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Tech Tips: Active Radius And Its Effect On Clutch Torque Capacity

Many newer-design clutch friction discs are


active radius 4.704When automotive technicians order parts for a ­repair job they know they need the right part, and a quality part as well.

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So it’s just a common procedure of any repair job that technicians will inspect the new part to make sure it is a correct part as well as a quality part, without any visual or functional flaws that would lead to a poor repair job and a customer comeback.

Many newer-design clutch friction discs are ­designed with a thinner band of friction material than the original disc that came in the vehicle.

The thinner band of material on the newer design may seem like a bad thing to some technicians as they compare the new and original parts that came out of the vehicle.

 Active radius 4.367

Conventional thinking would be that “more is better;” more friction material will mean a stronger clutch.

The active radius of a friction disc is defined as the distance from the center of the disc’s splined hub to the center of the friction material.

So, by using a thinner band of friction material on the disc, the active radius will be increased.

The active radius can be thought of as a lever. The longer the lever, the easier it is to move a given load.  

The above images illustrate how a thinner band of friction material will increase active radius and, therefore, the torque capacity of a clutch.

The disc on the top has a thinner band of friction material than the disc on the right.

This will increase the ­active ­radius and torque capacity of a clutch.

Courtesy of Schaeffler Automotive Aftermarket

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