Calculating Clutch Torque Capacity

Clutch Disc with Organic Friction Material

Let's talk about torque and how the torque handling ability is calculated for an automotive clutch.

The formula looks like this:

Torque Capacity (lb-ft) = F ● Rg ● N ●µ

The torque capacity of a clutch can be calculated if the following four values are known.

1) F = Clamp load applied by the cover assembly in pounds (lb).
2) Rg = The mean radius, or radius of gyration, of the friction material in feet (ft).
3) N = The number of friction surfaces, one disc has two surfaces.
4) µ = The coefficient of friction of the facing material.

Let's go through them in order.

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First of all, the clamp load applied by the cover assembly will be something that the manufacturer or distributor will have to give you. This number can range from (but is not limited to) 800 lb. to 3,200 lb. For our import car example we will use 950 lb. Special equipment is needed to calculate this number. Contact the company you bought your clutch from as they should be able to tell you the clamp load number for the cover assembly in your particular vehicle.

The second variable is the mean radius of the of the friction material in feet. This is also known as the radius of gyration. At first glance it might sound simple to calculate but it actually involves a few steps. I will go through an example of how to calculate it. The first thing you need to do is measure the outside diameter of your clutch disc (a.k.a. driven disc) in inches.

Then measure the inside diameter of the clutch disc (at the edge of the friction surface) in inches. The Radius of Gyration in Feet = Square Root of (Outer Radius in Feet Squared + Inner Radius in Feet Squared / 2) .
Note that this formula says RADIUS of gyration in FEET. Remember that your diameter numbers will have to be coverted from inches to feet and then divided by two to get the RADIUS in FEET.

For example, if you have a clutch disc with facing that has outer and inner diameters of 8.35" x 5.91" respectively, then that would translate into radii in feet of 0.348' x 0.246' respectively. Then we plug these numbers into the radius of gyration equation. The Radius of Gyration in Feet = Square Root of (0.348' squared + 0.246' squared / 2). The final number for this example would be 0.30 ft.

That pretty much takes care of the complicated stuff. Next you would need to know the number of friction surfaces. 1 disc = 2 surfaces, 2 discs = 4 surfaces, 3 discs = 6 surfaces...... You get the idea. The example here will have 1 disc (N= 2 surfaces).

The last number you will need to know is the coefficient of friction of the disc's facing material. In general, the following numbers can be used for most automotive clutches if you just know the basic material the friction is made of:

Stock Organic Facing Material = 0.25
Carbon/Kevlar Material = 0.28
Kevlar Material = 0.36
Ceramic Material = 0.38
Steel Material = .40

Coefficient of Friction numbers can vary from manufacturer to manufacturer but these numbers are well within reason for the types of material listed and should also provide a slight safety factor which means that each material might have a slightly higher coefficient of friction at lower operating temperatures. Our example will use a stock coefficient of friction of 0.25.

These 4 values can now be entered into the formula and multiplied to determine the torque capacity of this particular import car system.

Clampload = 950 lb.
Mean radius in feet = .30 ft.
Number of friction surfaces = 2
Coefficient of friction .25 (organic facings)


950 lb. ● .30 ft. ● 2 ● .25 = 142.5 lb-ft


As mentioned earlier, this value includes a designed in safety factor that is in excess of the expected maximum engine torque.

If you don't feel like calculating this on your own use our handy clutch torque calculator found HERE.



Related Page:

Clutch Torque Calculator