Putting power to the pavement is more than just stomping on the go-pedal, there are complex mechanical events that happen between your foot and the tires. It is a common thread in performance circles; the rear differential gets forsaken for the glory of the engine and transmission. Many street/strip cars get by on a stock posi-style diff, or God forbid, welded spider gears. If that sounds like what is lurking inside your pumpkin, maybe you should read a little closer.
Modern differentials come in many flavors—open, limited slip, locking, and spool (which is not even a differential). The open differential is not really “geared” for performance, so it’s out as is (see drop-ins below for open carrier upgrades). That leaves three. The spool locks the axles together permanently, similar to the low-buck welded-spiders, but in a safe way.
This eliminates the differentials main function of allowing one wheel to spin slower than the other when needed, specifically when turning. Spools are not the worst thing on the street, but they are pretty close. We are going to leave those out of this conversation too, sticking to true performance differentials— lockers and limited-slip (LSD).
Within these two styles there are quite a few different options. Each has their own pros and cons. Which one is best for your car? We talked to Jeff Saxton at Eaton Performance Products who helped us compile the details and put them right here for you to decide.
Eaton employs nearly 10,000 people in its Southfield, Michigan headquarters, its international Engineering, Research and Development centers in Marshall, Michigan; Turin, Italy; Baden Baden, Germany; Pune, India; and Shanghai, China and throughout its 30 manufacturing locations worldwide. Eaton also supplies nearly every automotive, vehicle and engine manufacturer in the world with its high-quality parts and services.
Besides the tires, the rear differential is the last line of defense against slow 60-foot times and lost traction, but like transmissions, differentials are steeped in mystery and voodoo magic, or are they?
Limited Slip Giving You The Slip?
Best known by the GM name “posi-traction,” the limited slip (LSD) has been used by just about every manufacturer at some point. This style of differential is designed to transfer power from “the wheel that slips to the wheel that grips,” or so the commercials used to say.
This is an upgrade from the open diff, which would send the power to the slipping wheel, not exactly the best thing for a tidy launch off the line. To do this, the LSD uses either gears or friction clutches to allow the axles to spin at different rates.
The LSD differential operates on three input torque states:
Under no load conditions, the LSD operates in static couple, similar to an open carrier.
What classifies the LSD is its overrun operation. Overrun is a sudden release of torque, such as hard on the throttle then jumping off. How the LSD reacts to overrun situations determines whether the LSD is a 1, 1.5, or 2 way; which is an important spec for a street car.
The shape of the ramp (or geometry of the tooth form) on the spider gears determines the LSD type. If the ramps are symmetrical, then the diff is a 2-way. If they look like saw-teeth (one vertical, one sloped) then the LSD is a 1-way. If both sides have sloped ramps but are asymmetrical, it is a 1.5-way differential.
A 1-way LSD is the quietest, with smooth, noise-free operation. The 2-way tends to be a little noisy, the cut of the gears and increased clutch pressure tend to “chirp” around corners. This is not tire chirp, rather the clutches slipping.
Saxton explained, “Gear tooth geometry directly impacts clutch system loading/compression (gear separation forces) but whether the clutches make noise or not is the result of clutch system design itself, lubricant used, etc. A properly designed clutch system doesn’t have to make noise – regardless of gear tooth type/geometry.”
Where this is important is in the driving characteristics. If the LSD releases the coupling as soon as the throttle lifts, then it is a 1-way. This is the safest type of LSD, as it allows the rear tires to spin as needed.
If the differential increases the coupling regardless of forward or reverse torque upon throttle release, it is a 2-way diff.
This does two things:
It allows the driver to be in control of the wheel spin, the differential is engaged, and in terms of drifting
It keeps the wheels spinning throughout the drift
An inexperienced driver can find themselves in an unwanted spin with a 2-way LSD. There is a middle ground, the 1.5, which has less deceleration lock-up over the 2-way, but retains the coupling, unlike the 1-way.
There are two main types of LSD:
We spoke with Eaton’s test engineer, Jeff Saxton to get some details; he explained, “Both designs have been around for over 50 years and have evolved in terms of advanced materials for added strength & durability but the principles of operation remain essentially unchanged.”
Clutch-type differentials are what you commonly find in OEM applications. According to Saxton, “Clutch compression is achieved by the outward (axial) force of the side gears which then compress the clutches.” As the torque increases, the more compression is put on the clutches, coupling the axles, reducing slip.
“The Eaton Posi (a clutch plate type) is the original positraction differential providing efficient transfer of power to the rear wheels equally so that acceleration is maximized yet controlled,” Saxton told us.
This is how they work—as the spider gears climb each other as the side gears react with the spider gears, pressure is put on the clutches, which couple the axles together. Notice the symmetrical angles on the spider gears, this is a 2-way differential.
Clutch-type LSDs are popular. They are inexpensive and smooth on the street. With the different configurations (1, 1.5 and 2-way), there are enough options to allow you to get the right style for your application. Where the clutch-type LSD misses is in longevity. Since they require clutches, as time progresses, the clutch material wears away, requiring a rebuild. If left alone, the clutches with eventually wear away and there will be no coupling of the axles, although Saxton refutes, “This is not 100% the case and is entirely dependent on usage. Outside of heavy competition, an Eaton Posi commonly lasts as long as the owner wishes to keep it.”
Each manufacturer has its own specific break-in procedures that must be followed in order to obtain optimum coupling.
Replacing the clutches is not a big deal, but the break-in is. Each manufacturer has its own specific break-in procedures that must be followed in order to obtain optimum coupling, although all of Eaton’s clutches are pre-broken-in, not requiring a “break-in period.” While not difficult, the break-in is crucial. Another caveat to the clutch LSD is the oil; all clutch LSDs require LSD gear oil.
Gear-driven LSDs are becoming more and more popular. Eaton’s TrueTrac was the first LSD to use helical gears over clutches, eliminating the consumable clutches and further reducing the noise of chattering clutches. Instead of controlling slip like a clutch/cone system, the gears are actually torque multipliers. As one wheel loses traction, the gears spin, transferring power to the wheel that is gripping.
Once a tire has completely lost traction, the LSD operates as if it is an open differential; the slipping tire receives no torque. In this situation, applying light pressure to the brakes (or e-brake) will put enough load on the axle to force torque application. This can be considered a drawback of the gear-drive, but not all gear LSDs have this issue. Some feature a bias plate that maintains a supply of torque to both wheels.
A geared LSD is well suited for drag racing, drifting and road racing. In street cars, ice can be a problem for the geared LSD due to the free-wheeling in no-traction situations. Saxton tells us “The Eaton Truetrac (a helical gear type) provides smooth power delivery in poor traction situations making it an excellent choice for hauling heavy payloads and trailering.
It is also an excellent front axle application choice for a vehicle that is both a daily driver and a regular off-roader.
For ultra-quiet and smooth operation, a gear-driven LSD is the best choice. Eaton’s True-Trac uses helical gears instead of clutches, so there absolutely no chatter. Geared LSDs actually work by multiplying torque, much like a torque convertor, rather than actually allow slippage.
Don’t Get Locked Out By Your Locking Differential
A locker does pretty much what it says, normally locking both axles together when torque is applied, ensuring both wheels spin regardless of traction. Where a limited slip diff changes how much torque is applied to each axle depending on traction, the locker sends the same power to both wheels all the time, under power. While that sounds like a spool, and it is very similar, the locker also has the ability to unlock.
When power output increases beyond the ability of an LSD, a full lock is needed. Strange Engineering offers their own version of the full-locker for racers using a Ford 9-inch rear. Built without using clutches, the new S-Trac is purely mechanical, designed to maximize traction and minimize wear and slippage.
There are two ways of achieving this- automatic and selectable. An automatic locker will open when the there is little to no torque applied to the differential. This happens in situations like cruising or low-speed cornering, where there is no power being applied.
As soon as you hit the gas though, for example, goosing the throttle out of a corner, the diff will lock. This can cause some unsettling handling issues, such as a sideways jerk as the locker engages. Lockers are streetable, they just take some getting used to.
“The Detroit Locker is the original differential of its type, dating all the way back to the 1940s. Over the decades it has been applied in heavy trucks, military vehicles, performance cars, street rods, NASCAR and all manner of off-road vehicles.
“This arguably makes it one of the most versatile traction modifying differentials ever invented. The primary reason for this popularity is its high torque carrying capability and resultant strength and durability.
“It has the resources and the engineering and historical expertise of the world’s largest manufacturer of high performance differentials behind it. No one else can make that claim,” Saxton explained.
Lockers are known for being noisy and harsh on the street, which is why the LSD design is so popular for street rodders—they are quiet and easy. A locker is more efficient and simply will not slip off the line like an LSD can. The most famous of the lockers is the Detroit Locker from Eaton.
First introduced in 1941 as a OEM product and later for the aftermarket as the Detroit Locker, it is known for its strength and reliability. It is also the most versatile, having the most applications. For those that prefer a quieter differential, Eaton has the Soft Locker, which uses the same principle with different springs to reduce the clunking and jerking associated with engaging and disengaging.
For extreme budgets or for the builder who doesn’t want to mess with (or have the experience) the critical gear settings, the drop-in locker is a viable option. While not as strong as a full carrier, units such as this Lock-Right from Powertrax (Richmond Gear) work very well. They offer positive locking in the straight lines, but allow the inside wheel to spin when needed around a corner.
The harsh realities of a locker require the driver to either adapt their driving style or simply live with erratic handling. Manual transmission cars show the most abrupt changes, as torque is applied, released and applied again at every shift, so there will be more clunking and banging. As you accelerate out of a corner, the awkward handling rears its head.
The locker will free-wheel like an open diff until you hit the gas, applying torque, and the locker couples. This can cause oversteer or understeer, depending on the suspension, speed, and the road itself. These characteristics are even more pronounced in wet and snow/ice conditions. The vehicle itself also makes a difference; short-wheelbase and lightweight vehicles are more susceptible.
The best aspect of a locker is that you get true, fully-locked axles for maximum torque application just like a spool in the straight lines and free-spin action for the corners. In terms of strength and durability, a full replacement locker like the Detroit Locker is the best. These units replace the entire differential carrier, which means that you get beefy parts that are made to take the abuse of a race car.
Drop-in lockers, sometimes referred to as “lunchbox lockers”, are a little different. They perform the same task as a replacement carrier locker, but allow the user to simply replace the stock spider gears or limited slip clutches with a locker section. This means that you don’t have to mess with setting pinion depth or backlash on the gears, which can be intimidating for a novice.
This biggest drawback of running an automatic locker on the street is the noise and poor handling characteristics. To get around that, you could opt for an on-command locker, like the ELocker from Eaton. This style uses a simple electromagnetic field to lock the axles together when activated, but operates just like an open carrier when it’s is off, truly the best of both worlds.
Drop-ins are user friendly; they do not require any special set up and are compatible with C-clip axles. Installing a drop-in locker can as little as an hour or so. They are also cost effective. Adding this style of locker does not mean you need to invest in tools to set up a gears, it is a cheap way add traction to your stock rear end.
We asked Saxton about choosing a drop-in over a full replacement carrier and he told us, “This always comes down to strength considerations. If the vehicle modifications include increased engine output, higher than factory ratios, large wheels and tires or other added mass (body armor), a full locker will in almost every situation offer a more durable set-up than a drop in. Of course high strength axle shafts must also be considered with these types of modifications.”
Drop-ins function in a similar manner to a replacement locker, the locking action is based on torque application, so the handling characteristics are the same. When Richmond Gear introduced the Powertrax Lock-Right drop-in, they were quite noisy due to the design of the ratcheting action. Over the years, they have improved on their design, reducing the noise and chatter.
The biggest drawback of the drop-in is strength. While the locker itself is very durable, the overall strength can be hindered by the OEM carrier they are installed in. This makes them less reliable, but only because the stock carrier may fail, not because of the locker.
A properly designed clutch system doesn’t have to make noise.
As the car becomes more powerful and modifications increase, the drop-in will become less reliable than a full-replacement locker. For an otherwise stock or mildly modified vehicle, a drop-in is a good (read cheap) place to start.
On-Command lockers offer the best of both a spool and an open carrier. An open carrier is the most street friendly design, there is no noise, chatter or clunking involved, but the classic “peg-leg” carrier is not good for drag racing. The locker’s sketchy handling can make them annoying on the street, but perfect for the strip.
The Detroit Locker is the original automatic locking differential. Over the years, more racers have used and abused the Detroit Locker than any other locking differential, and that is saying something.
To get both, you can go with on-command. This gives you an open carrier for street driven and at the push of a button or pull of a lever; you get a fully locked carrier just like a spool. With more function comes more labor on the install side and more cost. On-command lockers are typically air, cable, or electric solenoid operated, which means you need to run wires and hoses to the rear end.
The cost goes up with these units as there are more moving parts and functions that need to happen to make them work. On-command lockers are very popular with the 4×4 crowd, but they provide the best in terms of traction and drivability for a street-driven drag car.
Eaton has a new electronic locker, the ELocker. Originally developed for the H1 Hummer, this one is geared for the off-road but it may be just at home on the track. Where other on-command lockers require compressors and levers and such, the ELocker uses 12v power to activate the electro-magnetic field that forces the collars to move, locking the axles together.
So which is better, the LSD or the locker? In terms of straight line traction, the locker provides better coupling hands down, but going around corners will be noisy and unsettling. The LSD design is considerably quieter in the corners, and is easier to handle on the street. For a moderately powered street/strip car, the LSD is likely to be the best bet, but as power increases, so does the need for traction. So instead of firing up the ol’ MIG and welding those spiders, try swapping out that peg-leg for a real performance differential. Which one is up to you.