The first time my father took me out driving, it was behind the wheel of his rusty but still running six-cylinder ‘88 F-150. This was a bare bones truck that my father had bought for his contracting business, and standard equipment back then was a five-speed transmission with a shifter a mile long. The truck was as fast as a retired snail, but it could lay down an impressive smoke show if you dumped the clutch, which I quickly discovered on the day my dad took me out driving. Even as my father was yanking out the last few hairs on his head, I was blowing smoke shows up and down the elementary school parking lot – at least until I fried the clutch.
That was my first experience with both the importance of a good clutch, and the pain it can be to replace a torn up clutch. The clutch is the ambassador between the engine and transmission, transferring torque and power from the flywheel through the transmission to either the driveshaft or drive axles, and putting the power to the ground. But clutches can be tricky devils, and picking the right one for your application can have a dramatic effect on your drivability and power capacity. That’s why we turned to the pros at SPEC Clutches and Flywheels to answer ten questions about how clutches work, choosing the best part for your application, and what goes into designing and manufacturing high power holding clutches.
Who is SPEC?
Based out of Bessemer, Alabama, SPEC Clutches and Flywheels was founded by enthusiasts and racers who were motivated to build a better product than what was available to them. SPEC assembled a team of experienced engineers, clutch builders, machinists, and other automotive enthusiasts to build the company, which began cranking out clutches all day in a 5,000 square foot building.
They now reside in a purpose-built 30,000 sq. ft. state-of-the-art facility, where they design, test, and manufacture all of their products in-house. This means that your clutch, pressure plate, and flywheel are all designed under the same roof and are therefore optimized to work with each other, giving you the very best results. Shelly Norton of SPEC was kind enough to take some time to answer our questions about how clutches work and what goes on behind the scenes.
A single disc clutch kit.
PowerTV: What is the difference between single, dual, and triple disk clutches?
SPEC Clutch: The single disc setup is the most common design found in production automobiles and consists of one friction disc contacting two friction surfaces (the friction surfaces are the pressure plate and flywheel).
The dual disc adds a friction disc and set of surfaces (called a floater). The addition of a second friction disc and surfaces, given an equal load pressure and diameter, nearly doubles the torque capacity of the clutch assembly.
The triple disc consists of three friction discs and can increase capacity by nearly 200% beyond a single disc. The practical benefit of the multi-disc is a higher torque capacity without an increase in the aggressiveness of the unit. A properly tuned multi-disc will offer a much higher capacity than an aggressive single disc and with much better drivability.
PowerTV: How do you go about designing and building a clutch-pressure plate-flywheel package that can stand up to HUGE (800+) horsepower?
SPEC Clutch: The key to an effective performance assembly is to balance clamp load, friction, diameter, and rotating mass to achieve the best solution for a particular application. That’s why SPEC builds to order in-house. We encourage builders and end users to call us for a recommendation so we can have all the information possible about the vehicle.
We may have three or four different stages that will hold well, but when we know more about the environment in which the vehicle will be driven, we can fine tune the recommendation and offer an even better solution. For example, a dedicated road race 800hp Corvette should require a slightly different recommendation than an 800hp Corvette that is road raced but also driven to work and to the drag strip a few days a week.
PowerTV: What have advancements in technology allowed you to do that wasn’t previously possible in the past decade?
SPEC Clutch: Friction material technology has greatly evolved in the last decade, but cutting edge production machinery and test equipment have allowed us to accelerate and expand research and development. It is the knowledge from research and development that provides a direction for technology.
A triple disc clutch for serious applications that would break other clutches in twain.
PowerTV: How does one upgrade or downgrade a clutch? We’ve never heard of that. What specifically does that entail or cost?
SPEC Clutch: Upgrading or downgrading a clutch can consist of changing up to two possible elements, clamp load and friction disc. If someone did not order enough clutch for their application, they could upgrade the friction disc with a higher friction coefficient material, increase the clamp load of the pressure plate, or both.
A driver who purchased too much clutch for their application (meaning they want better drivability and don’t need all the capacity of their current unit) can change their friction disc to a less aggressive configuration and/or have the clamp load of the pressure plate reduced. The process entails sending the clutch to the manufacturer for the upgrade or downgrade. Not all manufacturers have the ability or willingness to execute such a task. The cost depends on the application, but is normally less expensive than purchasing a new clutch.
PowerTV: What does SPEC need to know to help someone pick the right clutch?
SPEC Clutch: They need specifics on their application, such as what they drive, where they drive, horsepower and torque output, and the nature of the modifications responsible for that output. Other factors are important as well, such as vehicle weight and gearing, if greatly different from stock.
PowerTV: How does SPEC recommend breaking in a new clutch?
SPEC Clutch: Different clutches require different break-in procedures. In general, the break-in procedure entails normal usage to seat the new surfaces. Normal usage does not mean babying the clutch, but simply refraining from burnouts and high RPM speed shifting until the clutch is ready to hold the torque. Premature aggressive driving can take wear life from the clutch, produce heat related issues that can effect drivability, or even permanently damage the clutch and surfaces.
PowerTV: I see you have eight stages of different clutches. What applications might a stage one use versus a stage four or five?
SPEC Clutch: We offer seven stages of single disc clutches and up to eight stages of multi-disc clutches. An application that requires a stage three would be more modified than an application that requires a stage one. It must be noted, however, that in a lot of applications a stage one can still hold power adders, so the stage one is not always restricted to a mildly modified setup.
The pressure plate, which grabs onto the clutch and transfers the power from the engine through the transmission.
PowerTV: How about the flywheel and pressure plate? What differences can those two pieces make in the grand scheme of things?
SPEC Clutch: The flywheel and pressure plate are 2/3 the equation, so both are VERY important. Our kits always include a performance pressure plate and sometimes include a performance flywheel. Billet flywheels are an optional item on most applications, but are always recommended for optimal clutch performance, safety, rotational integrity and balance, as well as additional power.
An aluminum flywheel. A lighter flywheels spins more easily, siphoning off less horsepower from the engine which means more for the wheels.
PowerTV: What are the different kinds of metals you use for flywheels, and what are the pros and cons of each?
SPEC Clutch: We use high carbon steel and aircraft grade aluminum. Given the quality of the materials, there are no cons to either material. However, the weight of the flywheel can change drivability characteristics, as well as add performance. A flywheel recommendation from a SPEC technician is recommended.
This is the beginning of the milling process for the flywheel, which will bore and drill mounting holes and dowel pin locations.
PowerTV: What process do your clutches go through to ensure they can survive multiple thrashings without burning up?
SPEC Clutch: The entire manufacturing process is engineered to ensure the clutches are built to survive the demands of a performance clutch application. In addition to our normal post-manufacturing quality control procedures, during which each unit is tested for clamp load, actuation, and concentrically, prototypes are put through the best test environment: real world use. We prototype and install in-house, as well as conduct abusive street and track tests with all new applications.
There is some great information here, but you may still be scratching your head wondering, “How exactly does a clutch work and why do we need them in the first place?”
It’s very simple really. When you press the clutch pedal down, it releases the pressure plate from the clutch disc and the flywheel, where all the engine’s torque and power are stored. Releasing the clutch pedal clamps the disc to the flywheel, transferring that power to the transmission and to your wheels, providing propulsion. The clutch disc is attached to the input shaft and is always spinning when the pedal is out, whether or not the car is in gear. Only when the clutch pedal is depressed does the disc stop spinning, allowing you to slip the transmission into gear.
The mechanics behind how a clutch works haven’t changed much in the hundred years of mass transportation, at least in any way that the typical driver would notice. Clutches have evolved from mechanical linkage to hydraulic, which provides easier and smoother gear shifts over older mechanical linkages.
Fork actuation has also been replaced by concentric hydraulic bearings, which eliminate the external slave cylinder, fork, and standalone throwout bearing. But as far as you and I are concerned, clutches work the same today as they did when Henry Ford first began cranking out the Model T for the masses.
If you need more technical answers or have questions regarding your specific application, the gearheads over at SPEC will be happy to help you out. You can call their tech line at 205-491-8581 and their experienced technicians will be able to sort out any clutch problems you may have. Just don’t ask them the meaning of life, that’s for Miss Cleo.
So What Have We Learned?
• Double disk clutches can double the torque capacity of a clutch, and triple disks can increase it by as much as 200% over a single disk.
• Upgrading or downgrading a clutch can help you find the right balance of drivability and power capacity for your application, usually at less cost than buying a whole new clutch kit.
• Prototype clutches are put through real world testing on the street and track to ensure they are durable enough for real world use.
• Stage one clutches can hold power even with a power adder, so a stage two clutch or beyond may not be necessary. Speak with a SPEC technician for specifics regarding your application.
• Flywheels and pressure plates are 2/3 of the clutch equation and can have a dramatic effect on performance, durability, and drivability, so make sure all your parts are in agreement with each other.
• Breaking in a clutch does not require babying the car, but rather merely refraining from burnouts and speed shifting until the clutch is settled in. Premature aggressive driving is a good way to destroy even the strongest clutches.