Clutches Explained: ACT, McLeod Racing, and SPEC Weigh In

CLUTCHESEXPLAINEDIf you’re a typical driver, the most thought you’ve probably ever given to how a clutch works, is when you’re stuck in bumper-to-bumper traffic in a manual transmission vehicle. Depress the pedal, pick a gear and in delicate unison, release the clutch pedal while applying power to create forward movement. Drive and repeat. The sheer volume of that repetitive movement may lead you to wonder – what’s really going on here?

Clutches, whether for manual or automatic transmissions aren’t only reserved for cars though. The clutch connects the two shafts so that they can either spin together at the same speed, or it disconnects them, allowing them to spin independently.

We had the opportunity to talk with the representatives for three of the leading clutch manufacturers, Advanced Clutch Technology (ACT), McLeod Racing, and SPEC Clutches. We asked them the same three questions and their thoughtful viewpoints and educational responses are guaranteed to advance your clutch selection IQ. 

An exploded view of a typical manual gearbox car clutch -

An exploded view of a typical manual gearbox car clutch –

The Function of a Clutch

In the simplest terms, in a manual transmission the driving force is the friction between the clutch plate, clutch disc and the flywheel. When the clutch pedal is pressed a cable or hydraulic piston pushes or pulls on the release fork, which presses on the middle of the diaphragm spring, causing the spring to release the plate from the clutch disc and disconnects the clutch from the engine. This interrupts the power transfer which allows the engine to keep spinning and lets you shift without munching gears.

When your foot is off the pedal, the springs push the pressure plate back against the clutch disc, which in turn presses against the flywheel. This reconnects the engine to the transmission input shaft, so that they turn together, and off you roll.

With friction force being the most important law of physics in this scenario it’s easy to understand that the most common problem with clutches is that the friction material on the disc wears out. As the lining on the clutch disc wears over time, the clutch starts to slip and later, forward momentum can be affected.

clutchIMG_86952014Friction force falls under the larger overall category of “contact force,” and it’s defined as the force exerted by a surface as an object moves across it, or makes an effort to move across it. There are at least two essential types of friction force, sliding and static friction and both exist in an automotive clutch environment. In short, choosing the correct clutch disc material for your car’s upgrade is significantly important, as is how that specific material is put into action, via full-face discs versus the various puck-style disc configurations. Selecting the proper clutch for your car of course then breaks down into how you intended to use your car. Is it comfortable as a daily commuter, or do you take it to the track on the weekends to fulfill your need for speed. Or, are you specifically searching for a clutch to deliver the ultimate racers edge and every mph of performance possible?

Briefly discuss the various clutch material your company uses, and how do these different materials distinguish themselves in terms of application and performance?

Chris Bernal – ACT: “While there are currently many, many choices for friction materials, the majority of ACT clutches are shipped with either Performance Organic (metal reinforced, woven material) or sintered ceramic (cerametallic composite). Some markets we serve require sintered iron, such as certain drag racing series and professional drifting.”

clutchDSC_0724-640x4252014“The Performance Organic option is best suited for street performance, where smoother engagement is important. Occasional aggressive launches or drag races are well-tolerated, yet excessive slipping will overheat any organic-based friction material. If not overly abused, ACT’s material will recover after it cools to serve as it did previously. Performance Organic is the best choice for slightly, to somewhat heavily modified street vehicles that must retain good drivability.”

“Customers choosing the Sintered Ceramic material trade off smooth, easy engagement for higher torque capacity and better thermal management near the limits. Typically, this material will handle repeated launches with no drop in performance. It takes a little more finesse to casually engage a clutch with this material, so it is not as friendly for street use. Also, street driving involves much more low-speed engagements, which wears this type of material faster than an organic. For cars that are highly modified and experience serious track time, Sintered Ceramic is often the best choice.”

clutchDSC_07232014“Sintered Iron is for hard-core racers seeking fine torque control through clutch modulation. This material has nearly a flat torque curve, meaning it responds the same time and time again. Perfecting a drag race launch is a much less complicated task with these discs. Drifters running turbocharged engines can keep those big turbo(s) spooled up by modulating the clutch more precisely with sintered iron.”

“Other materials have found use in special projects or R&D exercises, such as carbon-carbon, carbon-ceramic and other newer materials. We are very excited about the future of friction materials as manufacturing processes continue to evolve across various industries. We are evaluating some now that likely no one would have thought could have been friction candidates just a few years ago.” 

Billy Mieczkowski – McLeod Racing: “McLeod uses three different style of disc facings, Organic, Ceramic (semi-metallic), and Sintered Iron. The organic is for your base model entry level stock replacement clutch. This is good up to about 400 foot-pound torque at the crank. Then we offer an organic-ceramic mix, which is our 500 Series disc. This is for the customer that has modified their car slightly maybe heads and a cam, small shot of nitrous, a small supercharger good up to 550 pounds of torque at the crank.”

clutchDSCF60592014“From there we go to our 600 Series disc which is going to be ceramic semi-metallic on both sides of the disc. This is for the customer that has a little bit more nitrous or a little bit more boost, good to 650 foot-pounds of torque at the crank. And lastly we have Sintered Iron; this is for the full race application.”

“We offer two compositions, a 5135 and 5191 compound. The 5135 compound is typically used in a drag racing application where the coefficient of friction is greater and the window of modulation (slipping the clutch) is less. We typically see 1200 to 1300 hp and torque at the crank. The composition 5191 is typically used where someone needs to slip the clutch a lot, most commonly used in truck pulling. This disc has a low coefficient of friction and allows you to slip the clutch a lot more so it has a larger modulation window. This also has a torque rating of 1200 to 1300 foot pounds of torque at the crank but this one will wear a lot faster.”

David Norton – SPEC Clutches: “Yes. The disc material friction coefficient, coupled with the pressure plate clamp load, surface diameters, configuration and a few other factors, determine the torque capacity of the assembly.  A more aggressive material will have a higher friction coefficient; and, therefore, will hold more torque at the same clamp load.  Generally, higher friction coefficient materials produce reduced engagement quality, but recent material technology has produced high coefficient materials that are more manageable than old school high performance materials like ceramic and full metallics.” 

clutchIMG_44662014“All of our units are built for abusive driving at their ratings and even our Stage 1 and Stage 2 clutches are substantially stronger and more durable than stock units.  Our carbon graphite and Kevlar hybrids can produce the drivability offered by more stock-like materials, so the Stage 1 organic units and Stage 2 Kevlar units are not the only choices for smooth operation. We have a mission at SPEC to provide the best possible drivability with our units that can handle extreme torque levels.  So, any of our stages through 3+ are considered ‘street-friendly’ units, though they are built for extreme application.”

“Stages 4 and 5, due to the rigid hub configuration, are street-able but not street friendly.  In multidisc applications, some of the rigid configurations are more street friendly due to the progressive nature of double and triple the surface area.

Our Stage 3+ material is the flagship, offering the highest level of wear life, capacity and drivability at its rating. The carbon/carbon is the most advanced and costly to manufacture, offering an extremely lightweight option, high torque capacity, unparalleled heat threshold and softer engagement.  The most valuable characteristic of the carbon seems to be its ability to dampen driveline shock under the most extreme engagements.”

In regards to full-faces discs versus different puck configurations, what are their different impacts on power capability and engagement from your perspective?

clutchDSC_07262014Bernal: “A full-face disc provides the smoothest engagement properties and the most energy absorption potential. This is why almost all street cars are fitted from the factory with designs in this category. More energy capacity means lower temperature rise in the material for each engagement. Puck-style discs are advantageous when using heavier friction, like the sintered options. Reducing some of this additional weight helps regain some lost shifting speed that a full-faced sintered disc would otherwise cause. Less surface area does not necessarily equate to less torque holding ability, yet usually means more wear as each puck must work harder. It also results in more abrupt engagement, making it feel more like a full racing clutch.”

Mieczkowski: “The segmented or puck style facing is used where the customer may see more heat with his/her driving style. Such applications would be drag racing, drifting, auto cross etc. These facings have a narrower window of modulation compared to a full-face organic facing. They also have the potential for clutch chatter compared to their organic counterpart. The 4 paddle puck style disc is commonly found in a street/strip vehicle where drivability is not a concern. Our 600 Series, which is also a segmented style facing, has multiple smaller pucks around the circumference of the disc facing. This is a better choice for drifting and autocross where it works better with the more heat you put into the disc.” 

clutchIMG_44802014Norton: “Clamp load is distributed equally across the surface of the disc material. Reducing the number of pucks increases the pressure per-square inch of the material. Torque capacity effects are negligible, but 3 and 4 puck units produce quicker shifts due to reduced disc mass. Drivability is affected by increasing the rate of engagement, and ultimately thermal efficiency is decreased. Driving environment, modification level and surface area of the unit are factors that determine what puck configuration is most beneficial.”

Who would you say makes up the largest segment of your consumer base?

Bernal: “Back in the early days of ACT, the consumer base consisted largely of import drag racers, as there was a huge need for better equipment for those cars in the ‘90’s and not many places to turn to for workable solutions. We still proudly service many in that sector, yet have grown to include many American domestic market segments and a rapidly growing European base. In each vehicle segment, there are customers using their cars for all sorts of activities, so we cater to them in as many ways as possible. We find that the harder someone pushes their car and the more demanding the environment for the clutch and flywheel, the more likely they are to look to ACT for solutions.”

clutchIMG_86802014Mieczkowski: “Our number one market is our street market by far. We have been known in the industry to produce a great clutch for the track, but it is a smaller percentage of customers in the overall big picture. We really strive to make a great clutch for the high-performance street market. Drag Racing, Truck Pulling and Drifting comprise a smaller segment of our business, though just as equally important.”

Norton: “It is a combination of street and multipurpose street applications, meaning cars that are driven during the week and/or weekend and transformed for racing events. Drag racing (along with mile and half mile) and road racing usage lead the track environments, with drifting, rally, Auto-X and pulling to follow.”

As with any complex or hard to understand automotive part, increasing one’s knowledge and learning about what’s actually going on becomes the first step toward understanding. Thankfully, with valuable insight from three of the most prominent clutch manufacturers, each of us can be better prepared to make an educated decision the next time we are in the market for a performance clutch and choose the right one for our application.

About the author

Olaf Wolff

Olaf Wolff discovered the joys and passion of off-roading at age 11 and since then has made it a point to enjoy motorcycles, go-carts, buggies, and anything with a motor. He has been writing about off-roading and travel adventures for more than 30 years, and has published four DIY books. Olaf previously served as associate editor for Modern Cycle and Mini-Bike BMX Action, both national publications, and as a contributing writer with Rider magazine where he tested and reported on motorcycle products, and wrote travel features that took him through the U.S. and to Canada, Morocco, Turkey, and South America. Since 2009, he has written hundreds of online motorsports profiles and racer interviews, covering nearly every aspect of the motorsports racing industry.
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