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Clutch Tech - Friction To Move

Posted in How To on February 1, 2006
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On a vehicle with a traditional manual transmission, engine output is routed through a clutch mechanism to the geardrive assembly. A flywheel bolts to the end of the crankshaft on the rear of the engine. A pressure plate bolts to this flywheel and a clutch disc is sandwiched in between the two.The center of the clutch disc has a coupler with internal splines. These mate to the splined input shaft of the manual transmission. Thus, the clutch disc is solidly coupled to the transmission. It also has a set of friction plates on each side of the disc. These come in contact with the flywheel and pressure plate.

Both the flywheel and pressure plate are solidly connected to the crankshaft. When the pressure plate pushes the clutch disc against the flywheel, the entire assembly is tightly coupled and drive power is transferred from the engine to the transmission.

Whenever you press on the clutch foot pedal, you actuate a hydraulic master cylinder that in turn operates a hydraulic slave cylinder and moves the clutch fork on the transmission bellhousing. Some vehicles may use a mechanical clutch linkage instead of the hydraulic system, but the action at the bellhousing is similar. The clutch fork pivots in the bellhousing and a throwout bearing is attached on the end inside the bellhousing. The throwout bearing presses on the pressure plate, which flexes and releases the pressure being applied on the clutch disc. This action separates the drive of the engine from the transmission and allows you to stop the vehicle without stalling or shift to another gear.

The pilot bearing is a small roller bearing that resides in a recessed area in the center of the flywheel. A small tip on the transmission input shaft fits into the inside diameter of this bearing. The pilot bearing helps support and align the shaft to allow the splined clutch disc to move freely.

In general, clutch systems are quite reliable when used within reasonable limits. The components will last many tens of thousands of miles and can last more than one hundred thousand, depending on quality and how they are treated. Clutch plate wear is accelerated whenever the clutch is allowed to slip excessively, such as when towing heavy loads, running large tires with too high axle gearing, or simply not engaging the clutch promptly when leaving from a stop or shifting gears.

On a hydraulically actuated clutch, you should periodically check the fluid level to ensure there are no leaks at the master or slave cylinders. Also, if you live in a relatively wet climate, you should consider flushing and replacing the fluid in the system every couple of years. On a mechanically actuated clutch linkage, you should inspect the components for wear and lube pivot points as needed.

A clutch should be adjusted periodically to compensate for clutch plate and linkage wear. There should always be some slight freeplay at the pedal when the clutch is fully released. This ensures that the pressure plate is allowed full clamping force on the clutch disc. A clutch that is adjusted too tight with no freeplay can allow disc slip and premature wear of the clutch components. The pilot and throwout bearings should require no maintenance and come factory-lubed to last their service lifetime.

Typically when it's time to repair a clutch system, it will usually be because the disc is badly worn and is now slipping. In this case, the disc is replaced with a new one. Often, to complete the job, a new pressure plate is installed. Over time, the spring pressure on the pressure plate will decline and the fingers may also show wear from use and contact with the throwout bearing. The contact surface on the pressure plate may also become warped or uneven over a long period of use. Additionally, the flywheel surface can become distorted. In such a case, the flywheel is removed from the crankshaft and can be resurfaced by a local auto machine shop for a small fee. During a complete clutch install, new pilot and throwout bearings are typically installed.

When performing clutch troubleshooting or repair, be sure to check the condition of the clutch fork and make sure the pivot area is greased. It is possible for the clutch fork to crack and flex, causing poor clutch disc release and possible gear grinding.

Clutch Choice
There is a wide variety of aftermarket clutch components available for most 4WDs. In general, if you are running a stock or near-stock engine, a quality stock-type clutch assembly will often work fine for you. However, if you've significantly boosted your engine power, you'll probably want to consider an upgrade to a clutch assembly designed for greater holding power. This allows you to push more torque through it without slip or excessive clutch plate wear. Heavy-duty towing or large tire applications can also benefit from a clutch upgrade.

When choosing an aftermarket clutch for daily-driver use, you'll want to inquire as to how much, if any, additional leg effort is required to operate the clutch. A heavy clutch effort may be fine for a short trip down the dragstrip or for other racing applications, but won't be very pleasant on a slow trail or when you're stuck in congested traffic. Also note that high-performance aftermarket clutches may tend to grab more quickly than a stock clutch. This action can make engagement a bit jerky and make it harder to feather the clutch for a smooth takeoff from a standstill.

Another possible modification to your clutch system is to change the flywheel for one that is lighter or heavier than stock. A lighter flywheel allows the engine to rev faster and holds less spinning inertia. A heavier flywheel slows engine rev and can make power delivery smoother. Once the heavier flywheel is spinning, it will hold more momentum and the engine will be less prone to stalling at low speeds.

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