Dual-Disc Clutches 101: Does Your Hot Rod Need One?
The dual-disc clutch—often called a twin-disc or multi-disc clutch—is designed to increase holding power over an OE-style single-disc clutch. Not to be confused with a dual-clutch transmission (DCT), which uses two separate clutch packs (one for odd-numbered gears and one for even-numbered gears), dual-disc clutch designs are over 100 years old and were originally employed for industrial and agricultural use before finding their way into high-powered road-racing and drag-racing applications. Their big claim to fame is the ability to sustain huge amounts of torque throughput without slipping, and that’s why they’re increasingly used for high-powered street cars.
A clutch upgrade in a street-driven car is often needed once other improvements are made in the driveline, such as a power-adder, an engine build-up, or sticky racing tires. Whether it’s additional power from the engine, greater load from the drive wheels, or a combination of both, a car with a manual transmission will often begin experiencing clutch slippage once torque exceeds the clutch’s design limit. The clutch is intentionally the weak link in the torque chain and it will let you know when it can’t keep up, usually with an acrid odor, slippage, smoke, and, eventually, failure. You will need more holding force from your clutch, but there are a variety of factors to consider in choosing a new one, and it’s not a foregone conclusion that a dual-disc is the answer.
When Do You Need a Dual-Disc Clutch?
Many domestic cars, including the Corvette ZR1, Dodge Hellcat, Mustang Shelby GT500, and Camaro ZL1, already have multi-disc clutches from the factory and are quite streetable, proving that the technology has come a long way since being introduced to the racing community. Where dual-disc clutches were once the purview of hardcore, race-only machines, multi-disc clutches have risen in popularity as power levels from the factory have swelled. Increasingly, multi-disc clutches have been called upon to manage that power and aftermarket companies have moved to fill that need with a range of products. Current consensus among aftermarket vendors is that once torque exceeds 600 lb-ft, a dual-disc clutch should be among those clutches seriously considered as an upgrade, and certainly by 700 lb-ft, a dual-disc clutch is nearly a requirement.
How Does a Dual-Disc Clutch Work?
In cars with a manual transmission, the amount of the clutch’s holding power is dictated by three things: clamping force, the coefficient of friction, and surface area. These three factors are balanced to arrive at an appropriate level of torque throughput that takes into consideration not just the holding power of the clutch but also its pedal effort and overall drivability. The assembly consists of a floater disc (attached to the flywheel through straps or pins) that is sandwiched by two clutches splined to the input shaft, flywheel, and pressure plate. Engaging the clutch assembly is a throwout bearing that applies force to the fingers of the diaphragm (a sectioned Belleville spring) on the clutch cover and is activated by a lever (attached to a Z-bar or cable) or by hydraulic pressure from a master cylinder/slave cylinder setup. The product specialists at today’s dual-disc clutch manufacturers are experts at juggling these design parameters for a variety of operating conditions and vehicle types and we strongly advise consulting with the tech department of your preferred clutch maker before making a choice.
What’s the Limit of a Single-Disc Clutch?
When deciding to move from a single-disc to a dual-disc clutch, the tipping point depends on a couple of factors. When the engine’s torque output gets beyond a certain level—somewhere in the 600- to 650-lb-ft range—raising a single-disc’s clamping load and increasing the coefficient of friction with more aggressive material begins to cause drivability problems, with excessive pedal effort and balky, non-linear clutch engagement (chatter and lurching). Here, a good way to bring drivability back into reality is with a dual-disc unit. Having twice the surface area, a dual-disc clutch allows some breathing room with the friction material and clamp load, restoring drivability to a normal level of effort.
Dual-Disc Clutch Trade-Offs and Myths
Before jumping into a dual-disc clutch, you’ll want to become familiar with the pros and cons of dual-disc clutches. You may have heard things that are both good and bad about dual-disc clutches and we want to clear up the misinformation and erase some time-worn myths.
- Holding power—at the top of the list of positives for a dual-disc clutch is its inherent holding power. With double the surface area of a single-disc clutch, a dual-disc unit can preserve a street-friendly pedal effort using more traditional organic friction materials, where the acceptable alternative in a single-disc unit would behave more aggressively.
- Reduced rotational inertia—if your intention is to go drag racing or road racing, dual-disc clutches typically have a lower moment of inertia, so they tend to rev faster. This factor is more pronounced as the clutch diameter decreases. (Trivia: the stock Mustang Shelby GT500’s dual-clutch DCT transmission has 11 discs in all, but they are only about 7 inches in diameter for greatly reduced rotational inertia. )
- Engagement—The engagement of a dual-disc clutch has historically been compared to an on-off switch. This was a problem decades ago, when the engagement wasn’t slowed with a higher pedal ratio, a smaller clutch master cylinder, changes to the spring rate or shape of the fingers in the diaphragm, not enough marcel in the clutch discs (the amount of wave built into the friction substrate), or a lack of damper springs. In recent years, factory engineers have fine-tuned these adjustments in the many OE dual-disc clutches in production vehicles and you’d never know from driving them that they are dual-disc. These days, smooth engagement in dual-disc clutches is largely the norm.
- Cost—there’s no way around the fact that dual-disc clutches cost more than single-disc ones. Prices start around $800 for an entry-level dual-disc kit, and the overall cost will partly be determined by whether the kit includes a new flywheel. There’s just more hardware, more machining, and more assembly with a dual-disc kit, so prices will be higher than single-disc clutch kits. (Watch a cool video of how RAM builds its Pro Street dual-disc clutch system here.) On the positive side, a dual-disc clutch will last longer than a single-disc clutch, so maintenance intervals have the potential to be significantly longer.
- NVH—Dual-disc clutches employ an extra clutch disc plus a floater, and either a set of straps or retaining pins to link it mechanically to the flywheel. This can result in noisier operation particularly between gears, with the clutch depressed, or with the transmission in neutral. How much this bothers you will depend on the noise floor of your vehicle, personal taste, and the specific clutch model used. Today’s aftermarket dual-disc clutches for the street exhibit very little of the noise and harshness experienced with race-only units.
Who Makes Dual-Disc Clutches?
Virtually every aftermarket clutch maker offers a dual-disc clutch but if your thing is domestic rear-drive V-8 muscle cars, the field is dominated by the seven major players listed below. Some of these manufacturers make it a point to offer more than one kind of dual-disc clutch (Centerforce, McLeod Racing, RAM Clutches) which is helpful for optimizing the driveline combination for either street or track work. You may find, however, that other manufacturers are willing to customize a dual-disc clutch with a variety of friction materials (e.g. organic, Kevlar, ceramic, bronze, sintered iron), floater assemblies (strapped, pinned), flywheel types (standard or lightweight), various engagement strategies (damper spring stiffness, marcel, diaphragm design), and clutch diameter.
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