Most classic cars were equipped with an open rear differential from the factory. This type of rear differential is perfect on dry pavement, as it allows the axles turn at different speeds when cornering. However, an open rear differential is less than ideal when one or both tires lose traction. In these situations, the tire with no traction spins, and the tire with traction does nothing.
For those who recall the 1992 classic movie My Cousin Vinny where this situation was clearly described.
All situations except factory OEM electric lockers, a standard open carrier can be replaced with a limited slip or locker. Likewise, a locker or limited slip can be replaced with a standard open carrier. Limited slips and lockers can be interchanged. And of course, a spool can be installed in place of all of them. The main point is that the rear differential carrier unit design is independent of the housing it is being installed into (we are limiting this info to American rear wheel drive passenger cars, American trucks up to 1-1/2 ton, and Toyota Trucks). When changing rear differential carriers that support the ring gear, there are many rear-end designs that use more than one height carrier case. In these cases, the differential carrier case must match the gear ratio and ring gear thickness. If someone tells you that the housing you have will not accept the ratio you want, they are probably wrong, but that is a whole different story.
Standard/Open Carrier:
Whenever a vehicle negotiates a turn, the outside wheel must travel farther (turn faster) than the inside wheel. This is accomplished by spider gears that allow the outside wheel to turn faster (this can be either the left or the right but must be the outside wheel) than the ring gear, while the inside wheel turns slower than the ring gear. The net sum of the speed of the two wheels is twice as fast as the ring gear. That means if the outside wheel is turning 2 rpm faster than the ring gear, the inside wheel is turning 2 rpm slower than the ring gear. This differential action is very straightforward in a standard open differential and works great for more than 95% of vehicles on the road today.
When a vehicle with an open or a standard differential encounters a loose traction surface, it directs power to the wheel with the least resistance. The resulting action is the wheel on the loose surface spins, while the opposite wheel on the better traction surface stands still or barely transfers power. The reason a standard open differential works this way is because it is designed to provide equal force to both sides. If the wheel on either side loses traction, the other side suffers as well.
If, for example, the left wheel loses traction and cannot provide a resistance force back to the differential side gear, then the right-side gear cannot transfer a force to the right wheel to counteract the slipping left wheel. The main point to remember is that an open differential provides equal power to both sides, not equal speed.
An open rear differential works simply fine for 95% of vehicles on the road, since most people do not spin their tires often, if at all. For those of us who like to burn rubber or go off the dry pavement, there are now too many limited slips and locking differentials to choose from.
Posi-tractions & Limited Slips:
A Limited-Slip Differential generates more traction than the open differential. Most Limited-Slips, sometimes called posi’s or posi-traction diffs, use clutches to drive both tires, but allow the clutches to slip when the vehicle corners and the axles need to turn at different speeds.
Posi’s and limited slips transmit equal torque to both wheels when driving straight. However, when one-wheel spins due to loss of traction, a patch of ice, mud, too much throttle, etc., the unit automatically provides more torque to the wheel that has traction. Recommended for daily driving, works well in ice, rain, mud, and snow. However, in situations where absolute lockup is needed, a limited slip is not the best choice since limited slips do slip in some situations.
Posi-traction is a type of limited slip. The name “posi-traction” was used by General Motors for their limited slip differential and the name has been associated with limited slips for many years.
Lockers:
Locking differentials use various means to lock both axles, forcing both wheels to turn in unison regardless of traction conditions. Locking diffs are ideal for more extreme off-road driving and are available in automatic and selectable designs.
Automatic Locking Differentials
Automatic lockers lock and unlock automatically with no direct input from the driver. This type of locking differential forces both wheels to turn at the same speed when torque is applied. When the throttle is lifted and the vehicle is coasting, they are designed to allow each axle to turn independently for smoother cornering. Lockers (mechanical locking differentials) provide 100% transfer of power to both drive wheels. This power can be lost if both wheels spin, but a locking differential will never let one wheel spin free by itself while the other one sits still (unless the unit is broken). Automatic locking differentials are engineered to keep both wheels in a constant drive mode yet can unlock to permit a difference in wheel speed while negotiating turns.
Recommended for serious off-roaders or serious hot-rodders, they can be annoying for everyday driving. The main shortcoming of automatic lockers is the noise inherent in their design. When getting on or off the throttle, they tend to bang or clunk, or sometimes make noise while disengaging when negotiating a turn. These characteristics are due to the design and a few locker manufacturers have come up with designs that make less noise, but no one has developed a perfectly quiet automatic locker yet. One more effect of lockers that hampers every-day use is that they only drive the inside wheel on a turn, which can affect vehicle handling.
When driving the inside wheel, the vehicle tends to yaw from side to side slightly when negotiating a turn under power. When turning sharply while taking off from a stop, the inside wheel will tend to spin easily. This is especially prominent when taking off on a right turn, for two reasons: the torque on the rear-end housing due to the driveline torque un-weights the right wheel, and right-hand turns are tighter (because we drive on the right side of the road).
On-Command/Selectable Locking Differentials
Selectable lockers provide the best of both worlds. These units allow the driver to engage or disengage the differential with the touch of a button. This provides the smooth operation and cornering of an open differential on the street and the performance of a locker when needed in low traction situations. Many people want the benefits of a locking differential, but do not want the handling characteristics or noise of an automatic locker. There are a few on-command lockers available as an OEM option. These include the 1998 and newer Toyota Tacoma and Land Cruiser. These EOM designs use an electric motor or vacuum diaphragm to engage the locker. When it is not engaged, the unit acts like a standard open differential with no ill side effects aside from poor traction. When engaged, these units’ lockup 100% and act like a spool with no differential action or difference in speed between the left and right wheels allowed.
Spools & Mini Spools:
A spool is a solid carrier that does not allow axle differentiation. Both axles turn all the time. However, spools cannot disengage like a locker, so the tires will chirp when the vehicle corners. Tire wear will increase. Spools are most often used in racing applications, and occasionally in off road applications. For racing, serious off-road, or crazy hot-rodders, a spool is the ultimate traction differential. It can hardly be called a differential because it does not differentiate. Spools have several purposes. First, they lock both left and right axles solid together so that there can be no loss of power due one wheel spinning faster than the other. Second, they usually support the ring gear much better than most other differentials. Last, they can be lighter than a differential, which saves rotating mass as well as un-sprung weight that the suspension must contain.
The main shortcoming of spools is the fact that the outside wheel must turn faster when negotiating a turn if both wheels are to maintain traction. If the axles are locked together, one or both wheels must slip for the outside wheel to travel farther than the inside wheel. This can cause the rear of the vehicle to spin out, fish-tale, or can cause a lot of noise that sounds like posi-traction clutch chatter as the axles twist and release. Sometimes these effects are not all that bad, and they are less severe when 35-spline or larger axles are used. For those people on a budget, there are mini-spools available for many rear-end designs. A mini spool fits inside the standard open carrier, replacing the spider gears, and locking the left and right axle shafts together just like a spool does. A mini spool drives exactly like a full spool does, but they are not as strong as a full spool and do not add any extra support to the ring gear like a spool does.
When it comes to deciding which differential to use, there are now almost too many limited slips and locking differentials to choose from and it takes a knowledgeable salesperson to help the customer choose a design that will serve their needs.
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