Pull Type Clutches - The Future Of Clutch Actuation

An increasing number of new vehicles are being fitted with “pull type” clutches. And there has been a lot of interest in the market place about the differences between the new pull type and the traditional “push type”. Here is a summary of the differences and advantages.

The increasing torque output of modern engines has resulted in a requirement for clutches with higher clamp load. But because the clamp load is determined by the diaphragm spring, this means that a stiffer diaphragm is required to increase the clamp load. But of course the stiffer a spring is, the harder it is to bend, which means that the pedal effort is increased, resulting in what is called “heavy pedal”. And here is where is the problem occurs, because the tendency in modern cars is for greater comfort and less driver effort.

At this point please note that the words “pull” and “push” refer to the action on the diaphragm, not the method of moving the release lever.

Simple physics and the law of levers tells us that the greater the lever length, the less the effort required to move a mass. A traditional push type clutch utilises only about 70-75% of the available diaphragm finger length to act on a fulcrum point and cause the release bearing to “push” the diaphragm spring, thereby removing the clamp load from the clutch.
See figure 1 for push type clutch operation.

In a pull type design the fulcrum point is relocated, by hinging the diaphragm under the lip of the cover and thereby lengthening the lever by up to 30%. The release bearing “pulls” the diaphragm to remove the clamp load from the clutch. Figure 2 shows the operation of a pull type clutch.
This means that for a given lever length, the pedal effort will be 30% less than an equivalent push type design. Therefore in a pull type clutch, for the same pedal effort as a push type clutch, the clamp load of the clutch can be increased by 30% without driver comfort.

Most older model cars like the peugeot 505 use a traditional push type clutch, most car models used this clutch up until the early 90's then the clutch was changed to a pull type clutch for the increasing torque output of the modern engines.