The engageable front-axle drive gives more traction but not at the price of excessive additional weight.
MAN indicates that the HydroDrive has lower fuel consumption and higher payloads than conventional all-wheel-drive vehicles. The HydroDrive is ideal for situations in which extra traction is briefly needed on the front axle, such as when entering and leaving construction sites and dirt roads, on slopes, unsurfaced forest tracks and earth roads, on gradients and slippery surfaces. It is available on the TGS and TGX range with two, three and four four-axles. The HydroDrive does not impose any restrictions in bodywork height, manoeuvrability, choice of cab or axles.
The HydroDrive is engaged through the rotary switch for the differential lock, and can be activated while driving under load. It automatically cuts out from speeds of about 30 km/hr (19 mi/hr) upwards and if the speed drops below 22 km/hr (14 mi/hr) it automatically cuts in again. A display on the driver dash indicates to the driver that the front-axle drive is activated. When driving downhill with the HydroDrive engaged, the sustained-action brake also acts on the front axle, stabilising the vehicle.
The HydroDrive hydraulic pump is seated on the gearbox output shaft to feed the wheel hub motors on the front axle. The technology is rugged and comparable to conventional rear-wheel drive. Some of the advantages over conventional all-wheel drive include a weight-saving of 400 kg (880 lb), the transfer case is done away with, only the rear wheels are driven on-road and the hydraulic pump and wheel motors produce no friction. With all-wheel drive, the transmission elements keep turning even when the front-axle drive is disengaged. Source: http://www.truck.man.eu