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Friction Modifiers

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Friction modifiers are added to various lubricants to change their lubricating properties under special conditions. On a molecular level, they have a polarity that makes them want to cling to metal surfaces. In light friction environments, they provide excellent lubrication, but when the friction stress is increased, they are sheared off, and friction can occur. This makes them particularly useful in automatic transmission fluid, the planetary gear sets inside of the transmission have to be clamped on by bands to make the outer ring stationary, and friction is required to achieve that.

Friction modifiers are found in most automotive lubricants, but are each tailored to different scenarios that they will encounter, and are in less noteworthy amounts for some fluids.

Friction modifiers play a vital role in many limited-slip differentials, controlling how much slip is allowed. The amount of friction modifier added can be altered to achieve different slip-limiting characteristics, but limited to a certain range before too little or too much causes the limited slip effect to fail completely and sometimes generate noise. While limited slip friction modifier additives do contribute to gear oil odor, the sulphur in the extreme pressure additive package tends to be the bigger odor producer.

What Are Friction Modifiers? Video by Cars Simplified

In this video from January of 2016, Steve explains friction modifiers.