Clutch Max Torque capacity

[Bror]

How much torque can the A4 B8 (Facelift) manual clutch handle?

[audrobotic]

It’s pretty simple to calculate:

To calculate the torque capacity of the clutch we’ll have a look at the geometry of the clutch (friction) disc. Within the area of the friction surface we are going to represent an elementary area dx, at the distance x from the center.

Clutch torque capacity calculation
Image: Clutch torque capacity calculation
where:

Fa [N] – the normal force pressing the clutch plate
Tc [Nm] – the torque capacity of the clutch
r1 [m] – the inner radius of the friction surface
r2 [m] – the outter radius of the friction surface

The pressure p [Pa] acting on the clutch is equal with the ratio between the normal force Fa and the area of the friction surface S [m2]:

𝑝=𝐹𝑎𝑆(1)

Assuming that the area of the rivets is neglijable, the area of the friction surface is calculated as:

𝑆=𝑆2–𝑆1=𝜋𝑟22–𝜋𝑟21=𝜋(𝑟22–𝑟21)(2)

Replacing (2) in (1), we get the expression for the clutch pressure:

𝑝=𝐹𝑎𝜋(𝑟22–𝑟21)(3)

The elementary area dA is calculated as:

𝑑𝐴=2𝜋𝑥𝑑𝑥(4)

The elementary normal force dN, acting on the elementary area is calculated as:

𝑑𝑁=𝑝𝑑𝐴(5)

Replacing (3) and (4) in (5), we get:

𝑑𝑁=2𝐹𝑎𝑥𝑑𝑥𝑟22–𝑟21(6)

The elementary friction force dF is calculated as:

𝑑𝐹=𝜇𝑑𝑁(7)

where μ [-] is the friction coefficient of the clutch disc.

Replacing (6) in (7), we get:

𝑑𝐹=2𝜇𝐹𝑎𝑥𝑑𝑥𝑟22–𝑟21(8)

The elementary friction torque dT is calculated as:

𝑑𝑇=𝑥𝑑𝐹(9)

Replacing (8) in (9), we get:

𝑑𝑇=2𝜇𝐹𝑎𝑥2𝑑𝑥𝑟22–𝑟21(10)

Integrating equation (10) from r1 to r2, we get the mathematical expression of the torque capacity of the clutch:

\begin{equation*} \begin{split} T_c &= \int_{r_1}^{r_2} dT & \\ &= \frac{2 \mu F_a}{r_1^2 – r_2^2} \int_{r_1}^{r_2} x^2 dx \\ &= \frac{2}{3} \mu \frac{r_2^3 – r_1^3}{r_2^2 – r_1^2} F_a \end{split} \end{equation*}
𝑇𝑐=∫𝑟2𝑟1𝑑𝑇=2𝜇𝐹𝑎𝑟21–𝑟22∫𝑟2𝑟1𝑥2𝑑𝑥=23𝜇𝑟32–𝑟31𝑟22–𝑟21𝐹 𝑎

The resulting mathematical expression for the torque capacity of a single plate clutch is:

𝑇𝑐=23𝜇𝑟32–𝑟31𝑟22–𝑟21𝐹𝑎(11)

For a multi-disc clutch expression (11) becomes:

𝑇𝑐=𝑧23𝜇𝑟32–𝑟31𝑟22–𝑟21𝐹𝑎(12)

where z [-] is the number of friction plates (discs).

Assuming a mean radius rm [m] of the clutch calculated as:

𝑟𝑚=𝑟2+𝑟12

we can deduce a simplified expression, with an acceptable error, for the torque capacity of the clutch:

𝑇𝑐=𝑧𝜇𝐹𝑎𝑟𝑚

[elpeterson]

Yeah see? Super simple to calculate.

In all seriousness I would say stock is good up to about 300ft/lbs. It can probably be pushed harder but I think at that point you are getting into the breaking point of other stock parts.

[Smac770]

The 2007 A5 SSP lends some specifics, relevant to that time.

Use ZF Sachs dry clutch, one-stage dual-mass flywheel, single 240mm plate clutch without torsion damper
0B1, ML311-6F, 350Nm (2.0 TSI I4, 2.0 TDI I4, and 3.2 FSI V6)
0B2, ML311-6Q, 350Nm (2.0 TSI I4, 2.0 TDI I4, and 3.2 FSI V6)

Use a LuK dry clutch, one-stage dual-mass flywheel, single 240mm plate with torsion damper
0B3, ML351-6F, 400Nm (for 2.7 TDI V6) - 0B3 105 266 (for B8.0 2.7 TDI V6 FWD), 0B4 105 266 G (for B8.5 3.0 TDI V6 FWD)
0B4, ML451-6Q, 440Nm (for 4.2 FSI V8, later 3.0 TSI V6) - 0B4 105 266 G

Use a LuK dry clutch, two-stage dual-mass flywheel, dual 228mm plates with torsion damper
0B4, ML451-6Q, 500Nm (for 3.0 TDI V6) - 0B4 105 266 H

240mm and 228mm, not to be confused with the 280mm overall diameter noted in ETKA, I assume.

CJEB uses the a LuK 0B1/0B2 unit, first 0B1 105 266 AG then 0B2 105 266 AD. The 0B1/0B2 transmissions as a whole are 350Nm limit, so I see no reason to believe the clutch was designed to handle anything more than that. Particularly since the engine is only rated at 320Nm max torque output (CNCD, Gen3 2.0T uses the same, and is 350Nm max output). I assume by this time, Audi was sourcing all varieties of clutch unit from both vendors.

Considering the clutch units are all made by LuK or ZF Sachs, I imagine if you determine the actual OEM part number, it would provide some reference to check with them regarding specs. But it doesn't really matter since you need the whole transmission for the car to work, and the system as a whole has a design limit.

[Bror]

Thank you all very much for the useful info!

Can I just ask if anybody knows what the S-tronic transmission dual clutch for the same model is rated for?

[Smac770]

The 0B5, DL501-7Q, as a whole is rated in the Audi SSP for handling 550Nm (S6/S7 4.0T V8 output limit); wiki page calls it 600Nm. US only has DSG on the S models; so for low end A/Q models in RoW, who knows if they used a lower end clutch unit or such.