Know Your Differentials
| 31 October 2004
The topic this month is an overview of differentials in rear wheel drive cars. As you know, the name “differential” comes from the need for a vehicle’s drive wheels to turn at different speeds when the vehicle is rounding a turn. For example when your Mustang makes a left hand turn, the right-side wheels turn faster than the left-side wheels and vice versa. To accomplish that on the driven axle without causing a mechanical bind, the rear axle drive gears, known as the differential assembly, must provide a means to compensate. There are several design variations to handle the task; each design is related to a different use profile.
The open differential, although seldom used today, was the standard design up through the early 80s. An open differential applies the same amount of torque to both rear wheels. This sounds like exactly what you want but there’s a kicker. And that would be that the open differential works to the lowest common denominator for traction. If one rear tire has less traction than the other rear tire, the wheel with the least traction sets the standard for the amount of torque going to both wheels. In a worst case scenario, the tire on one wheel would have no traction. It would spin and it wouldn’t move the vehicle. It would define the lowest common denominator (zero, in this case) so no torque would be delivered to the other rear wheel even though its tire might have good traction. The open design is good for general street use but it’s not well suited to performance applications and it doesn’t do well when slippery conditions exist.
Limited slip differentials are designed to overcome part of the lowest common denominator problem. Also known as posi-traction, traction lock, or torque sensing differentials, they’ve been around since the early 50s but they were generally only offered as an option prior to the mid 80s. Since the mid 80s, limited slip has been standard on Mustangs.
The limited slip unit is similar to the open design except that it has a spring loaded clutch pack that locks together the drive mechanism for both drive wheels when the vehicle is moving straight ahead. When a turn is made and differential action is required, the friction of the clutches is overpowered and they are forced to slip. The clutches stop slipping when the vehicle straightens out again.
Using the example of one rear tire having poor traction and the other having good traction, the limited slip clutch pack will try to maintain the lock between both drive wheels thus allowing the tire with good traction to propel the vehicle. This works until the clutches begin to slip. When that happens, the limited slip begins to act just like an open unit. But assuming that vehicle movement is not severely restrained, the clutch pack in a limited slip unit should have enough friction to allow the tire with traction to move the car.
Limited slip is a good device for everyday driving where slippery conditions may be encountered. The device is smooth and quiet and, with proper maintenance, durable enough for occasional use on the race track or drag strip.
Moving on from limited slip differentials, the locking differential is a popular device for racing applications or extreme street applications. The “locker” relies on an architecture that’s completely different from either the open or limited slip design. When driving straight ahead, toothed gears interlock to form a mechanical coupling between the axle shafts going to each drive wheels. Both drive wheels receive equal amounts of torque and short of physically breaking the assembly, there is no way to overpower the coupling. So in straight ahead driving with both wheels always receiving the same torque, it’s not possible to spin one wheel exclusive of the other. If one spins, both will spin in unison.
When a vehicle with a locker goes around a turn, a cam and ramp assembly unlocks the drive to the outer wheel, thus allowing differential action. The inner wheel remains locked and provides the driving force as long as the vehicle is turning. When the vehicle straightens out again, both sides return to a locked configuration.
Last on this list of differentials is the spool. A spool is not really a differential although it resides in the same location under the vehicle and it serves as the final drive just like a real differential. Spools have a simple design and serve to lock both axles together; there is no provision for differential action. Their function is to provide drive for vehicles that almost always travel in a straight line, like a drag car. They are not at all practical on the street and when used in that manner, they can result in axle breakage plus dangerous vehicle behavior when turning.
One feature common to all differentials is the ability to multiply torque through the internal gearing. This is what we variously call the gear ratio, rear end ratio, or just plain gear. The ratio is the number of times the drive shaft turns for each single revolution of the rear wheels. Manufacturers of typical people-mover automobiles choose the ratios carefully to offer the best compromise between performance and fuel economy. On the other hand, it’s not uncommon for performance vehicles to run a gear ratio with a higher numerical value to gain more mechanical advantage for better acceleration, albeit at the expense of economy and top speed. Race cars use a gear ratio chosen such that their engines run at peak performance under the most extreme conditions. The extreme condition, for example, could be found in the speed trap on a drag strip or at the end of a given straight away on a race track.
Ratios are expressed as a numerical value related (always) to the number one, for example 3.27 to 1 (3.27:1). The number one always stands for a single revolution of the tire and 3.27 in this example stands for the number of revolutions made by the driveshaft for each revolution of the tire. A number higher than 3.27 means the drive shaft will make a greater number of revolutions for each single tire revolution while a lower number means there will be fewer drive shaft revolutions. Higher numbers provide more mechanical advantage and lower numbers provide less. Although it may be confusing, the performance and racing communities say they’re using a lower gear when the number is higher and a higher gear when the number is lower. In very much over simplified terms with all else being equal, the lower gear provides better acceleration and the higher gear gives better economy and greater top speed.