The Eaton Detroit Locker

Wayne Scraba

3 Oct 2017

Click Here to Begin Slideshow For plenty of old drag racers (yours truly included), the muscle car era was filled with interesting and often memorable hardware. The Detroit Locker, typically found under big power Fords with a Drag Pack, was one example. And as it turns out, the Detroit Locker was likely the toughest, meanest, gnarliest differential that ever turned a tire on the street. Fair enough, but that was a long time ago, and what does this have to do with today’s high performance cars? As it turns out, the Detroit Locker is still the most durable and dependable locking differential available today. And it is certainly not limited to 9-inch Ford applications either, but we’ll get to that in a bit. Way back when, the Locker had some decidedly naughty manners to go along with it too (of course, that didn’t hurt the bad-boy reputation one bit): The Detroit Locker functions as an automatic locking differential designed to lock both wheels of the axle together automatically, with power input, when forward or reverse torque is applied. This means both driving wheels provide 100% of the power to the ground. When “locked,” it’s like a spool that solidly connects both axles (and, consequently, wheels) together. When torque isn’t applied, the Detroit Locker “unlocks.” The locking and unlocking wasn’t exactly invisible (typically, you’d hear and feel clunking and banging). Sure it was a bit naughty, but the Locker also had a reputation for brute strength. We’ll get to the way a modern Locker works down the page. Today, Eaton owns Detroit Locker, and most of the bad manners have been banished. The latest versions, dubbed “soft lockers,” still let you know when they lock and unlock, but not quite with the same ferocity as the older models. Comparatively speaking, they’re actually rather refined, but we’ll get to that in a bit. Originally born as a “Thornton Drive,” the Locker was first manufactured by Detroit Automotive Product Corporation. During the forties, the fifties and the sixties, the Thorton Drive was made available as original equipment in any number of light and medium duty trucks. By the sixties, the Thorton Drive came to be known as the Detroit Locker. It was then that Ford Motor Company fit the Locker into 9-inch differentials – particularly those in Drag Pack cars. Over time, Detroit Automotive Product Corp transformed into Tractec (the name changed in 1979). In 2005, Eaton Corporation purchased Tractec Holdings, Inc. With the purchase came the Detroit Locker. Today, Eaton Corporation offers Detroit Lockers for a wide array of rear end applications, including Dana 60, Ford 9-inch, Dana 44, Chrysler 8-3/4, GM 10 (various configurations) and 12-bolts (standard and aftermarket 33 or 35-spline), along with a few other applications. How does the Detroit Locker operate? First, consider the major internal pieces, which are essentially the side gears, outer springs, driven-clutch assemblies, spider assembly and center cam. The center cam is held in the spider assembly with a snap ring, but is free to rotate. When a Locker-equipped car is under power and moving forward, both axles (and thus both wheels) are locked in position and turn at the same speed - rather important if you’re interested in “straight line acceleration contests.” The key to the unit is the series of teeth found on the spider assembly along with the driven clutches on either side of the spider. The driven clutches ride between the side gears, which are splined to fit the axles and turn at the same speed as the axles. The Locker spider assembly takes the place of spider gears found in a conventional open differential. So it’s locked. But what happens if you go around a corner? That’s something a fully locked differential (or spool) isn’t too happy with, of course. The crux of the Locker operation is this: While in a turn, the outside wheel must travel a distance greater than must the inside wheel. This causes the wheels to turn at different speeds. The teeth on the spider and driven-clutch assemblies are cut at a negative angle. Here the center cam (inside the spider gear) locks into position and acts as a ramp to disengage the right driven-clutch teeth from the spider gear. This allows the right wheel to rotate faster around the corner. Once the driven-clutch rotation equalizes and both wheels are traveling at the same speed, the exterior springs force the teeth to mesh (and of course, “lock”). That’s why a Detroit Locker feels and sounds like it’s locking and unlocking - because it is. Now, since Eaton took charge of the Locker, they’ve managed to remove some of the harshness from the locking and unlocking cycle. But hey, these are high performance pieces. You can still expect some of the lock/unlock sensation when you’re driving a Locker-equipped car. When all is said and done, the Detroit Locker was, and still is, the toughest limited slip automotive differential ever built. If you need brute strength from the differential in your extreme power street-strip car, don’t look any further than the Detroit Locker. It’s one bad to the bone piece. For a closer look at a Locker, check out the accompanying photos:

The Eaton Detroit Locker — Click Here to Begin Slideshow For plenty of old drag racers (yours truly included), the muscle car era was filled with interesting and often memorable hardware. The Detroit Locker, typically found under big power Fords with a Drag Pack, was one example. And as it turns out, the Detroit Locker was likely the toughest, meanest, gnarliest differential that ever turned a tire on the street. Fair enough, but that was a long time ago, and what does this have to do with today’s high performance cars? As it turns out, the Detroit Locker is still the most durable and dependable locking differential available today. And it is certainly not limited to 9-inch Ford applications either, but we’ll get to that in a bit. Way back when, the Locker had some decidedly naughty manners to go along with it too (of course, that didn’t hurt the bad-boy reputation one bit): The Detroit Locker functions as an automatic locking differential designed to lock both wheels of the axle together automatically, with power input, when forward or reverse torque is applied. This means both driving wheels provide 100% of the power to the ground. When “locked,” it’s like a spool that solidly connects both axles (and, consequently, wheels) together. When torque isn’t applied, the Detroit Locker “unlocks.” The locking and unlocking wasn’t exactly invisible (typically, you’d hear and feel clunking and banging). Sure it was a bit naughty, but the Locker also had a reputation for brute strength. We’ll get to the way a modern Locker works down the page. Today, Eaton owns Detroit Locker, and most of the bad manners have been banished. The latest versions, dubbed “soft lockers,” still let you know when they lock and unlock, but not quite with the same ferocity as the older models. Comparatively speaking, they’re actually rather refined, but we’ll get to that in a bit. Originally born as a “Thornton Drive,” the Locker was first manufactured by Detroit Automotive Product Corporation. During the forties, the fifties and the sixties, the Thorton Drive was made available as original equipment in any number of light and medium duty trucks. By the sixties, the Thorton Drive came to be known as the Detroit Locker. It was then that Ford Motor Company fit the Locker into 9-inch differentials – particularly those in Drag Pack cars. Over time, Detroit Automotive Product Corp transformed into Tractec (the name changed in 1979). In 2005, Eaton Corporation purchased Tractec Holdings, Inc. With the purchase came the Detroit Locker. Today, Eaton Corporation offers Detroit Lockers for a wide array of rear end applications, including Dana 60, Ford 9-inch, Dana 44, Chrysler 8-3/4, GM 10 (various configurations) and 12-bolts (standard and aftermarket 33 or 35-spline), along with a few other applications. How does the Detroit Locker operate? First, consider the major internal pieces, which are essentially the side gears, outer springs, driven-clutch assemblies, spider assembly and center cam. The center cam is held in the spider assembly with a snap ring, but is free to rotate. When a Locker-equipped car is under power and moving forward, both axles (and thus both wheels) are locked in position and turn at the same speed - rather important if you’re interested in “straight line acceleration contests.” The key to the unit is the series of teeth found on the spider assembly along with the driven clutches on either side of the spider. The driven clutches ride between the side gears, which are splined to fit the axles and turn at the same speed as the axles. The Locker spider assembly takes the place of spider gears found in a conventional open differential. So it’s locked. But what happens if you go around a corner? That’s something a fully locked differential (or spool) isn’t too happy with, of course. The crux of the Locker operation is this: While in a turn, the outside wheel must travel a distance greater than must the inside wheel. This causes the wheels to turn at different speeds. The teeth on the spider and driven-clutch assemblies are cut at a negative angle. Here the center cam (inside the spider gear) locks into position and acts as a ramp to disengage the right driven-clutch teeth from the spider gear. This allows the right wheel to rotate faster around the corner. Once the driven-clutch rotation equalizes and both wheels are traveling at the same speed, the exterior springs force the teeth to mesh (and of course, “lock”). That’s why a Detroit Locker feels and sounds like it’s locking and unlocking - because it is. Now, since Eaton took charge of the Locker, they’ve managed to remove some of the harshness from the locking and unlocking cycle. But hey, these are high performance pieces. You can still expect some of the lock/unlock sensation when you’re driving a Locker-equipped car. When all is said and done, the Detroit Locker was, and still is, the toughest limited slip automotive differential ever built. If you need brute strength from the differential in your extreme power street-strip car, don’t look any further than the Detroit Locker. It’s one bad to the bone piece. For a closer look at a Locker, check out the accompanying photos:

The case found on Detroit Lockers is extremely beefy. No special setup is required with this limited slip. In many examples, you can use stock replacement bearings. In the unlikely event you break a Locker, it is possible to repair it. Eaton offers a full range of replacement pieces and service parts.

This is an exploded look at a Detroit Locker. From left to right, the major components that make it work include the side gears, outer springs, driven-clutch assemblies, spider assembly and center cam. The cam in the center is held in the spider assembly by way of a snap ring but is free to rotate.

From this angle you can see the carrier splines. This example is engineered for use with 35-spline axles. As far as lubricant for the Locker is concerned, you can use any quality petroleum/mineral based rear axle lube. Friction additives/modifiers are not required. Eaton does not recommend synthetic lube for the Locker.

Detroit Lockers are available for 3 and 4-series gear sets. The Locker locks up 100% when operating in a straight line or if the car is spinning, which means power is transmitted to both wheels. In a turn, the unit unlocks for the wheel turning the fastest.

As a Detroit Locker has “backlash” or “slack” between the drive and driven teeth, you will hear it “working” in everyday use going through corners and when going from drive mode to coast mode. Eaton notes that with the car on the ground and the transmission in neutral you will have 1/4 to 1/3rd of a turn of lash in the driveshaft; this is completely normal.

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