Causes of Slipping Clutch Condition
Slipping is one of the most common clutch related concerns. A slipping clutch condition can occur after years of service or immediately after installation of a new clutch and is caused by various scenarios.
One of the most commonly seen and easily prevented causes of slipping is improper step and cup dimensions after a flywheel has been machined or resurfaced.
To understand the importance of flywheel step and cup dimensions and how they relate to slippage in a clutch system, let’s first describe the wear characteristics of the two types of diaphragm clutches and how they work:
Self-Adjusting Clutch (SAC)
• Self-adjusting clutches extend the service life of the clutch disc.
• The clutch diaphragm fingers remain in their original position throughout the life of the clutch.
• The clutch adjusts as the disc wears, allowing for consistent pedal feel and release.
• As the disc wears, the clamp load is consistent throughout the life of the clutch.
• Lever-style clutches experience similar characteristics as these non-SAC, traditional clutch systems.
• The disc wears thin, causing inconsistent pedal feel and travel.
• Traditional clutches have a shorter service life than a SAC.
• The clutch diaphragm fingers, or levers, raise as the clutch wears.
• A traditional, or non-SAC, clutch wears over time and the clamp load lessens as the disc becomes thin.
Gauge is what engineers refer to as the total thickness of stacked clutch components within the clutch, disc and flywheel assembly. The components are manufactured within strict tolerances to assure proper gauge.
The clutch diaphragm fingers will appear flat to slightly raised when the clutch and disc are installed and torqued properly with properly matched gauge on a clutch and disc.
The clamp load on a new clutch assembly is sufficient to transmit 30% more torque than the vehicle’s OE engine torque rating as a safety factor against slipping by design.
Thick Disc Condition
Thick disc condition is a result of improper flywheel machining causing either too tall of a step or too shallow a cup. This is similar to using too thick of a disc on a perfectly flat, machined flywheel. The diaphragm fingers on the clutch will recess past flat, or go inverted in this condition.
This diminishes the clamp load and may lead to a slipping condition when driving the vehicle due to the characteristics of the diaphragm springs.
A thick disc condition may also create a no-release condition as the pressure plate will not raise high enough to clear the disc and allow release.
Important: The OE dimensions must be maintained when reusing or resurfacing a flywheel with a step or cup to ensure that slipping, no-release or other driveability concerns do not occur.
Checking the manufacturer’s specifications before performing any repairs or adjustments is a necessity.