Free Wheeling Hubs
Part time four wheel drive vehicles come in a variety of configurations with a variety of means for selecting 4WD. One of the more common options are vehicles fitted with free wheeling hubs.
How do Free Wheeling Hubs Work?
Free wheeling hubs or Locking Hubs engage or disengage the front half shaft (drive shaft) from the hub of the front wheels. This allows the 4WD to behave in the same fashion as a conventional two wheel drive car with only the rear wheels being 'driven'. It is only when 4WD is selected and the hubs set in the 'locked' position that the vehicle operates as a four wheel drive - with power being applied to all wheels.
The free wheeling or locking hub contain a spring loaded collar that slides inwards and meshes with the front half shaft when the actuator is set to 'Lock'.
When the actuator is set to 'Free' the front wheel spins independently of the front drive shafts and differential and the car is virtually a 2WD. When the actuator is set to 'Lock' the front wheels, drive shaft and differential are coupled as one unit. Once the transfer case has been engaged and High 4x4 or Low 4x4 selected then the car is operating in 4WD.
Selecting 4WD in a car fitted with free wheeling hubs involves two separate actions. The hubs must be 'locked' and the transfer case must be engaged to either H4x4 or L4x4. If 4x4 has been selected inside the car and the hubs are still in the free position the front differential and drive shafts are turning and being 'driven' but the car is still in 2WD because the hubs have not been coupled with the rest of the drive train.
It makes no difference which action occurs first - locking the hubs or engaging four wheel drive.
Automatic Locking Hubs
Automatic hubs work on the same principle as free wheeling hubs without the need to exit the car in order to turn the actuator in the hub of the front wheels and many newer 4WD's come with these as standard equipment. Very simply these units self-engage 'on the fly' when 4WD is selected within the cab. The are usually disengaged by reversing once 4x4 has been deselected. Aftermarket units exist to convert hubs for either operation. Free wheeling hubs can be converted to automatic and vice-versa indicating that the jury is out on which is the better solution.
Some camps point to the inbuilt safety factor in automatic hubs. With free wheeling hubs it is possible to select 4WD inside the cab thinking that the hubs are 'locked' and then crest a potentially dangerous hill only to find your hubs are in the free position and you are still in 2WD.
The devotees of manual locking or free wheeling hubs point to the situational control the driver has over the vehicle.
Why have Free Wheeling Hubs at All?
Free wheeling hubs were designed to reduce friction and the amount of moving parts in four wheel drive vehicles. In theory a car uses more fuel to turn the drive shafts and the extra drag created by powering the front tyres. Our experience suggests that the savings aren't huge by running with the hubs in the 'Free' position but as we pointed out in Touring and How to Reduce Fuel Consumption, every little bit helps.
Having fewer moving parts also results in lower noise levels and a quieter ride with less vibration from the drive train.
When to Lock your Hubs
Your free wheeling hubs can be permanently set to the locked position without any adverse affects. This means the vehicle is always ready to engage 4WD and you don't have to alight from the car to turn the actuator. Many station and mining vehicles spend their entire existence with the hubs locked.
If you drive mostly on bitumen and only choose to run with the hubs in the free position then it's recommended to lock the hubs occasionally to provide 'splash lubrication' to the drive shafts.
The Nissan owners manual in the Outback Crossing ute recommends driving at least 16km per month to provide this lubrication. The manual also insists that the hubs are run in the same position on each front wheel i.e.: both locked or both free.
In practise we lock the hubs whenever we anticipate the potential for having to use four wheel drive and generally whenever we drive on gravel (even though we tend to drive in 2WD on gravel roads).
There is nothing worse than having to climb over the bonnet or wade knee deep in mud to lock the hubs because you didn't get through that last washout in 2WD.
We will normally drive on the highway with the hubs in the free position and lock them between stops about once a week, more if we are racking up the kilometres.
When Not to Engage Four Wheel Drive
Part time four wheel drive vehicles are designed to operate in conditions of poor traction, on steep hills and on loose or unreliable surfaces.
Making tight turns on bitumen or concrete can damage drive line components and wear out tyres.
A part time four wheel drive rotates the front drive shafts and rear drive shafts at the same rate. The front wheels need to rotate at a greater rate than the rear wheels to make a turn.
Because the front and rear drive lines are coupled and operating at the same rate things begin to get ugly. Steering becomes difficult as the front end 'understeers' while the drive line starts to 'bind'. Something has to give and this 'driveline binding' will result in jerky movement, tyre spin and eventually component failure. Usually expensive components like axles, differential gears or transfer gears.
Driving in four wheel drive on sealed surfaces is to be avoided unless the car is designed as a constant 4WD. If your car has free wheeling or automatic locking hubs it is a part time 4x4.
We check the position of the hubs every day just by keeping them clean. A dab of paint on the indicator arrow and the preferred position (either locked or free) makes them easy to glance at when you walk past.
Regularly drive with the hubs locked to lubricate the drive shafts.
Lock the hubs whenever you anticipate having to use four wheel drive and be absolutely certain of their position before attempting any risky manoeuvring.
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