Friction Modifier
An additive blended into gear oil to adjust sliding friction and prevent noise or chatter in limited-slip differentials.
A friction modifier is a specialized chemical additive mixed into automotive lubricants—primarily gear oil and automatic transmission fluid—to alter the sliding friction characteristics of metal and clutch surfaces. They are most commonly used in limited-slip differentials (LSD) and wet-clutch transmissions to ensure smooth engagement.
In a limited-slip differential, friction modifiers allow the internal clutch plates to slip slightly during tight turns, preventing them from binding and grabbing. Without this additive, the clutches would engage aggressively and release in a rapid stick-slip cycle, creating a harsh vibration and chattering noise. The chemistry of these additives is designed to align polar molecules on the friction surfaces to smooth out the transition from static to dynamic friction.
Symptoms of missing or depleted friction modifier in an LSD include a loud chatter or shuddering noise from the rear axle when making sharp turns, such as when turning into a parking space. When servicing an LSD differential, technicians must either add a separate bottle of friction modifier or use gear oil pre-blended with the additive to prevent clutch chatter and premature wear.