clutch

Clutch

When you're gunning your car down the road, a lot of variables have to converge properly in order to complete that fast and furious gear change. Putting the power to the transmission means relying on a disc that's a mere 5mm to 8mm thick. The disc is abused in unfathomable ways. At launch, it's at a standstill one instant, then it's smashed between two pieces of billet-aluminum and spinning at 4000 to 6000 rpm. The shock, friction and heat generated by this act make the clutch one of the more remarkable automotive component systems.

Bare Basics

The basic clutch setup consists of two sub-assemblies--the clutch disc and the clutch cover--the latter of which is home to the prized pressure plate and diaphragm spring. The clutch disc lives between the flywheel and the pressure plate and its front and back surfaces are made up of friction material. The disc is ground zero, where the engine and transmission are physically joined. The disc is connected to the transmission input shaft via splines in its center section. The clutch cover is connected to the flywheel.

The metal ring on the backside of the clutch cover is the pressure plate. When it's forced onto the clutch disc and pushes the clutch disc onto the flywheel, the engine and transmission are joined. Full-surface contact between pressure plate, clutch disc and flywheel is key. The clutch cover is bolted to the flywheel and it always spins at engine speed.In a static state, the engine and transmission are connected. When the driver pushes the clutch pedal in, either a hydraulically actuated piston or cable release pushes in on the release fork, which presses the throw-out bearing (aka release bearing) against the center portion of the finger-like diaphragm spring. As the spring is compressed inward, an array of pins or other mechanisms on the outside of the unit work to lift the pressure plate off the clutch disc. Think lever effect and you're on the right track. In this instance, the clutch disc is freewheeling on the transmission shaft, where it slows considerably, allowing for gear changes.

When the driver takes his foot off the pedal, the pressure plate re-engages the clutch disc and the engine and transmission are connected. The center hub of the clutch disc is fitted with springs, which help lessen the shock of re-engagement.

Troubleshooting

What causes clutch chatter?
A high coefficient of friction, bad motor mounts, an incorrectly machined flywheel or faulty pressure plate, spring-centered race disc (aggravates chatter), and contamination (oil, grease, or rust).

Chatter is basically the engine winding up in the mounts as the clutch is engaging and then bouncing off the mounts, disengaging the clutch for a brief moment before engaging again and winding up again. No matter how aggressive a clutch is that would normally chatter severely on a normal street car, it generally won't chatter at all on a car with solid mounts and a rigid disc because there is no wind-up effect.

What causes clutch slippage?
Oil contamination, worn friction material, inadequate friction or clamp load for power.

What results in poor shifting, a shifter with a clumsy feel to it?
A faulty pressure plate (won't lift from disc), damage upon installation causing a bent disc, failure to adjust a clutch pedal or cable properly, or poor choice of parts (sintered-iron disc, wrong double-disc setup).

Why is clutch break-in so important?
Recommend 200 to 300 miles of mellow usage for street discs. The purpose of breaking in a clutch is to engage the disc in a controlled, consistent manner to ensure the disc, pressure plate and flywheel all make full surface contact. It's also a chance for the friction material to experience normal heat cycles.

Replacing of Clutch
Approximately 100,000km or early. When you realised it slightly difficult to engage in gear.