To the eye, all SUVs that have a 4X4 written on it look alike and it seems that they all do the same thing. However, that’s not the case and you should know that not all cars can do off-roading and neither can you cannot drive all four wheels in all conditions in case of every 4WD car. So what’s the difference? We understand in this article the nuances of four wheel drive systems.
Part time four wheel drive
A car with a part time four wheel drive, you have a Hi gear and a Lo gear setting, it’s by the name of 4Hi and 4Lo, in addition, there’s a 2 Hi mode that lets you drive the car by two rear wheels. The drive from engine is sent to a transfer case and then onwards to the front and rear differentials.
The power is split 50:50 between the front and rear axles when you engage the four wheel drive, else the car is driven by rear two wheels only. You cannot turn them well on hard pavement at high speed since there is no centre differential to provide different torque between front and rear axles, if you do so, it can cause transmission windup and can even breakdown.
Full time four wheel drive
In case of a car with a full time four wheel drive, you get the 4Hi and 4Lo mode of operation. The 2Hi mode is generally not available (that means that you cannot use two wheels for driving) and there are three differentials.
One differential splits the power between the front two wheels while the second splits the power between rear two wheels. There is a differential present between the rear wheels and front wheels (called centre differential) that allows different speeds for front and rear axles.
The centre differential is generally an open type with locking mechanism or an LSD (limited slip differential) which in case of slip transfers power to axle which has traction. A car with a Full time four wheel drive system can be used on pavement as well off road.
All wheel drive
Cars equipped with all wheel drive system drives all wheel at all times and is much like a Full time four wheel drive system, with the only difference that a 4Lo mode is not available in case of an All wheel drive and these are not meant for proper off road use.
In case of an all wheel drive car, the power is generally split 60:40 between the rear and front wheels using a quasi differential / planetary gear set or any other mechanism.
Automatic all wheel drive
It’s much like an All wheel drive system however it does not use all four wheels for driving all the time. Normally, only two wheels are driven and they receive 100% torque for driving. In case the axle that’s normally driven loses traction, the power is sent to the other axle until the primary axle recovers from slip.
Since everything is controlled using advanced gadgetry. Power to the secondary axle is sent using a multi-plate clutch or viscous coupling.
Some vehicles even have the provision to lock the clutch manually for permanent engagement of both axles. Whichever way, this is the least capable four wheel drive system.