Ever wonder what is perhaps the second-most important thing that makes your car move? You probably guessed that the engine is the most important thing so naturally, the clutch is the second-most important part of the equation. Although the clutch is the most critical component in your drivetrain, most enthusiasts have very little knowledge of how a clutch works, what are important attributes to look for when selecting a clutch, and how to pick the right clutch for their application. The right choice will get you moving and keep you moving reliably. The wrong choice may lead the clutch to self-destruct, quickly wear out, not work according to your expectations, or worse yet, break other parts.
How do you choose the right clutch and how does a clutch work? There is very little information available on this subject as a clutch is not as sexy as a 1,100hp B-series motor, nor is it shiny and out for all to see like an exhaust system. To get some help on this subject, stay tuned and we'll help guide you through all things clutch in the next couple of installments in this series.
What does a clutch do?Not to offend anyone by being too simple, but starting with the very basics, a clutch is needed because a car's engine is always spinning, even when the car's wheels aren't. A clutch is a device that couples the engine to a manual transmission. The clutch is a direct coupling device that solidly locks the engine to the drive wheels under power and uncouples the engine from the wheels while at a stop, during engine start-up, and while shifting. This is a very important and difficult job.
The clutch should engage with a smooth and progressive action so the car is easy to drive smoothly without shuddering and stalling from a standing start. Smoothness also helps to keep the stresses out of the transmission and differential when shifting. The clutch should also hold the engine's power under any load when engaged without slipping. The best clutch for this normally is the stock clutch. However, all this goes out the window when you modify your engine or drive a car in competitive events.
With today's technology, modifications such as forced induction and nitrous make it possible to double, triple, or even quadruple your car's horsepower over stock. It is fine to go fast but there is no way your poor stock clutch is going to be up to that task. Your stock clutch was not designed to do repeated drag launches, consistent shifts from redline, and other tortures that enthusiasts place upon them.
The clutch has to absorb a tremendous amount of abuse and heat, especially when doing drag racing starts and speed shifting. Doing clutch kicks to initiate a slide in drifting are also incredibly punishing. Road racing means many high-rpm shifts over long periods of time. Even slipping the clutch to carefully creep your race car onto the trailer to take it home after the races is extremely hard on the clutch.
How do you know if your clutch isn't up to snuff? The main indicator is common sense. Just installed a 100 shot of NOS? Just finished installing a turbo kit? I can tell you right now that your stock clutch won't last long and chances are as soon as you push the "too soon, junior, button," you'll hear the engine rev with no increase in acceleration and a horrible burning smell to match. Not sure what was up with that, so heck, try it again and your clutch will probably really be toast the second time around.
What do you do now? Well fear not, there are heavy-duty clutches made for nearly every compact car on the market, and if not, there are a few companies that specialize in making custom clutches to solve your problems. The trick is picking the right clutch for your application. Sit back and listen; I'll explain some of the ins and outs of clutches to help you decide. Let's first look into the basic construction of a clutch, starting with the most basic of its parts.
Parts of the clutchA clutch consists of two parts, the disc and the pressure plate. The disc contains the friction material and is coupled to the input shaft of the transmission. The pressure plate contains the diaphragm spring, the pressure ring, and the cover. The pressure plate bolts to the flywheel, which bolts to the engine.
The pressure plateThe pressure plate design that just about all compact cars use is called a diaphragm type. The main advantage of the diaphragm pressure plate is that it has a light pedal feel for the amount of clamping load delivered as well as smooth, linear engagement. Domestic cars, trucks, and pure drag racers can sometime use other pressure plate designs such as the Borg and Beck or the Long type designs. We won't get into those because the diaphragm clutch is what most performance clutches are nowadays.
Diaphragm clutches use a circular, cone-shaped spring aptly called a diaphragm spring to apply clamping force to the pressure ring, a heavy circular plate of cast iron that pushes the clutch disc against the flywheel. The flywheel is a solid piece of metal that bolts to the engine's crankshaft. The clutch disc has splines that engage the input shaft of the transmission.
The clutch assembly is contained within the clutch cover housing. It is usually made of stamped steel that bolts to the flywheel. Some racing clutches have covers made of machined billet aluminum for stiffness and lightness in weight. The diaphragm spring is squished under the cover when it is bolted tightly down to the flywheel. This presses the pressure ring and disc hard against the flywheel, forming a direct coupling from the engine to the transmission. Now power can flow from the engine to the transmission. In this state, the clutch is engaged.