The strange geometry of
Gleason's Impossible Differential

Constantly responding to changing road conditions, this unique differential automatically varies the distribution of torque to a vehicle's rear wheels. Result: two-wheel-drive traction approaching that of a four-wheel drive. It could make its Detroit debut in 1985.

As I approached the stop sign, I noticed an icy patch on the street in front of the left side of my International Scout. Several cars had already spun their wheels on the ice, and it was polished to a slippery, glassy smoothness. I had recently installed one of the new Torsen (torque-sensing) differentials in my four-wheel-drive Scout, and this was a perfect opportunity to test the maker's claims. In two-wheel drive, I gently accelerated and began to turn. The left rear wheel was now on the ice, but it didn't slip. I pushed harder on the accelerator. The rear tires grabbed, and I maintained a perfect are through the intersection as if there had been no ice at all.

Throughout the winters of 1981 and 1982, 1 never had to use four-wheel drive. The amazing improvement in my Scout's performance was due to the Gleason Torsen differential. This uniquely designed differential applies torque to both rear wheels and distributes torque as required. It will deliver as much as 90 percent of the torque to one wheel, with 10 percent going to the other.

The new differential has been proving itself in vehicles ranging from Mario Andretti's race car to the U.S. Army's Jeep replacement. And Detroit is definitely interested. The way engine power is conveyed to the wheels by the drive train affects the way a vehicle gets traction on a road surface. Most of us know about the first two components of the drive train: the engine and transmission. The third part, the differential is not nearly as familiar. That part has traditionally been a major source of mysterious traction problems for many cars.

To understand the characteristics of the Gleason differential, it is necessary first to review the "differential problem," as engineers describe it. The problem stems from the basic nature of power-driven wheels on axles. The best way to propel a vehicle is with power to both wheels. But in many situations, the wheels are not turning at the same speed. For example, when a car makes a left turn, the inside (left) wheel makes a smaller are than the right wheel. The right wheel must travel farther, and the differential must "differentiate," or compensate, between these two arcs. If both wheels were solidly driven by the drive shaft (as on some dirt-track racing cars) and the vehicle took a sharp turn, the tires would skid, squeal, wear unevenly, and possibly throw the vehicle off a curve at high speed.

Until now, there have been three basic ways to handle this problem. The first is the conventional differential. In normal operation (driving in a straight line), it distributes torque equally to both wheels. But because of its internal gearing, it has a built-in preference for the wheel with less rolling resistance (traction). This allows the wheels to make turns, but it has traction drawbacks. A conventional differential can't tell whether you're losing traction on a slippery surface or turning.

The "limited-slip" differential tries to overcome the conventional differential's limitations. It's been offered as an option on many makes of cars for over two decades. Through the compression of clutch packs or the binding of internal gears, limited-slips put about twice as much torque (engine power) on the wheel that isn't spinning. It's an improvement in so far as it transfers torque to the wheel with the greater amount of traction.

But limited-slip types also have their problems. The clutch-pack limited-slip has a nasty habit of locking up at the wrong time: When you're cornering on a road in the rain or passing over slick surfaces, for example. The reason: Sometimes these conditions don't provide enough traction to compress the clutches. Limited-slips have other drawbacks: Clutches wear out, special lubricants are needed, fuel economy is reduced, tires wear faster, and the differentials themselves are noisy. Both the clutch-type and gear-type limited-slips have a time lag as the clutch packs compress or the gears bind. In addition, they have a safe maximum torque bias of only 2.5:1. That means they will provide one wheel with no more than two-and-a-half times as much torque as the other. They are also considered to be dangerous in front-drive passenger cars.

The "locker" represents the third type of differential. It responds to wheel slip by locking in both wheels simultaneously. If one of the wheels can get a grip, this differential will pull you through. But traction is lost during turns, when the outer wheel must disengage to travel through a wider arc. It also can lock up at the wrong time, and it's not recommended for highway use. Its torque bias of 100:0 is useful only in a limited number of situations. This same characteristic can cause a safety hazard if one of the axle shafts breaks.

The "differential problem" seemed an unsolvable one, and by the mid-1960s, the Society of Automotive Engineers speculated that a differential that was capable of overcoming these drawbacks would require a computer that would fit within the confines of the differential. This would create a "fourth-generatioe' differential that could constantly monitor and distribute torque to both of the wheels in all traction conditions.

But there proved to be an easier way. The Gleason Torsen is a new type of differential, using a gear geometry never seen before in mechanical engineering. Gene Stritzel, engineering manager of Gleason Works'Power Systems Division, calls it a "mechanical computer." Just as an automatic transmission constantly adjusts engine rpm to vehicle speed, the Torsen distributes torque according to the demands of slippery surfaces, uneven terrain, and turns.

The Torsen is the brainchild of Vernon Gleasman from Cleveland, Ohio, an inventor and mechanical engineer who holds more than 100 patents. Gleasman realized that in all previous attempts, the standard bevel-gear differentials. limited-slips, and lockers, the gear complex was designed to lock onto one side while sacrificing the other. The standard differential delivers all the input power to the terrain side, then loses the engine side out to the spinning wheel when one wheel encounters a slippery surface. Limited-slips offer additional traction, but they have to "clutch out" the terrain side to lock in the engine. Lockers, the third type, completely sacrifice terrain changes to maintain engine power.

"Putting the forces in the opposite side of the gear complex led me in the right direction, "Gleasman says. In other words, he intended to lock onto the engine side to maintain engine torque at all times. A gear complex that would be flexible on the terrain side would allow the differential to maintain power on both sides of an axle while turning, without allowing one wheel to spin.

Doing the impossible
What was needed was a gear arrangement that would work in a one-way fashion. Years of experience pointed Gleasman toward the worm gear and wheel. Engineers have long known that the teeth of a worm gear and wheel can be cut at a helix angle such that the worm gear can turn the worm wheel, but the worm wheel cannot turn the worm gear. This is unlike more conventional circular gears that can rotate each other. It's why worm gears and wheels are used in winches, in which the gear turns the wheel of the drum, but the drum's worm wheel can't turn the gear and the cables on cranes and hoists can't unwind.

But to devise a worm-gear, worm-wheel arrangement that would fit into an automotive axle, Gleasman had to overcome many technical and engineering challenges:

  • Making the worm wheel smaller than the worm gear.
  • Understanding how these gears behave in a static condition. (Engineering handbooks contain formulas for worm gears and wheels only in dynamic interaction.)
  • Designing the gears with as few as six teeth without undercut. (Engineering handbooks claim that it can't be done.)
  • Working out a new stress analysis to understand the torque loads sustained by the gears.
  • Designing a new hob (gear-cutting too] for worm gears and wheels) to cut the new gears at extreme angles.
  • Designing new gear-cutting machinery to make the gears.
  • Virtually every aspect of Gleasman's new gear technology involved ideas that did not exist in traditional engineering handbooks. For example, engineering handbooks caution that with less than 14 teeth on a gear, severe undercutting results, undermining the teeth and weakening them. But Gleasman says, "I can cut gears with six teeth and have a wider tooth section at the bottom than normal gears have at 30 teeth."

    No gear-cutting machinery existed that could cut the gears he designed, so he bought cutting machines and re-designed them and the hob to cut gears to his new requirements. "I use six teeth on the worm wheel of my differential, and because of its function (to stop wheel spin) it has to withstand almost twice the load of a conventional differential in the same space. The entire geometry of all the gears is changed so that no undercutting exists. All the gear formulas say you can't do it, All the handbooks say you can't do it."

    Gleasman responded to those who shook their heads in disbelief, "You just can't check my work with the traditional formulas because I couldn't use that information to design the gears in the first place.'

    Gleasman goes to Gleason
    The differential's first patent was granted in 1958, and Gleasman started to manufacture the differential as a sideline, using the same machinery used to cut the prototypes.

    Originally dubbed the Dual Drive, the unique differential was later manufactured by Triple-D Inc. of Cleveland and was sold as an aftermarket accessory during the 1970s to owners of Toyota Land Cruisers, Pickups, Chevrolet Blazers, other four-wheelers, and racing cars. Its success in that market prompted Gleasman to look for a company large enough to manufacture the device for Detroit and the world market.

    He went to Gleason Works of Rochester, N.Y, in 1982. Gleason Works is the world's foremost authority on the engineering and manufacture of ring, pinion, and bevel gears used in differentials. Their machines cut 90 percent of the bevel gears used by auto makers throughout the world.

    The Torsen is currently available as an aftermarket replacement. Retail prices range from US$356 to US$482. Installation of the Torsen is identical to that of a conventional differential. Any competent mechanic or drive-line shop can do the work. To the price of the Torsen, add the cost of labor, bearings, and gaskets needed to switch differentials. Swapping can cost from US$400 to US$650 or more. No special adjustments, equipment, or tools are needed other than those required to swap a conventional differential.

    According to Gleason, the Torsen is very easy to maintain. It requires only occasional gear-oil changes. Repairs on the unit, if required, must be done by the Gleason Works in Rochester. That's because the Torsen, although simple in design (it uses only eight moving parts), fits together like a Chinese puzzle: There's only one way to do it right. I've had a Torsen in my International Scout for two years with no trouble at all. Operation is still quiet and smooth. In addition to the increase in traction, the handling has improved enormously.

    In fact, dollar for dollar, the Torsen probably provides a greater traction improvement than any other after market accessory. Because its torque bias provides 200 percent more traction than the best limited-slips, it will do more for traction than large tires, extra horsepower, or high-lift suspension kits.

    Who's using it
    Response to the new differential has been enthusiastic from a number of quarters. Mario Andretti used one at the Indianapolis 500 in his Newman-Hass T-700 Lola. Actor-driver Paul Newman installed a Torsen in his Bob Sharp Datsun. During the 1982 racing season, Newman had the most successful record of his driving career.

    Production sports cars, like racing cars, also have stringent handling and torque demands. The limited-production Vector, a California built, 600-horsepower, $150,000 sports model, uses the Torsen.

    Import-auto makers are also interested in the Torsen. Maserati , for example, submitted the differential to every test it could think of, including 3,800 miles of the roughest roads in Italy. Its decision was to make the Torsen standard equipment on its 300- horsepower Quattroporte and on the Bi-turbo sports car.

    The Army's new all-purpose Jeep replacement, the High Mobility Multi PurPose Wheeled Vehiqle [HMMWV,' PS, June '82] will use two Torsens, one in each axle. Before it was accepted by the Army, the HMMWV and the Torsens in each of the vehicle's two differentials were subjected to a 20,000 mile off-road endurance test an rugged terrain. Engineer Tjong Lie, of AM General, an American Motors division, has worked on the HMMWV project since 1979. He described some of the advantages that the Torsen offers. "First, it differentiates all the time at any torque and speed. Second, it doesn't require any special lubricants or adjustments, as do clutch-pack differentials. Third, the bias ratio can be increased from the initial 3:1 to provide additional traction. Finally, it has the lowest possible weight for its size.

    The Torsen could make its Detroit debut during the 1985 model year. "Some of our units have already passed the qualifying tests of one auto maker", says Paul Dandrea, vice-president and general manager of Gleason Works Power Systems Division. One thing that makes the Torsen differential so attractive to Detroit is that it can function in the same transmission fluid used by both automatic and manual front-drive transaxles.

    Limited-slip differentials have a clutch between the differential case and side gears. When the clutch is engaged, it limits movement between the case and side gears. This forces both axles to rotate with the cam. Any time one wheel rotates faster than the other, torque is transferred.

    In a conventional differential, problems occur when one wheel loses traction. The side gear of the wheel with traction becomes stationary The differential gears then rotate on their pinion shafts as they revolve around the stationary side gear. Power is then lost out the side with the least amount of traction. The limited-slip locks the case and side gears together to prevent this.

    How It works:
    In the Torsen, as in any other differential, the power of the engine is transferred to the differential housing via the ring gear. The Torsen then uses pairs of worm wheels (from two to three pairs, depending an the size of the differential) mounted on the differential housing to turn the worm gears splined to the axle shafts. The left worm wheel of each pair turns the left axle shaft, and the right worm wheel of each pair turns the right axle shaft, Because the worm wheel cannot turn the worm gear, it locks on the gear and turns the axle shaft, propelling the vehicle forward. The right and left axle shafts (and right and left wheels) turn simultaneously. Each wheel then rotates at the same speed.

    However, when the vehicle makes a turn, each wheel rotates at a slightly different rpm. For instance, during a left turn, the left wheel will slow down by two rpm, and the right wheel will speed up by two rpm. One axle shaft always slows down at the exact rate that the other one speeds up. This difference in rpm is transferred to the worm wheels (because the worm gear on the axle shaft can turn the worm wheel and equalize the other side via the 1:1 spur gears, which act as balancing gears). So the engine is "Locked" or engaged on the axle shafts, while allowing for differential action when negotiating turns.