[Ahmad Pervez]

Ahmad Pervez

May 8, 2008 at 4:51 pm #83

Got Traction?
In the sport of off roading almost inevitably the conversation turns to improving performance. When off road an important keyword is traction. Better traction generally boils down to four components, your tires. Quality tires designed for off road make a world of difference in your ability to find traction and have fun. But if a tire is not turning due to a lack of power being sent to that tire, the best tires in the world won’t help you. That’s where the right differential makes a difference. This article will attempt to explain what role your differential plays in traction and the different types of differentials available on the market and how they will affect your vehicle’s traction and handling characteristics.
The Differential
The differential in a vehicle is located in what is sometimes called the “pumpkin”, or that center section of the front or rear axle that intersects with the drive shaft. Within that center part of the axle is contained the differential.
In a vehicle, the differential usually consisting of a set of gears, that allows each of the driving wheels to rotate. The gears convert the rotating motion of the driveshaft or drive train and split power to each of the driving axle shafts of that axle. In 4 wheel drive vehicles there are two differentials, one in the rear axle and one in the front axle.
The differential has three jobs. It directs engine power to the wheels. It acts as the final gear reduction in the vehicle, slowing the rotational speed of the transmission (and transfer case of 4 wheel drive vehicles) before it hits the wheels. The differential also transmits the power to the wheels while allowing them to rotate at different speeds, thus the term “differential”.
The main purpose of the differential is to allow each half of the axle (each tire) to spin at different speeds, while supplying an equal amount of force to each wheel in that axle. The need for the wheels to rotate at different speeds is especially apparent when turning corners. When cornering the inner wheel travels a shorter distance than the outer wheel. With an open differential they both propel the vehicle forward with equal force, so long as both wheels remain in contact with the road and have traction. However if one wheel slips, for instance on ice, more torque is sent to the wheel that spins. If that slipping wheel completely looses traction, all power will be sent to that wheel and you have no forward momentum. When offroad, this is where the common open differential fails to remain effective. When offroad there are many situations where a wheel will spin free. In most stock 4×4 vehicles the common Open Differential can be found in both the front and rear axles. When a wheel in the front AND a wheel in the back are allowed to spin free due to the Open Differentials, that 4×4 is essentially a 2 wheel drive vehicle. One front wheel, and one back wheel. This is where other types of differentials will make drastic improvements to traction.
Different Types of Differentials
Differentials can be generally classified into 4 categories. Open Differentials, Limited Slip Differentials, Locking Differentials and Spools. Spools are really just the elimination of the differential, so really, there are three categories.
Beyond the open differential, the various types of “non-open” differentials will provide varying degrees of limiting of the spin or slip of an open differential. What also varies is the feel of these differentials, which translates into varying degrees of handling characteristics on road and offroad.

Open / Standard Carrier Differential

The standard differential, or what is referred to as an open carrier, is what comes with most OEM vehicles. The open carrier holds the ring gear in place and within the open carrier is generally a set of gears called spider gears. These spider gears are responsible for allowing a vehicle to negotiate a turn and allow the outside wheel to travel farther and turn faster than the inside wheel. This type of open design works great for most of vehicles on the road today. However when a vehicle with an open differential meets a lack of traction, it directs power to the wheel with the least amount of resistance. The result is the wheel on the traction-less surface spins free, while the opposite wheel of that axle on the better traction surface provides little or no power.

Limited Slip Differentials, Posi-Traction (Posi, Posis)
Limited Slip and positraction (posi) differentials are designed to “limit” the tendency of open differential to send power to a wheel that lacks traction and redirect the power to a degree to the other wheel of the axle. The Limited Slip and Positraction differential will send power to both wheels equally when traveling straight, however when one wheel spins due to a lack of traction, the differential will automatically provide torque to the other wheel with traction. Limited Slip and Positraction (posi) differentials limit the loss of torque to a slipping wheel through various mechanisms such as clutches, gears cones, and other methods dependant on the unit. The limited slip and positraction will not provide 100% lock up of the differential in extreme situations such as when a wheel completely looses traction. Limited Slip and Positraction (posi) differentials are recommended for daily driven vehicles and are used in many applications where traction is sometimes needed as in emergency vehicles. They are also ideal for front axles of 4×4 vehicles that are not equipped with front hubs that can be disengaged. The term “positraction” (“posi” for short) was used by General Motors years ago for their limited slip differential and has been used to refer to limited slips since.
CLUTCH-TYPE LIMITED SLIP
GEAR-DRIVEN LIMITED SLIP
Lockers, Locking Differentials
A locking differential or “Locker” uses a mechanism that allows left and right wheels to “lock” relative to each other and turn at the same speed regardless of which axle has traction and regardless of how little traction a slipping wheel has. In this state, the axle acts more as a “Spool”. This means traction can be sent to a wheel that may be planted firmly on the ground while the other wheel of the axle is completely off the ground. In this situation an open differential will spin the free (lifted) wheel sending absolutely no torque to the wheel in the ground. A limited slip in this situation will send some torque to the wheel on the ground but possibly no enough to provide any forward momentum.
Lockers use various mechanisms to provide lock-up and can be divided into two categories, Automatic Lockers and On-Command, or selectable Lockers.

Automatic locking differentials are designed to lock both wheels of an axle automatically when torque is applied so that both wheels are providing power. When torque is not being applied such as when the clutch is press down, the differential is allowed to unlock, permitting a variance in wheel speed while negotiating turns. Automatic lockers tend to create odd handling characteristics on the street as they lock and unlock and take some getting used to.
On-Command Lockers (Selectable, Manually Operated):
On-command lockers are the best of both worlds providing the benefits of a locking differential and an open differential. An on-command locker uses a switch activated electric motor or vacuum diaphragm or a cable / lever to engage the locker. When an on-command locker is not engaged, it acts like a standard open differential with none of the quirky handling characteristics of an automatic locker. When the on-command locker is engaged, the differential locks the axle shafts together where it is now more like a spool with no differential of speed between the wheels of that axle. Some OEM on-command locker designs are available on the market including 1998 and newer Toyota Hi-lux and Land Cruiser and the Jeep TJ and JK Rubicons.
Spools, Mini Spools
Spools are actually the lack of a differential. Spools are a 100% lock-up between both wheels of an axle all the time. Spools are generally used for racing and serious offroad use where little or no street driving is seen by the vehicle and a stronger, lighter rear end is needed.

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