Quick answer
Yes, friction modifiers can degrade over time with heat cycles and shear forces. As the fluid ages, their effectiveness fades, leading to renewed chatter or slip. That’s why replacing gear oil and re-adding additives periodically is essential for maintaining smooth operation in LSDs or certain transmissions.
Detailed answer
Friction modifiers are chemical additives that reduce metal-to-metal or clutch-pack friction under certain loads. However, like most additives, they don’t last forever. Heat, shear, and oxidative stresses gradually break down their chemical bonds. As a result, even the best friction modifier loses effectiveness with mileage. Think of it like sunscreen—you need to reapply.
Why They Wear Out
1. High Temperatures: Differentials and gearboxes can run hot, especially under towing or spirited driving. Heat accelerates additive degradation.
2. Shear Forces: The gears and clutches physically shear the oil film. Over time, friction modifier molecules can fragment.
3. Oxidation: Prolonged exposure to oxygen and contaminants in the oil can change the additive chemistry.
Symptoms
– Resurfacing chatter or noise in LSDs.
– Slightly rougher shifting if it’s in a transmission.
– A feeling that the LSD lock-up characteristics are “off.”
Routine Maintenance
To keep the LSD or gearbox smooth, plan on regular fluid changes. If you’re using a separate friction modifier product, add it fresh each time. For gear oils that come with LSD additives pre-mixed, swapping the fluid typically replenishes those additives.
Service Intervals
Some manufacturers suggest every 30,000 to 60,000 miles for diff fluid changes, depending on usage. If you do track days or heavy towing, you might shorten that interval. Old fluid can lose both its base lubrication qualities and the friction modifiers’ slip-control properties.
Testing
If your LSD chatters again after a few years of quiet operation, that’s a good clue your friction modifier has likely worn out. Doing a partial drain and adding fresh additive can help. However, fully draining and refilling is often a cleaner solution.
Key Point
Yes, friction modifiers do wear out. They’re not permanent. Replacing them on schedule ensures you maintain ideal friction balance—quiet operation at low load, reliable lock-up under higher torque. So, if you start hearing those dreaded diff noises, it might be time for new fluid and a fresh dose of additive. That’s just the nature of chemicals under stress: eventually, they break down, and your LSD or gearbox begins to act up again.
