Courtesy the Manufacturers
July 18, 2011

Discs

Here’s where the water gets a little cloudier in terms of selection. You literally have a gazillion choices (well, maybe not that many) when it comes to disc design and friction material. If you happen to choose the wrong type of disc for your application, you could very well end up with a jacked-up mess, or at the very least, end up wishing that you were driving a slush box. (You guys trying to creep into the Valvoline oil change bay with an unsprung hub, paddle style, sintered metallic clutch know what I’m talking about). Let’s look at the anatomy of a disc itself. This will help you to understand how each part affects the manners of your clutch kit.

Each disc is basically made up of a friction surface and a hub. The hub of course is what slides onto the input shaft of your transmission, so it’s a given that you need that hub to match the diameter and splines of your particular input shaft. Hubs are also offered in sprung and non-sprung versions. The springs surrounding the center of the hub act like a harmonic balancer; in the fact that they absorb some of the shock of the clutch engagement. Most street and street/strip versions are sprung as they reduce NVH and are easier on the drivetrain. A non-sprung, solid hub disc should be used for race applications, as they are business-only, and can be very harsh upon engagement.

Essentially, the rest of the disc is the friction surface. The diameter of this friction surface needs to coincide with the diameter of the pressure plate to get the most effect. There are many types of friction materials used for clutch discs. This includes, but is not limited to, organic, sintered bronze, sintered aluminum, sintered iron, ceramics, and Kevlar. Each material has its own coefficient of friction and thus its own performance rating. You’ll also notice that many of these friction materials are arranged in evenly spaced pads around the circumference of the disc.

The organic material is what you would consider an OEM or stock replacement material. These generally have excellent street manners, not exhibiting any form of clutch chatter after break-in. When you get into the dual-friction discs (organic/higher-friction material) and discs composed entirely of the same high-friction material, then there is always the chance of chatter or less-than-perfect behavior. Lighter cars and lower rearend ratios (3.73, 4.10, 4.30, and so on) tend to help that situation, while heavy cars and higher rearend (3.00, 3.27, 3.50, and so on) ratios tend to aggravate it. However, unless you go the twin-disc route, you may have to face it as a necessary evil.

Since a clutch kit is made up of several different components, it’s a little more difficult to put a ranking system on discs alone, but if we had to come up with an order on the most popular choices, they would be classified in this way in order of increasing grip and decreasing street manners:

  1.  Organic
  2.  Dual-friction (Organic with ceramic, sintered metal, or Kevlar)
  3.  Full ceramic/metallic discs (i.e. both sides sintered iron, sintered bronze, and so on)
  4.  Paddle-style, non-sprung hub, sintered metallic/ceramic faces

So Which Clutch Do I Need?

The majority of drivetrain-related emails that we get revolve around picking out the right clutch for his/her application. However, if you arm yourself with a knowledge of static pressures and disc materials, and then combine that with a mental image of what you’re willing to put up with and what the application is, then you can make a very educated decision on where to spend your money. There are a lot of variables that need to go into clutch selection, including vehicle application (street, street/strip, or competition), vehicle weight, rearend ratio, transmission specs, mechanical or hydraulic clutch actuation, what you’re willing to put up with in terms of street manners and pedal pressure, pressure plate pattern, and physical size—i.e. what your flywheel and bellhousing will accept—and of course, price.