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I'll give it a shot. MFWD is easiest. Mechanical means it's taking power through gears and a drive shaft from the transmission to the front axle, transferring that power to the wheels via a differential. Power going to the front wheels is proportional to the power going to the back wheels, as power is transferred from the engine through the same gears to the rear axle and the front output shaft on the transmission for the MFWD. HFWD requires a hydraulic pump to create flow to the front axle which uses hydraulic motors to transfer power to ground. HFWD is dependent on engine speed. As the engine speed drops, so does the flow to the front wheels. One problem with HFWD in tractors is that at higher speeds, the flow can't keep up with ground speed, effectively turning the tractor into a 2WD that wasting HP trying to move enough oil to keep the front motors turning. Another problem is if you get into too tough a spot, the back pressure created on the hydraulic motor can blow the relief valve on the pump, rendering the HFWD useless. You'd have to get it really stuck to do it, but it has happened. Hydraulic rear wheel assist works well in machines like combines because they are almost constantly running at high engine speed to keep the thresher going while moving at a steady speed through the field, in the 4-6 mph range (some machines are now much faster). The constant high engine speed keeps adequate flow to the rear wheels, and since the engineers know that the combine will be used mostly in the 4-6 mph range, they can set up the system to provide the most traction at that speed with the flow produced by the pump at threshing rpm. Tractors, on the other hand, work in a variety of gears, at a variety of throttle settings, depending on what they are doing. MFWD is a better setup for tractors, as it provides traction at any throttle setting or in any gear (although using it in road gear is not recommended.) If anybody wants to clarify, correct, or add to what I've said, feel free.
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