February 20, 2013

Selecting Friction Material

Know the Terms

Friction material properties and quality are often discussed and debated, but how often are they really understood? A good grasp on some important terms makes all the difference:
  • Static/Breakaway Coefficient:
    Quantifies the ability of a material to avoid slipping. The higher the coefficient, the more force (torque) it takes to begin slipping
  • Dynamic/Midpoint Coefficient:
    Quantifies the relative force required to maintain a constant slip. The higher the coefficient, the less force (apply pressure) and time it takes to modulate or control slipping
  • Thermal Capacity/Durability:
    The amount of thermal distress a material can tolerate before it can no longer function as intended. In short, can it take the heat?
  • Elasticity (Shudder Resistance):
    The ability to conform to variations in running surfaces. The elasticity of the material impacts surface contact (area) and loading (force). It is sometimes referred to as Shudder or NVH Resistance.
  • Shear Strength:
    The amount of force the friction material can handle before it fails or fractures.
  • Porosity/Density:
    The amount of oil the friction material can hold.

Know What Matters

The terms above have one thing in common: they are all well-established physical properties by which any piece of friction material can be analyzed and evaluated.

What matters? Which is of greatest importance? That depends on how you intend to use the friction material. For example, in a straight on or off lockup system, static coefficient and shear strength would surely top the list, and thermal capacity and porosity may be on the lower end of importance. As you move further toward a system designed to slip continuously, static coefficient and shear strength become less important considerations compared to thermal capacity and porosity.

In general, these are the levels of importance within the three general lockup categories:

Static Coefficient Dynamic Coefficient Thermal Capacity Elasticity Resilience Shear Strength Porosity
On-Off Lockup High Medium Low Low High Low
Limited Slip Lockup Medium Medium Medium Medium Medium Medium
Continuous Slip Lockup Medium High High High Medium high

Know the Facts

Once you know what properties are important, how do you select the right friction material? The charts below represent a comparative analysis of a variety of BorgWarner friction material offerings (HTE, HTS, HTL and XTL), along with analysis of materials offered by a major competitive brand (T, K, HC, PT). The superior materials offered by BorgWarner – used appropriately based on application criteria – will consistently provide you with the best quality lining for your torque converter rebuilds.

Static/Breakaway Coefficient

A higher static coefficient is better. This characteristic is a primary consideration in On-Off lockup systems.

Dynamic/Midpoint Coefficient

A higher static coefficient is better. This characteristic becomes more important as the system approaches continuous slip.

Thermal Capacity/Durability

High Speed Durability 33.5 KJ & 1.04J/mm2 @ 60 sec stop time
Systems designed to slip require increasingly higher thermal capacity scores.

Elasticity (Shudder Resistance)

As the amount of designed slippage increases, shutter resistance, or a high elasticity score, becomes more important.

Shear Strength

When a system is fully locked up, a higher rating means the material can withstand more force before it shears internally.


The higher the rating, the more lubrication is available to reduce heat. This quality is critical in systems designed to slip.

Learn more about BorgWarner friction rings.

Chart data provided by BorgWarner.

While Sonnax makes every effort to ensure the accuracy of technical articles at time of publication, we assume no liability for inaccuracies or for information which may become outdated or obsolete over time.