Eddy-Current Limited Slip Differential

The typical car is equipped with some device that generates rotational force driving two of the wheels. Because drivers are fundamentally indecisive, cars often change direction*. As the net motion of a car is usually a circle, the wheels on different sides of the car travel different distances. To prevent embarrassing tire noises and strange handling, cars are fitted with a differential <link>. In most cases the problem is solved, it's pretty robust, we can pack it with grease and retire to the pub safe in the knowledge it will last decades.

However, should you get onto a surface with variable traction, snow/ice being the usual culprits, then the wheel with the least friction will spin around uselessly while the wheel with the traction does nothing at all. What we need is a way of allowing some differential movement, but when there's a lot, a way of transferring some of the torque.

Several methods currently exist, the simplest is a clutch <link>. These are somewhat uncivilized and wear out. Torsen<link>, the main competitor here, is good but expensive, so much so, that even most "4wd" cars stick with open differentials. In radio-controlled racing cars which are so light and ludicrously over powered that they have essentially no traction most of the time solve the problem by fitting enormous differentials and filling them with variable viscous oils that act as a velocity-sensitive damper. At full size, this would overheat and require too much maintenance.

So, how do we get the velocity-sensitive damping without the mess? Enter the eddy-current brake <link>. If you try and move a magnet adjacent to a conductor, you will induce a current and produce a force. Examples of this include the dropping a magnet down a pipe experiment <link>. The principle is used as a power-independent brake on roller coasters and on earlier generations of bullet trains. The beauty is, that nothing wears out and it self operates.

To incorporate this, we have the drive shafts fitted with a thin disk, this is close to the differential case. The differential case has powerful neodymium magnets in, when the drive shaft spins quickly relative to the case (one spinning wheel) the magnets will generate eddy currents in the disk transferring torque to the case and therefore, over to the other driveshaft/wheel. There, a relatively mild LSD that should work forever and cost relatively little to make.

Secret bonus, if you ever drop a screw, it will likely be found safe and secure on the diff casing.

*This is something of a scandal, investigation reveals car journeys to be oscillatory in nature, with nested oscillations of various periods. Within a 24hr period, cars leave and return to the same location. Often, the ultimate destination of a car may be the location from which it was purchased. Significant energy savings might be afforded by simply leaving the car stationary after purchase, and having a bloody good think about where you actually want to be, which is unlikely to be at a car dealership.