2wd vs 4wd

The usual type of stock differential is an open differential, a simple arrangement that is inexpensive and works well on surfaces with sufficient traction. The problem is that when traction is not always sufficient; in those situations one tire spins, one tire is stationary, and the vehicle is stuck.

One way to meet this challenge is to use a locking differential (LD). A locker can be changed, electronically or by air pressure, between two states:

• an open diff configuration for normal use; and
• a locked diff for use in low traction scenarios. When locked both driving wheels rotate at the same speed, ensuring the tire with the best traction is getting enough force to make use of that traction.

Another way to meet the challenge is to use a limited slip differential (LSD), whether physical or virtual. A physical or traditional LSD uses clutch packs or worm gears to limit the difference between wheel speeds. A certain amount of spinning of one wheel relative to the other results in a clamping effect which can either lock them together (practially speaking) or limit the difference, as in a torsen type LSD.

The sensors and controls associated with ABS brakes allow for virtual forms of limited slip. A subset of Electronic Traction Control (ETC), Brake-lock differential (BLD) can control wheelspin with an otherwise-open diff by selectively braking the wheel that is spinning. In addition to stability controls that will not be addressed here, many ETC systems can be configured on the fly:

• traction control completely disabled
• brake-lock differential (BLD) only, no throttle management. This may be best when forward momentum is critical or traction at both tires is bad, as when stuck in mud or snow. Note that differential braking with higher wheel speeds can increase heat and wear on brake components.
• full traction control - Engine throttle management + BLD. This may be best when accelerating when tires are on surfaces with different traction: one tire is on gravel or ice, etc. This mode will wear brake components less than BLD only. It is common for a dash light to come on when traction control is active.