In1827, a Frenchman, M Pecquer, created problems for four wheel drivers in Australia, 170 years into the future.
How ? He invented the differential.
Althoughthe original inventor would surely have been impressed with themetallurgy and machining in the modern diff, he would still seebasically the same gear set up he painstakingly worked out 170 yearsago. His invention made it possible for 2 wheels, rigidly fixedto axles, to each accept drive from a common central point yet turn at different speeds when necessary.
There are three characteristics of that basic design that are the problem for four wheel drivers today.
- Thegear train directs torque along the path of least resistance. Thatmeans that as soon as one driving wheel is on a poor traction surface it will get all the torque because it is easiest to turn.
- Thedifferential equalises torque across the two axle shafts. That meansthe torque available to the wheel with traction is exactly the same as, and so controlled by, the one without.
- Thestandard detail (with the planetary gears equidistant between the axlegears) causes the spinning wheel to rotate at twice the speed of the other. That means:
- Whena wheel in the air returns to a 100% traction surface there is instant tyre wear or damage and substantial shock to the transmission
- Loss of traction down hill causes the vehicle to be propelled forward at twice the apparent speed.
Remember,a four wheel drive vehicle with open diffs is a two wheeldrive. It is,in effect, 2 x two wheel drive vehicles joined togetherby a solidtowbar. Either one can spin one wheel, the other may or may not be able to push or pull its twin out of trouble.
There have been many attempts over the years to control the open diff's annoying
characteristics, the notable ones are:
- The
NoSpin truck differential, subsequently developed into the smaller
Detroit Locker. This design allows differential action but directs 100% torque to the wheel or wheels with traction.
- There have been numerous designs, variably successful, using friction in clutch packs to control the torque flow.
- Thereare ways to stop differential action, and thereby wheelspin, by lockingthe mechanism solid using air or electric actuation. These interfere with steering simply because they stop differential action.
- Thereare gear based slip limiting designs.Some have not coped well with thehigh torque output of low geared 4 wheel drives and/or diesel engines. A real exception to that is the
Truetrac geartype slip limiting differential. The Truetrac has been in service formore than 20 years in recreational, motor sport and light commercialduties. The Truetrac has proved its point - it works and it stays the distance.
