On some 4WD vehicles the driver has choice of a "4-high" and "4-low" (AWD does not have low range
) - when would you use which one?
Four wheel drive high (4 high or 4Hi)
You would select 4WD high on snow and for easy to moderate off-road (dirt roads, easy trails). You would select it to have better use of traction (better than 2WD) to get going and to maintain forward movement with a reduced risk of slipping and spinning wheels.The added lateral stability of 4WD makes it less likely to slip sideways.
However, depending on the conditions, you may still encounter slipping wheels. Here is why..
Some vehicles have automatic traction control now and that somewhat reduces the risk of slipping wheels. A limited slip differential (optional on some cars) also reduces the chance of spinning wheels. Emphasis with both is on "reduce" - none of them will prevent wheel spin. Only differential locks could do that, and they are extremely uncommon. They are more a tool for 4x4 professionals.
If you have a vehicle with full time 4WD (or permanent 4WD) there is no more need to chose between 2WD and 4WD. Full time 4WD can safely be used on all surfaces.
Some auto makers still offer a choice between 2WD, full time 4WD and part time 4WD though (Jeep) - very confusing for the owners. Other makers combine different settings as well
Four wheel drive low (4 low or 4Lo)
When "4-low" is selected the wheels create substantially more torque
(on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower than in 4Hi on a Jeep Grand Cherokee).
Important: "4-low" does not create more traction - it creates more torque and that can be detrimental when traction is marginal. Slipping tires are more likely in "4 low" than in "4 high"!
For that reason 4 low is not a good choice for snow or ice - and not a good choice for mud either!
On most vehicles the 4 low setting is "part time 4WD" (both drive shafts are forced to rotate at the same speed). That means it must not be used on dry pavement. Here is why..
Some vehicles have 4WD low also available in "full time 4WD" (Mercedes M-Class and G-Class). They of course can use 4low also on dry pavement.
On pavement the 4 low setting is an advantage for drivers who need to tow and maneuver a heavy trailer (ideally it would be full time 4WD low for towing) etc. and for drivers who at one point or another may want to negotiate difficult off-road terrain, when more torque and/or slower speed is needed (when off-road, part time 4WD low is best - full time 4WD low will not do well, unless the center differential can be locked).
Like in 4WD high you still may encounter slipping and spinning tires in 4WD low. Most automatic traction control systems will have a hard time to control that due to the high torque in 4 low. Only "professional" traction control (manual differential lockers
) will prevent wheel slip in 4 low. Few have it stock (Jeep Rubicon and Mercedes G-Class) - on many it can be added as an aftermarket option (ARB, Detroit Locker etc.)
How do you engage 4WD?
How and when do you engage your axle diff locks?
When would you engage your center differential lock?
(description coming soon)
Per definition AWD cars do not have a choice between high and low. What defines AWD is, that their full time 4WD does not have low range. Confusing but true: some vehicles have a combination of part time and full time 4WD systems, or even a combination of 2WD, automatic asymmetric AWD and part time 4WD (low range). Impossible to give those custom mixes a name that would indicate what they really are. manufacturers will lable them as whatever they like and that's the confusing part for consumers - how do you know, what you really have in there?.
Learn from professionals to get the most out of your 4WD :
2012 hands-on 4WD "real life" training courses