The hydraulic drive used on the 2x2x2 is of the 'positive displacement' type- commonly called "hydrostatic".  It is a common misconception that the drive is achieved by shooting the oil at a turbine wheel of some sort, as does a torque converter in a conventional automotive automatic transmission- this is not the case.  An internal combustion engine (in this case a 250cc two-stroke made specially for the job) drives a hydraulic pump rather than a gearbox as in a conventional motorcycle.  The pump pressurises oil to around 4500psi which is then directed to hydraulic motors located in each wheel hub which convert the high pressure oil's energy into rotary motion.  By freezing time for an instant you can visualise one piston in the pump directly applying force onto a piston in a wheel motor via a long column of oil.  Oil is effectively incompressible- so for a split second you can imagine the oil as a solid steel bar causing one pump piston to push on one motor piston.  The speed of the bike is determined by the flowrate of the pump which is infinitely variable. 

The advantages of hydraulics are many- particularly for an experimental vehicle.  With a pressure reducing valve you can limit the overall torque output of either wheel motor - or split the pressure right through the range giving torque split.  A flow dividing valve will on the other hand will prevent either wheel spinning. This applies to a 'parallel' flow system where the flow is split into 2 streams after the pump. This was the setup that I ran on my 2WD -although I didn't start experimenting with valves to limit torque or wheel spin.  My 2WD will spin either wheel if it comes off the ground – leaving the other wheel with zero torque (or near to) - a flow dividing valve is definitely necessary if the design was to be developed further.

The other option with 2 wheels you can also use is to run the motors in 'series' - i.e. - feed the oil into the rear motor first and then into the front motor.  This is the reason the Dryvtech 2x2x2 uses a bigger front wheel -a high pressure motor will leak up to 5 % internally at full load - so the front wheel must rotate 5 % slower or it will 'cavitate' (the quickest way to kill hydraulic equipment).  I didn't get around to using series flow- basically it gives twice the speed but half the torque- used at high speed.

Hydraulic drive results in a few idiosyncrasies - the first being that when the engine is off the transmission is locked, a one-way valve is required so that the bike will free wheel- but this means you have no engine braking.  It is still impossible to push the bike in reverse - not usually a major problem with a motorcycle.

Interestingly you can't keep too many secrets in hydraulics - the diameter of the pressure feed (particularly hoses) to a motor gives away its power delivery.  Whilst you might think that there are too many variables to do this actually there are only two- namely pressure and flow rate.  There are also really only two pressure ranges- low (1000-1500psi) used in agricultural stuff and high (3500-6000psi) used in industrial and aircraft applications.  So a 25mm outside diameter hose may carry high flow/low pressure (with a thin wall thickness) or low flow/high pressure (with a thick wall).  Either way the outside diameter will give the approximate power rating.

The Dryvtech 2x2x2's drive itself is outstanding, with wheelspin virtually impossible even in sand or mud.  The steering and drive combine to make it impossible to slide the back out- the bike will actually highside the rider in mud! 

I started on a shaft drive 750cc 4 stroke twin version of the desert racer (still 2WD but front steer only) but this was sidetracked by the work on the 750-V8.  The desert racing rules seem to be favouring single cylinder bikes - so the twin will be shelved.  A 1000cc single cylinder shaft drive 2WD sounds good!  As for the prototype itself- it was sold in 1998 and now resides in the Donnington Motor Museum in the UK, in spite of the fact that several friends threatened me with physical harm if I sold it.