Click for Coverage
Exclusive Content
Original Shows, Motorsports and Live Events
Try it free for 14 days
Due to the EU’s Global Data Protection Regulation, our website is currently unavailable to visitors from most European countries. We apologize for this inconvenience and encourage you to visit for the latest on new cars, car reviews and news, concept cars and auto show coverage, awards and much more.MOTORTREND.COM
  • JP Magazine
  • Dirt Sports + Off-Road
  • 4-Wheel & Off-Road
  • Four Wheeler

Are Old Lockers Better Than New?

Posted in How To: Transmission Drivetrain on April 27, 2018
Share this
Photographers: Keith Gleasman

The need for traction began the first time the first engine turned the first wheel and it spun helplessly for the first time. We all know the cure…get more than one wheel providing traction. What may be a surprise is how much some of those early traction aids were like the products we all use today. Some of the old inventors and their inventions should be enshrined in the four-wheeling hall of fame because they impacted our sport and created products we still use today. Here are a few of the most pivotal innovations in the development of performance traction products that have led to the lockers you use in your rig today.

1913—M & S/Scurlock Worm-Gear Differential
In May of 1913, William F. Muehl applied for a patent on a cross-axis, spiral-gear differential, and it was granted in March of 1914. According to period reports, the first design dated back to 1911. Lewis Scurlock became involved, mostly on the business side, and they formed the M&S Gear Company of Kansas City, Missouri. The M&S diff appeared just in time for the legendary Jeffery Quad 2-ton 4x4 military truck, the first truck designed primarily as a military truck to military specifications, and also the first mass-produced American 4x4 with standard traction-enhancing differentials.

The M&S was a spiral-gear differential using four pinion gears. Both the pinion and side gears (they called them crown gears) were spiral-cut at a 45-degree angle. The side gears could drive the pinions but not vice versa, and the unit was very effective at around a 3:1 bias ratio. Because the 45-degree gear contact was small, the unit was potentially weak, but since most of the torque multiplication took place in wheel hubs of the Quad that wasn’t an issue, but might have been in other applications. Also, the Quad had four-wheel steering so all the wheels followed the same path and there was little drivetrain windup in turns, despite the full-time transfer case with no center diff.
The Quad had a small bevel gear ring-and-pinion (#7 & #20) with only a 1.8:1 ratio. The M&S differential—parts 12, 16, 17, 18, and 19—was a compact unit. The Quad’s small ring gear and differential mounted in a housing atop a cast beam axle with shafts out to spur gears that drove a gear on the inside of the brake drums. The ratio in the wheels was 3.86:1, and combined with the 1.8:1 ring-and-pinion ratio, a total reduction of 6.94:1 was achieved. The Quad was one of the first trucks to have four-wheel drum brakes as well.

The contract with Jeffery was lucrative and continued into the early 1920s, even after Jeffery was bought by Nash. Eventually, Muehl left the company and Scurlock patented a different version of the diff, offering them under the Scurlock Differential name. The most notable Scurlock application was for the Ford Model T, but they were available for Chevrolet, Dodge, and many other cars, trucks, and tractors of the era. It’s seen in various forms into the ’30s. Overall, the M&S/Scurlock is the most historically significant of the very early traction aids, of which there were many.

Pictured is a Jeffery Quad with the USMC in Haiti in 1916. The Quad debuted in 1913 and was one of the U.S. Military’s earliest 4x4s. Quads served with distinction well into the 1920s. Interestingly, Jeffery (later Nash) is part of the AMC lineage, so it’s a shirttail relative to the Jeep.

1939—Thornton Automatic Locking Differential
The long-reigning “King of Lockers” is the Detroit, which has been known by other names, including Thornton Automatic Locking Differential and No-Spin. It all started in 1931, when Ray F. Thornton (1898–1971) debuted the Thornton Tandem Drive, a tandem rear axle conversion for medium and heavy trucks. One of the key features was an inter-axle differential, or compensator, between the axles to prevent tire scrubbing in turns. The compensator unit was developed into a locking differential for use inside axles and became available as early 1939, marketed separately as the Thornton Automatic Locking Differential.

Here is the original patent filing for the Thornton “axle compensator,” dated September 21, 1938.

World War II brought the Thornton locker into battle. During the war, Thornton became Detroit Automotive Products, so you see them referred to as both “Thorntons” and “Detroits” in this period. A complete list of locker applications in that period is not available; we know kits were produced for passenger cars and light trucks. International debuted their 2 1/2-ton 6x6 trucks in 1941, produced primarily for the Marine Corps and Navy—the M-5H-6, a high-mobility “Deuce and a Half” that featured Thornton lockers in the two back axles and the super-flexy Hendrickson rear suspension. A unit was also developed for the Dodge WC series 6x6 trucks.

When this 1939 Thornton Locker brochure came out there were only two applications. In a very short period that had expanded significantly. It’s known these units were used in Ford staff cars, possibly sold to the British for use in North Africa. Every once in a while, an old N.O.S. Thornton locker turns up for some obscure ’40s application.

After the war, the Detroit Automotive Products Company continued to market the No-Spin in larger trucks. In 1969, the No-Spin went mainstream. The applications were expanded to extensively cover the light-vehicle market, and those new applications wore the “Detroit Locker” name. The rest is history, and though the corporate ownership has changed several times, the Detroit Locker is still very much a pillar and mainstay in the recreational 4x4 market.

In 1943, the Dodge WC-62 and WC-63 1 1/2-ton 6x6 trucks were commonly used as prime movers for the 57mm anti-tank gun. With the towed gun, ammunition, and guncrew, the trucks were having mobility issues cross-country, so in 1943 Thornton developed a locker kit for them and the Army bought 500 for retrofitting. Dodges so equipped saw first use in the battle for Italy, where the severe winter of 1943/44 made the campaign a logistical struggle. A few of these Thornton-equipped trucks are known to have survived to this day.
This January 1956 Dana Corporation bulletin offers the Powr-Lock for passenger cars, station wagons, and light trucks. Its message is: “Spicer Thornton Powr-Lock keeps power geared to both driving wheels.”

1955—Spicer-Thornton Limited Slip/Spicer Powr-Lok
Ray Thornton started work on a limited slip around 1952, and by 1955 he had a design that would be manufactured by Spicer as the Powr-Lok. The Powr-Lok would become the first commonly available limited slip—and the first with broad market coverage. Starting with Studebaker-Packard in 1956 (“Twin-Traction”) and Jeep in 1957, the Thornton unit would be snapped up by numerous manufacturers. Soon Chrysler (“Sure-Grip”) would be on board, then Rambler (“Twin Grip”), Pontiac (“Safe-T-Track), and Lincoln (“Directed Power Differential”). It would even be used in some GM cars under the “Positraction” name.

Many people don’t realize the first Powr-Loks used cone clutches. They had the familiar ramping device with Belleville springs to increase bias ratio according to available traction torque, but it acted on a pair of cone clutches rather than the more familiar plate clutches. By 1958, the plate clutch version had debuted and began to replace the cone clutch types in all applications. Enthusiasts in the gearhead community soon learned the Powr-Lok could be tuned mild or wild according to how the plates were stacked. The Powr-Lok was installed into OE vehicles well into the ’70s, when the milder-mannered and less expensive Trac-Lok debuted. The Powr-Lok design is still in production by various aftermarket companies.

1958—Gleasman Dual Drive/Gleason Torsen
Vernon Gleasman (1912–2004) isn’t a household name, but his contributions to four-wheeling are significant. In fact, they go beyond four-wheel drive into everyday motor transport and motorsports. In 1955, Gleasman submitted a patent for what was termed a positive-drive differential that used the worm gear principle in a way that had not yet been done. He’d been working on it since the late 1940s, and thinking about it since the 1930s.

As a young man in the logging industry, Gleasman observed the Walter and Scurlock differentials in operation. He eventually came to the conclusion they nibbled at using the worm gear principle, but didn’t fully utilize the concept. Gleasman realized the roadblocks were in the tooling, and he solved the problem by designing and building special tooling to grind the gears in a way many engineers said was impossible. In doing so, he created a differential that came close to differential perfection: a high-bias unit with easy differentiation and with considerable inherent strength. In other words—beefy, very effective, and with impeccable manners.

Photo: Keith Gleasman
In 1952, Vernon Gleasman began testing the Dual Drive differential in a WWII surplus Willys MB jeep. One of the demonstrations ran the jeep up a ramp with a normal surface on one side and rollers on the other. An open-diff jeep would be completely stopped. This image was pulled from a 16mm movie shot in 1954. The Gleasman family still has this jeep!

By the time the patent was granted in 1958, Gleasman was ready for business. He quit his job as an engineer at White Motor Company to form Dual Drive Differentials in Cleveland (soon known as Triple-D). He quickly landed a contract with Eaton Axle to supply Dual Drives for big Air Force tugs moving fully loaded B-52 bombers in all sorts of weather. From there, he fleshed out a list of popular applications and began producing them. Along the way, the Dual Drive ended up as popular with the street performance crowd as with the 4x4 crowd because it delivered traction in turns without any adverse handing effects. In fact, with a Dual Drive, lap times on a curvy course were faster than without.

Triple-D continued through the ’60s, ’70s, and into the ’80s, then getting on the ground floor with the AM General who was developing the High Mobility Multipurpose Wheeled Vehicle, the Humvee. Competing for a contract to supply traction aids, the Dual Drive significantly outperformed the others in contention. It became a vital part of the HMMWV performance equation and helped AM General win the contract, so it owns a slice of the Hummer legend.

Triple-D almost lost the deal when AM General realized they were actually a very small company. According to Keith Gleasman, son of Vernon and a part of the company at the time, Triple-D manufactured 9,000-10,000 units per year at this time, and AM General needed a guaranteed ready supply many times that. As part of the deal, AM General demanded they either gear up to produce many more units, and do it in a very short time, or sell the design to a company that could take over high-volume production. Vern Gleasman was 70 years old at the time and ultimately decided to sell. Gleason Corporation, best known for producing gear-grinding equipment, stepped up, hired some of Triple-D’s engineering staff, and hit the ground running. Gleason was also the only company at the time able to build the special tooling for cutting the gears for the Gleasman design. So it was a good match. The name Torsen was applied soon after, for torque-sensing differential, and the rest is history.

The Romeiser Differential Brake deserves an honorable mention for what may be the first plate-clutch limited slip. The German army had already tested Wilhelm Romeiser’s device when he patented it here in 1914. A clutch pack, preloaded with a coil spring, is within the differential case and will keep both axles turning until enough breakaway torque is reached to slip the clutches and allow differential action to occur. The big difference from today is that the plates are not behind each side gear but connecting the shafts together in the center. As a result, it cannot use the gear separation forces of the side and pinion gear to add braking/clamping force to the clutches. It isn’t known how far this unit went in a production environment. There is no evidence it was used in the USA, but applications in Germany are unknown.
This is a dual drive from the early ’60s. The Dual Drive used the worm gear principle, but the secret sauce was in the way the gears were cut. The worm gear principle shows that the worm gear (attached to the axle) can turn the worm wheel, but not vice versa. The problem in the past was that there was too little gear contact, so while the early units worked well as torque-biasing diffs, they were not capable of high-torque loads. Gleasman managed to develop gears that had a lot more tooth contact and by adding spur gears to the worm wheels, the unit could easily differentiate in turns while delivering torque to both axles equally. In low-traction situations, when one side spins and the other is stationary, the worm wheels on the spinning side will try to rotate the worm wheel on the stationary side and because the worm wheel can’t drive the worm gear, the unit locks up and torque is supplied to the stationary axle through the case. For one of the best illustrations on how the Torsen T-1 works, go to: Bear in mind the Torsen T-2 and T-3 were developed using a different principle, namely parallel axis gearing. Parallel axis diffs are a bit stronger than the T-1, but they do not share the virtually transparent manners and do not lock up quite as well.

Connect With Us

Newsletter Sign Up

Subscribe to the Magazine

Browse Articles By Vehicle

See Results