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Differential...Know The Difference


Zoomed in M4 Differential

You regularly change your engine oil, but when was the last time you changed your differential oil?


As most BMWs are rear wheel drive, the rear axle requires your attention too. Don’t wait until your differential fails before you change your oil. Before we explore why you should change the differential oil, let’s explore what exactly does the differential do.



What?

The differential is a very important invention because it allows the car to take corners without skidding. Why does the car skid without differential? Imagine a car takes a right turn. While turning, the right wheels will travel less distance than the wheels on the left side of the car. If the drive wheels both travel at the same speed, the inner wheel will skid. There are 3 categories of differential:

Open The open uses a combination of ring and spider gear to send power differently to both axles. This allows both wheels to spin at a different velocity. The weakness of this design is power will be directed to the wheel with the least grip.

Limited-Slip The limited-slip differential or LSD adds a clutch system into the differential. LSD locks the left and right axles together if wheel slip happens.

Torque-Vectoring The torque-vectoring system builds up on the LSD system where the clutch pack is controlled by a motor and an array of sensors. Electronic controls are now used to control how much power each axle receives.




When?

As a general guide, you should change your differential oil between 50000KMs to 80000KMs. Do change the differential oil as per manufacturer’s recommendation. The images below illustrates a dirty vs clean differential oil.


Dirty Differential Oil


Clean Differential Oil



The differential oil is usually has at least 80 viscosity. Rear wheel drive differential has its own casing and is located at the rear of the car. The differential oil lubricates the ring and spider gears that transfer power from the driveshaft to the wheel axles. If your car is fitted with a limited-slip differential, it also keeps all the moving parts in that assembly healthy. Changing this oil is just as important as changing your engine's oil, and for the same reason. Metal-to-metal contact wears down surfaces and creates heat from friction, which inevitably weakens the gears and leads to failure.



Damage Cone bearing due to poor lubricant quality.

How?

1) Prep the Area – car needs to be jacked up as the rear differential is well hidden at the rear underneath the trunk. There may be a need to remove components that blocks access to the drain plug. Remove the plug when you're ready.






2) Fill to the Brim - Use the highest-quality gear oil you can afford to fill the differential. One sample high quality differential oil would be the RAVENOL VSG SAE 75W-90. Fill the differential directly from the bottle if you've got clearance, but if space is tight, you can get a pump or extension hose to make the job easier. The bottom of the plug hole is the maximum fill line, so when oil starts dripping out, you're finished.




3) Seal It Up - Install the plug, tighten the bolts to your vehicle's specs with a torque wrench, and you're good for tens of thousands of KMs.




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