| Get a Grip! - Clutch Guide
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.
Squishing the diaphragm spring to the pressure ring and disc is what provides the clamp load that keeps the clutch disc from slipping when power is applied to it by the engine. The pressure ring is held to the cover by thin, flexible metal strips called drive straps. The drive straps transfer the engine's torque from the pressure ring to the cover and helps the pressure ring retract, holding it against the diaphragm spring to keep it from rattling around when the clutch is disengaged.
To disengage the clutch to allow the engine to spin freely, as when coming to a stop or shifting, the clutch pedal is pushed in. The clutch pedal moves a pivoting arm in the transmission case, called a release fork, via a cable or hydraulics. The release fork pushes on the throwout bearing, which is a simple thrust-type ball bearing. The throwout bearing pushes on the center of the diaphragm spring. The diaphragm spring is visible in the center hole of the clutch cover and has multiple "fingers" that the throwout bearing rests against.
The throwout bearing pushes on the center of the diaphragm spring, bending it inwards. The diaphragm spring pivots about on a fulcrum, which is either a rim stamped into the clutch cover, riveted or bolted-in pedestals, or a round wire held in place with rivets around the inside of the clutch cover. The outer end of the diaphragm spring is attached to the pressure ring. When the spring pivots on the fulcrum as the diaphragm spring is pushed in by the throwout bearing, the clamp load is released on the pressure ring and the clutch disc is allowed to spin free, disengaging the engine from the transmission.
The worst thing that a clutch can do is slip under power when the clutch is engaged. A clutch that slips means that the engine's power is not getting to the drive wheels but is being converted into heat energy and not driving the car forward. To reduce the chances of slipping, a heavy-duty (HD) clutch will generally have a pressure plate with a higher clamping load and a clutch disc with a higher coefficient of friction and greater heat resistance.
To prevent slipping, high-performance pressure plates have stiffer diaphragm springs to give a higher clamp load. The more clamping force, the harder the pressure ring smashes the disc to the flywheel and the less likely the clutch disc will slip. Most aftermarket clutches have a thicker diaphragm spring or a stock spring that is re-stamped more conically for more preload and/or heat-treated for more spring tension. Some extreme clutches even have a double diaphragm spring, which is usually two stock springs stacked on top of each other.
Beware of going too crazy with clamping force; some clutches have so much clamping force that they can cause excessive wear on your crankshaft thrust bearings. A general rule of thumb is: Do not increase the clamp load more than 50 percent over stock.
Some HD clutches have beefier, thicker pressure rings to resist warping with heavier clamp loads and high heat. Several HD clutches have a cover stamped of thicker gauge steel to hold up to the increased clamping pressures of thicker diaphragm springs without flexing. This gives more consistent clutch action and improved clamping.
Some pressure plates have a ring of weights on a cable attached to the fingers of the diaphragm spring. These weights are supposed to offer a centrifugal assist, increasing clamping pressure at high rpm. Although this is an innovative idea, care must be taken to avoid overcentering with this sort of device.
Overcentering is when the diaphragm spring's fingers get pushed past the normal centerline of the diaphragm spring. When this occurs, centrifugal force acting on the weights and diaphragm spring keeps the fingers of the spring bent down causing the clutch to stay disengaged during high-rpm shifts. This may cause your motor to overrev or damage the transmission. Overcentering can occur with any diaphragm clutch, but centrifugal-assisted clutches are especially sensitive to this. If you can feel the pedal get lighter with increasing revs, that is the clutch wanting to overcenter. All diaphragm-type clutches are prone to overcenter to some degree so you want to be sure that your clutch is adjusted for proper throw.
Although the penalty for having a slip-resistant high clamp load clutch is a stiff, hard-to-drive clutch pedal, many HD high-performance clutches use some tricks to keep the clamping force high but the pedal pressure low. The most common trick is to increase the throwout bearing's leverage ratio by moving the diaphragm spring's fulcrum inboard so the throwout bearing has more leverage to bend the spring inward. This also makes the engagement slower, which can help make slipping a grabby clutch off the line easier. Another trick is to use a smaller throwout bearing. Some clutches come with a clutch slave cylinder with a larger bore to reduce pedal effort. The only disadvantage to this is that it can slow clutch release by a few milliseconds and therefore slow shifting somewhat.
A less common type of pressure plate is called a pull-type. Pull-type pressure plates are found on cars like the EVO, STi, some Porsches, and the Skyline GT-R. Pull-type pressure plates are also common on full race applications. A pull-type pressure plate releases by pulling on the diaphragm spring instead of pushing on it.
The throwout bearing is attached to the inside of the diaphragm spring. Pull-type pressure plates have a lighter pedal effort for a given clamp load and are more efficient in developing clamp force because the diaphragm spring's fulcrum is at the outer edge of the cover and the outer diameter of the spring. Because the clamp load of the diaphragm spring is on the outer edge of the cover, there is less stress and thus less flex in the cover. With less flex, a higher clamp load can be applied to the pressure ring. This reduction of flex allows a pull-type pressure plate have up to 30 percent more actual clamping force per pound of load that the spring makes over a conventional push-type pressure plate.
The pull-type pressure plate also has a much lighter pedal effort because the outboard fulcrum gives the release arm more leverage to flatten the spring and take load off of the pressure ring. Because the pull-type pressure plate's cover is near the bolted down periphery it reduces bending load on the cover. As such, the cover can be both smaller and lighter in construction, thus reducing rotating weight. These features are the reason why many true racing clutches are pull-type.
Now that you are familiar with the operation of the pressure plate, in our next installment we will discuss the ins and outs of an even more complex subject, the clutch disc.