HOW IT WORKS: DIFFERENTIALS

WHAT IS A DIFFERENTIAL
By: Anthony Pascarella

When a vehicle makes a turn, the wheel on the outside of the turn has a larger turning radius than the inner wheel does. The outer wheel must turn faster than inner wheel in a turn. The power from the engine must also be available to both wheels. The differential, in the drive axle, allows either wheel to turn at different speeds while still receiving power from the engine. The drive axle may be the rear, front or both axles. Differentials may be an open type or limited slip type (LSD). An open type is the most common; however, it has one disadvantage. If one tire is on a slippery surface such as ice, snow or mud, and the other tire has good traction, the power will take the path of least resistance to the tire with the least traction and that wheel will spin. The LSD will direct the power to the tire with the best traction.

RING and PINION GEARS
All differentials have a ring gear and a pinion gear. The ring and pinion gears in a rear- wheel drive differential are hypoid gears. These gears, in a rear-wheel drive axle, change the direction of rotation 90 degrees and transmit the power of the engine, through the differential gears to the wheels. Ring and pinion gears are manufactured together as a matched pair. If it is necessary to replace the ring and pinion gears, you must replace them as a pair.

GEAR RATIOS
In math, a ratio is a relationship between two numbers of the same kind, usually expressed as “A to B” or “A:B”.
In the case of ring and pinion gears, the first number indicates how many revolutions the pinion gear turns, to one turn of the ring gear.
A ratio with a larger first number means that the engine will turn faster at a given speed than a ratio with a smaller first number.
Ring and pinion gears are available in many different ratios. EX: 4.11:1,, 3.73:1
With the 4.11:1 you will get more low-end torque. With a 3.73:1, you will get more top end speed. In mathematical terms, the larger the first number is, the higher ratio is.
In automotive terms, when the ratio has a larger first number, the gears will be called a low ratio.

CALCULATING THE GEAR RATIO
There may be a tag on the differential with the ratio information on it, or the gears may have numbers stamped into them, if not, refer to one of the following procedures.

A. 1. Count the number of teeth on the pinion gear and the ring gear.
2. Divide the number of teeth on the ring gear by the number of teeth on the pinion gear.
Example: 41 teeth on the ring gear divided by 11 teeth on the pinion gear equals 3.73:1
The pinion gear turns 3.73 times for every one turn of the ring gear.

B. 1. If the car has an open differential, jack up one side of the axle to raise one rear tire off the ground and place a jack stand under that side. Leave the other tire on the ground.
2. Mark the tire and the driveshaft with a chalk mark.
3. Place the transmission in neutral.
4. Turn the tire two full revolutions and count the number of times that the driveshaft turns.

C. 1. If the car has a limited slip differential, jack up both sides of the axle, place jack stands under both sides.
2. Mark one tire and the driveshaft with a chalk mark.
3. Place the transmission in neutral.
4. Turn one wheel one full revolution and count the number of times the drive shaft rotates.
NOTE: One-quarter turn of the drive shaft equals .25, one-half turn equals .50, and three quarters of a turn equals .75.

REPLACING RING and PINION GEARS
If you replace the ring and pinion gears with ones of a different gear ratio, you must recalibrate the speedometer for it to read accurately. In a rear-wheel drive car, with a cable driven speedometer, gears at the tail-shaft of the transmission drive the cable. The tail-shaft of the transmission and the pinion shaft turn at the same speed. The driveshaft is between the tail-shaft of the transmission and the pinion shaft of the differential. The correct speedometer drive gears will run the speedometer at the proper rate. In a modern car, the speedometer is electrically driven. There are several ways to calibrate these speedometers.

LUBRICATION
The first consideration is proper weight, or viscosity. The next is the GL rating. It is best to use only what is specified for your differential. If the manual calls for a GL-5 rated 90W gear oil, then use that oil. Using the wrong oil may cause expensive damage.
Another important consideration when it comes to differential gear oil is the requirements of the limited slip, or traction-sensing differential. In a normal or "open" differential, the torque, or twist, created by the driveline is always applied to both wheels, regardless of whether one of those wheels happens to be spinning helplessly on an icy surface. A limited slip differential, or LSD, will sense this loss of energy and redirect torque to the wheel that has the most traction.
While there are various types of mechanisms used to accomplish this miracle of redirected traction, most of them require friction modifiers unique to their own design to work correctly. An LSD without these special lubricants will at best not work correctly and at worst fail outright - ending up in an expensive repair. If your vehicle has a LSD, always be sure to use gear oil that contains the correct friction modifiers for that particular LSD. The proper level for the oil is at the bottom of the fill hole when the differential is cool.

DIFFERENTIAL REBUILDING (A brief look)
1. Replace all bearings
2. Replace all oil seals
3. Adjust the preload on all bearings
4. Set the pinion gear depth
5. Adjust the backlash. This is the clearance between the teeth of the ring and pinion gears
6. Check the contact pattern

ACKERMAN ANGLE AT STEERING AXLE
It is also necessary for the front or steering wheels to be angled properly to allow the outside tire to follow a larger radius in a turn. This is achieved by the angle of the arms on the steering knuckles. This angle is called the Ackerman Angle.

The information in this article is meant for information purposes only. Please refer to specific manuals and information for the vehicle you are working on. Be sure to follow all safety procedures.

AUTHOR NOTE: Anthony Pascarella has a mechanical background that spans over 35 years in the bus and aircraft industries. He has held many managerial and teaching positions throughout his carreer.  "Tony" as he is known by his friends, has a passion for antique vehicles and is an active member in the North Shore Old Car Club, where he is a Technical Advisor.