Clutch Torque Testing
A different way to test your clutch's ability to control torque, and apply it to the ground
During your next engine rebuild, or perhaps the next time you pull your engine to replace a flywheel seal, here is a basic test that you can perform to check your clutch’s condition. Normally, about the only way you can check your clutch’s life span is to measure the thickness of the clutch disc, looking for wear, or stepping on the pressure plate collar with your foot to feel the resistance. But unless you measured it when it was new, whatever thickness or readings you may come up with may be meaningless. Professional engine builders keep a log book on every engine they build, and within the pages of these notes are engine specifications that are stored for future reference. Included in these notes should be the thickness of the clutch disc, and the clutch and disc’s “torque breakaway point,” as we will show you here.
As you know, when your engine is running, and the clutch pedal is not depressed, the clutch is engaged, or locked up. In other words, the pressure plate is applying enough force to the disc to prevent it from spinning freely, thus transmitting the engine’s torque directly to the transaxle. Now, once you depress the clutch pedal ,the pressure plate disengages (releasing pressure applied to the disc), and that disc is now free to spin independently. Since the disc is hooked directly to the transmission, via the splined input shaft, when the disc is spinning freely the transmission is no longer under a load, making shifting gears possible.
As the condition of the pressure plate deteriorates (both in spring pressure and surface smoothness), and the thickness of the clutch disc reduces, the clutch loses its ability to control engine torque and provide smooth transition from engaged to disengaged. When engine torque overcomes the clutch’s ability to control it, you have slippage. The result is poor shifting or grinding of gears, jerking, or all-out engine free wheeling!
Each clutch and disc combination has its own “breakaway point” if you will, since not only is the pressure plate spring pressure important, but also how aggressive the disc surface is, along with flywheel smoothness and depth. Since there is a factory tolerance for this depth, a flywheel that has worn beyond it offers less preload than one that is still at the proper height. This is also true of the disc thickness. If you change out your disc with one that is different (either thicker or thinner), the pressure plate will effectively change its clamping power.
So to get things rolling you need to locate a used VW Type 1 input shaft, and cut it in half. Next, find a good impact 1/2-inch-drive socket, and MIG or TIG weld it to the end of the newly cut shaft. You should end up with a shaft that has the tapered point with splines on one end, and the drive end of a socket on the other. Next, you will need a good quality 1/2-inch-drive torque wrench to measure the gripping force of the clutch, one that reads up to 250-ft.-lbs. The idea is to install the disc and pressure plate, and tighten them down as normal. Next, insert the end of the modified input shaft into the disc splines, and push it in all the way. You will need to lock the flywheel with a suitable device, to prevent it from rotating, just as you would when you tighten down the flywheel gland nut. You are now ready to check your clutch.
If you are using an adjustable torque wrench, turn up the setting to at least 80-ft.-lbs. If you are using a high performance clutch, you will probably have to raise that number up as you go along. The idea is to find the point at which the disc breaks free from the pressure plate’s clamping force. As you attempt to apply force to the disc, if the torque wrench is set too low, you will not be able to move the disc at all (remember that the flywheel is locked). So, readjust the wrench and try it again. You will eventually reach a point that the disc breaks free, and at that moment, check your torque wrench’s setting, and record that number.
If you have a reference number for a brand new clutch and disc, you can compare the two and see where you now stand. Racers who use an adjustable clutch, such as the J&G, take measurements constantly to keep track of their clutch’s condition. Since repeatability is utmost important, as the clutch wears this type of setup can be adjusted to make up for that wear and maintain a constant pressure.
What we found by checking a number of different 200mm setups is that a fresh heavy-duty Sachs clutch’s breakaway point is about 145-ft-lbs. A KEP Stage 1 is approximately 160-ft.-lbs., a Stage 2 about 170-ft.-lbs., Stage 3 185-ft.-lbs., and a Stage four about 195-ft.-lbs. All of these readings were taken with a Bugpack Pro-Grip disc. If we changed that disc to a Copperhead disc from ACE (Air Cooled Engineering), those readings raised about 40-ft.-lbs. If we added a Bugpack 3-puck racing disc, those readings basically stayed the same as the Pro-Grip. And, just for drill, we bolted up a well used stock clutch and those readings dropped down to just 95-ft.-lbs. — 2/3 of what a new unit measured. Time to replace.