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Clutch Construction 101 (archive)

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Posted by David on April 24, 2000 at 07:20:06:

In Reply to: Re: Maybe you shouldn't limit clutch pedal travel. posted by kwillmorth on April 23, 2000 at 23:28:41:

It seems that not everybody understands how a clutch works. The friction disc is the meat in a sandwich, being between the flywheel in front and the pressure plate behind. The disc (with lining on both sides) is free to slide fore and aft along the splines of the transmission input shaft. The pressure plate is a metal ring with a flat face that's pressed against the back face of the disc by a powerful spring. The release (or throwout) bearing sits around the center of the shaft and is on the end of the clutch fork. When you step on the clutch pedal, you press the bearing forward against the center of the pressure plate, where a radial lever system of some kind causes the pressure plate's bearing surface to move away from the rear of the disc (there are several different kinds of pressure plates, the commonest one today being a circle of radial spring "fingers" - old English cars have three discrete levers with riveted pivots and coil springs).

The entire clutch assembly is bolted to the back surface of the flywheel and spins with it. Power is transferred to the transmission through the splines on the input shaft. Depressing the pedal does not move the disc directly. It releases the pressure hlding the disc against the flywheel, decoupling the two. Although the disc does move away from the flywheel some thousandths of an inch, the gap itself is not important as long as there's no pressure between disc and flywheel. The last inch of pedal travel may be releasing the last bit of pressure.

Over-travel of the throwout bearing against the pressure plate can actually cause pressure against the hub of the friction disc. In some cars, the back of the rotating friction disc will rub the release mechanism (throwout bearing etc) and cause damage and an AMAZING noise.

I hope this helps you understand my point of view.

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