The Friction Zone and Multiple Clutch Manufacturer Designs

Figure 1 - Friction Zone

The friction zone is the range of driver control from a totally released clutch to full engagement of the clutch. It is used to start a vehicle in motion from a standing start both forward and reverse. This directly determines proper clutch feel and function in addition to the ability of it to transmit power from the engine to the drivetrain. During this time the driver must be in control of the vehicle, not a chattering clutch in control of the vehicle. This is when chatter would occur, and it interferes with the ability to totally control vehicle speed and position.

Figure 1 at right represents the pressure plate just beginning to touch and compress the friction material facings and the marcel cushion springs in between them. This compressible section along with the friction material creates what many refer to as the “Friction Zone”.


Form, Fit & Function

Figure 2 -  Variations of Cover Assembly Designs
of the Same Part Number

Figure 3 - Lever to Table Dimension

One of the issues customers are often confused about is why the parts removed do not look like the ones being supplied in the new clutch set. Take a look at one of the most popular applications, the Ford Ranger. Due to the popularity of this range of trucks, almost every company that manufactures clutches in the world offers a cover assembly for this application. This allows for a lot of potential confusion in the field when a customer attempts to match the old to the new. Take a look at the examples in Figure 2.

The collection in the image shows seven different part numbers from six different countries. One other variation is not shown so that would make it 8 variations for this one application!

How does this affect the customer, countermen and installer? Only two of the pressure plates (aka cover assemblies) look alike. The others are all built from the standard designs that these various companies like to use. All of them fit the flywheel and all work within their respective designs and specifications, but most of them would raise an eyebrow or two at the counter. This is where the fun usually begins.

 Out comes the 12” ruler, tape measure or even a pencil as a gauge. The customer and countermen should be truly commended for trying to figure this part out, but it cannot be done on the counter by comparing measurements of un-installed clutches. The cover assembly must be correctly mounted to a flywheel using either a new clutch disc or precision steel gage blocks like used on a clutch testing table in a lab.

Then the clutch should be cycled (engaged and disengaged) several times. Only after cycling, can one accurately check the Lever to Table dimension or diaphragm spring lever tip height (see Figure 3) and plate lift at the specified release travel and see if the system releases. Note that the plate lift at the specified release travel is not a published number.

One would have to tear this setup down and set up the other seven systems and repeat the entire process to get true comparison numbers. If the customer has any doubts about the fitment of parts please do not hesitate to call the tech line at the company from which you bough the part. They can provide you with all the information you need to verify a part and save you lots of time.

The only bench comparison that is possible to measure is the inside diameter (ID) and outside diameter (OD) of the pressure plate casting. Also, it is possible to compare the bolt pattern and dowel pin pattern. Even this simple comparison can cause confusion due to staggered or offset dowel pin patterns (i.e. Nissan, Honda and Toyota).

Do not forget that GM, Ford and Dodge often use 2 or 3 smaller bolt holes that act as pilot holes for special shoulder bolts. One of the easiest ways to compare confusing bolt or dowel pin patterns would be to trace the pattern on a piece of cardboard using a fine point pen or pencil. Note that dowel pin holes are usually evenly spaced between the bolt holes, or adjacent to the bolt holes. Once traced, place the cover being compared on the pattern and rotate the clutch to find the matching pattern.

Clutches can be very confusing to understand and it can only help the process if the parts do the best possible job in terms of function. It is a real plus when they match Form and Fit at the same time.