that your wheels turn at different speeds? During a turn, the outer wheels drive farther than the inner wheels, and this is an important function of the differential.

 Differentials have three main functions. To begin with, they lower the overall drivetrain gear ratio, usually somewhere between 2-to-l and 3-to-l. This allows the transmission to operate in and around direct drive and reduces the load on the transmission. 

Secondly, many differentials rotate the power flow by 90 degrees. That's because most rear-wheel drive cars have lateral transmissions — transmis­sions that run down the length of the car. The power flow leaving the trans­mission is going the wrong way. The differential uses a ring-and-pinion gear set to turn the power flow 90 degrees so the drivetrain can drive the car properly. 

One of the most important functions of a differential is to allow the two drive wheels to turn at different speeds. That difference is necessary for turning corners. Here's why: When you're driving straight down the road, the wheels on both sides of the car are traveling the same distance, so they turn at the same speed. However, that changes when you turn a corner.

 

Think of it this way: When a car turns, the wheels form an arc, or part of a circle. Because of that, the inner wheel — the wheel on the side closer to the center of the "circle" — doesn't travel as far as the wheel on the out­side of the turn. To make up this difference, the outer wheel has to turn faster than the inner wheel. 

If the drive wheels were locked together directly, the tires would skip or slide across the roadway when you made a turn. That's where the differen­tial comes in. The differential connects the two wheels through four gears:

two side gears and two spider gears. These gears transmit power to the wheels, while allowing them to rotate at different speeds when necessary. 

While the differential is necessary, it can cause another problem. Imag­ine a situation where one drive wheel is sitting on dry roadway, while the other is on a patch of ice. Since both drive wheels can turn at different speeds, the differential will allow the wheel sitting on ice to spin, while the one on dry road just sits there. 

To combat this condition, some manufacturers offered a limited slip differential. Limited slip means there can be only so much difference between the speeds of the wheels on either side of the car. There's plenty of slip for going around a turn, but a limited slip differential won't allow one wheel to spin while the other just sits there. 

However, limited slip is an option on most cars and trucks. So, if you drive in areas where ice and snow are likely, consider this option when buy­ing a vehicle. 

Differential Service Procedures

If your car has a standard differential, the only service procedure necessary is to have the fluid level checked regularly. Since this involves raising the car and removing a side plug, it's probably best left to the repair shop. Unless it becomes contaminated, there's no reason to ever change the oil in a standard differential. As long as the level is OK, it's fine. In general, you should have the differential fluid level checked when you have the oil changed. Most shops will do that automatically, but it won't hurt to ask for it. 

If your car has front-wheel drive, the differential probably is part of the transaxle. In most cases, the transmission and differential will share a common fluid collection pan, commonly known as a sump, so if the transmis­sion fluid level is OK, the differential fluid level is, too. 

Limited-slip differentials do require service beyond simply checking the fluid level. If your car has a limited slip differential, the fluid will have to be changed on occasion. Forgetting to service the differential could cause it to skip and make loud groaning noises on turns.

 What's important about this is the fluid used. The wrong fluid in a limited-slip differential also will cause it to skip and groan on turns. So if your differential starts making noise right after being serviced, take it back to the repair shop and ask them to make sure they used the right type of fluid.

 

Also see

Your Car's Mechanical Condition

Dealing with Transmission Problems -learn about some common transmission problems for manual and automatic transmissions.

Maintaining the Suspension and Steering -relatively dependable and trouble-free; however, there are a few things you should do to keep both in good working order.

No Monkey Business Allowed Here!- Steps for Choosing a Mechanic  -some must-know questions to ask before choosing a mechanic.

Putting the Stop to Your Brakes -is your car exhibiting any of this potential warning signs?  Find out more.

 

This webpage is brought to you for general information purposes only and there are no warranties as to accuracy, completeness, or results obtained from any information posted on this or any linked website.


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