ASK ARB – Air Locker
vs Centre Diff Lock

Dear ARB,

I have recently taken my family Toyota LandCruiser 80 Series on a 4WD track with my wife and children. I wanted to test the capabilities before I take them Outback onto more demanding terrain. The vehicle responded really well on the steep, uneven, broken stone terrain; however I have a query about the different types of diff lock.

In my vehicle, the diff lock automatically comes in when in low ratio, and a push button for high ratio. I don’t believe that this gives me individual traction on the four wheels. Could you please explain the difference between diff locks (as above) and the ARB Air Lockers?

Kind regards,

Manuel Pace and family

Dear Manuel and family,

The ‘diff lock’ you are referring to is not an axle differential lock like an ARB Air Locker, but rather what is correctly referred to as a ‘centre diff lock’. It is a very common misconception that they are the same thing, when in fact they are quite different in function and benefit.

The centre diff lock you have serves a similar purpose to ARB’s Air Locker except that a centre diff is there to ensure equal traction between your front and rear axles only. All traction losses from side to side (from left to right wheel across the same axle) are not accounted for at all by a centre diff lock until you add an Air Locker to one (or both) of your axles.

Note: A centre diff is not even included on most part-time 4WD vehicles (i.e. vehicles with manual front hubs) and therefore their centre is always locked.

The diagram on the right may help clarify this.

Here are a few examples:

• Have you ever noticed that if you turn off your centre diff lock your vehicle will spin its front axle as you climb up a steep hill? This happens because most of your vehicle’s weight is bearing on the rear axle while climbing uphill. A locked centre diff will save you from losing this traction from the front axle.
• Have you ever noticed how 2WD vehicles pulling a boat out of the water on a slippery ramp will usually spin the right hand tyre? This is from torque bias caused by the torque from the vehicle’s drive shaft, and is remedied when you turn an Air Locker on as both wheels are forced to spin at the same speed.
• Have you ever brought the whole vehicle to a halt just because one side of the track is slippery (e.g. from mud)? Without an Air Locker, a traction loss on one side of the vehicle equals a traction loss on both sides. Locking an ARB Air Locker will save you from losing traction on the side of the track that still has a good surface, and most of the time this is enough to keep you going.
• Have you ever brought the whole vehicle to a halt as you attempt to climb over a bump on only one side of the car? You will find that this occurs when the tyre that is NOT elevated on the bump loses traction and spins. This happens because the other elevated wheel is lifting the vehicle upward and reducing the down force on the opposite wheel. A locked Air Locker will never allow one wheel to spin freely – even if it is hanging in mid-air.
• Have you ever found it difficult to steer when driving up or down steep hills? This happens because of varying traction conditions on different tyres which allows one side of the car to slip while the other side holds firm. The spontaneous steering effect that results can dangerously get you sideways at the worst possible time. An Air Locker will assist you with steep ascents and descents as it will maintain an equal rotation on all locked wheels, which has a natural tendency to track the vehicle straight, thus preventing your vehicle from yawing from side to side as the road surface conditions change.

Hopefully my reply has clarified the difference between centre diff locks and axle diff locks like ARB’s Air Locker. You should also be aware that because the locations of the centre diff lock and Air Locker axle diff locks are totally different, you are always able to add an axle diff lock (or two) onto your axles to work together with your centre diff lock, to get the maximum grip available to your vehicle.

Daniel Bongard
(Engineering Supervisor)