Full-time four-wheel drive is one of those annoying things that’s great to have until you really need it. At that point it stops working, unless you undo one of the things that lets it work well the rest of the time.
I am talking about what happens to a vehicle that always drives all four wheels, when the going gets so slippery that one wheel spins on the spot. At that point, all the other wheels stop. Which is not what you’re hoping for.
They stop because they are driven through special gear sets known as differentials, or diffs. A diff lets one engine drive two car wheels at different speeds. Three diffs let one engine drive four car wheels at different speeds. The different speeds are needed whenever you’re turning a corner, because the wheels on the outside must go further than those on the inside, and the forward wheels will go further than those at the rear.
If it weren’t for diffs, you wouldn’t be able to turn your full-time 4WD. But no diff will drive any wheel if its opposite wheel (or axle) is spinning.
Four-wheel drive vehicles built for very rough and slippery going get around the problem with differentials that lock. The locks can be switched on and off, and each stops a diff from working. You can engage them for short periods when you don’t need to steer, or when the terrain is so loose that you can steer by skidding.
What if you replaced the differentials with something that let the driven wheels turn at different speeds, like a diff does, but also prevented the wheels from spinning, like a diff-lock does? That’s the idea behind a revolutionary 4WD system dreamed up by an engineer in Germany, and shown for the first time recently at Agritechnica, a major farm machinery exhibition in Hanover.
At this point the Line Traction system has been optimized for use on 4WD agricultural vehicles that need to negotiate very steep, very rough, or very sloppy surfaces, and at Agritechnica it drove a prototype tractor built by Swiss brand Aebi.
It does away with differentials and instead places a set of planetary gears at each wheel. A shaft from the engine turns the planetary gears, which drive an internally toothed, outer ring gear. The ring gear is affixed to a hub, and the hub turns the wheel.
What’s most innovative about the system is the way in which the ring gear is tied to the hub: it connects through a hydraulic circuit. The circuit can fix it in place or can let it rotate, and its rotation speed can be controlled.
Rotation of the ring gear allows the wheel to turn more slowly than the driveshaft, while receiving all of the drive. Hub speeds are adjusted automatically, with no special input required from the driver.
The developers describe operation this way: “When the ring gear is fully locked, no oil flows. The locking is controlled by a software-controlled proportional valve. In this state, the Line Traction transmission behaves like a conventional planetary gear with a fixed ratio.
“When cornering, the proportional valve is opened in a targeted manner so that oil circulates within the system. This releases the ring gear in a controlled manner, which – due to the transmission geometry – leads to a speed overlap.
“As a result, the planet carrier to which the respective wheel is attached slows down. By recording the wheel speeds, a control loop can be closed to control each wheel speed individually.
“The outermost wheel in the curve is referred to as the ‘master’ in the Line Traction system. For this wheel, the ring gear always remains completely fixed. The associated proportional valve is completely closed. For the other wheels, which are to run at reduced speed in the curve, the proportional valves are opened until the respective target speed is reached.”
How does Line Traction know how much to slow the other wheels?
“By continuously monitoring the steering angle and taking the vehicle's geometry into account, the tracking lines are calculated in real time. Combined with the speed specified by the driver, this results in target speeds for each wheel, which serve as input parameters for the Line Traction System to control the wheel speeds.”
It appears that Line Traction also uses a sensor-informed feedback loop that monitors external stress on the components, allowing it to compensate for demands on the wheels that its control software hadn’t predicted.
The result is a full-time 4WD system that dispenses with diffs and diff locks, and that behaves like one which – impossibly – is using both simultaneously. All wheels drive all the time. Even if two wheels are off the ground, they will continue to rotate at their prescribed speeds while the grounded wheels propel the vehicle.
A further benefit claimed for Line Traction is less damage to what’s being driven over. Wheels can’t spin (unless all do), and the vehicle can always turn without skidding. The system is robust enough that vehicles can be driven in 4WD on sealed roads.
Line Traction is also claimed to simplify driveline engineering, and if that sounds unlikely, take a look at the insides of a diff. And it reduces demands on the driver, who does not have to think about which diff locks to engage, and when to engage them.
The system’s inventor, Werner Muller, patented his idea in 2015, and developed it in collaboration with engineers at the Karlsruhe Institute of Technology, in Germany’s south-west. Line Traction won a gold award for innovation at Agritechnica 2025, and it is expected to see production soon propelling Aebi’s Terratrac tractor, aimed at use on steep slopes.
Many modern SUVs that drive all wheels address the wheelspin problem with solutions that are less intrusive – if ultimately less effective – than diff locks. Commonly, for example, a traction-control system limits wheelspin by braking individual wheels. Some manufacturers of advanced electric cars dedicate a motor to each wheel, again using software to adjust speeds as the car turns.
Line Traction publicity says it is “flexible and ready for new vehicle concepts”. It’s not clear whether applications exist outside heavy machinery, given that the hub-mounted gear-sets likely add unsprung weight that could degrade suspension performance – among other possible drawbacks. In its initial market, however, it seems to resolve effectively, and efficiently, a long-standing problem.
Check out the system in the video below.
Source: Line Traction
