What Is Clutch Slip?
Clutch slipping is a common phenomenon where the clutch disc is not grabbing the flywheel surface as firmly as it should, resulting in slipping between two surfaces and failure to transmit power properly to the drive shaft. Although the first victim of clutch slipping is the friction disc itself, it can cause damage to your flywheel and transmission if left unattended.
How to Determine If Your Clutch Is Slipping
Even if you are not an expert, it is almost impossible to miss these telltale signs of clutch slipping:
- Pungent, burning smell when revving
- Poor acceleration
- Difficulty changing gears
- Squeaky or rumbling sound when pressing the pedal
- Discrepancy between Tachometer and Speedometer variation when accelerating (RPM’s shoot up quickly with the speed not matching)
Common Clutch Slip Causes
Worn out friction disc
It is normal for friction discs to wear over extended use. They can also wear prematurely due to other reasons such as not properly adjusting the clutch pedal after installation.
- Incorrect clutch selected for your modified vehicles torque output
- Incorrect driving style such as keeping the vehicle in gear while at a stoplight, riding the clutch, lingering while changing gear, flooring the gas pedal at low RPM, etc.
- A stiff clutch actuator, a.k.a hard clutch caused by air bubbles in hydraulic line or worn-out pressure plate
A worn-out clutch disc has a twofold contribution to clutch slipping:
- Wearing smooths out the friction surface, causing slippage.
- Wear of the friction material reduces the disc thickness, making the downforce insufficient for firm contact and hence causing slippage If your clutch is slipping because it’s worn out, the only obvious choice is to replace it with a proper clutch.
Oil contamination on the friction disc
Another major cause for clutch slippage is oil or grease contamination on the friction disc. Even a small amount of oil grease reduces the friction coefficient of the disc materials and causes considerable slippage and chatter. Therefore you should NEVER touch the friction material with your bare hands.
Finding the location of the oil contamination on the clutch disc often leads to the source of leakage and thus helps prevent it. A clutch disc should not be replaced before identifying the leak.
- Examining the clutch disk center for markings helps to define the type of leakage.
- Dark streaks mostly result either from a leaking transmission input shaft seal or a leaking concentric slave cylinder
- Silver streaks are mostly resulting from solids particles of anti-seize compounds
- Streaks from the hub toward the friction disc can mean too much grease was used when installing the clutch
- Examining the pressure plate: Inside the pressure ring – Black oily residue is most likely from a bad crankshaft oil seal
- Overlubricated bearing is another source of contamination. In most cases, pilot bushings come pre-lubricated. Even if you want to lubricate them, a light application of high temp grease is enough.
- Concentric slave cylinder failure is a common reason for oil or grease contamination. Typically, you can see oil all over the exterior of the slave cylinder if it fails.
Improper Pedal Adjustment
Too often vehicle owners buy a shiny new clutch and slap it on the flywheel without any proper pedal adjustment. Skipping the quickest portion of the job can lead you to a very large expense of replacing your clutch kit again.
- Locate locknut and adjuster nut (usually above the clutch pedal)
- Pull up clutch cable slightly
- Determine the point of fork engagement
- Keep pressure on the cable
- Tighten the locknut and adjuster nut
These clutches use a hydraulic clutch pedal and although at times, self-adjusting, they do need adjustment from time to time.
- Loosen pedal stopper
- Loosen push rod lock nut at the clutch master cylinder
- Turning the push rod clock-wise wll give you more pedal height, turning counter clock-wise will give less pedal height
- Adjust pedal stopper once you are done
Improper Torque Specs used on bolts
Installing a new clutch or tightening the pressure plate requires proper torque when tightening bolts. Below practices can cause misalignment issues, leading to excessive wear and clutch slipping.
Important Note: It is always a good practice to replace the bolts when replacing the clutch. Even if they are not TTY bolts, pressure plate bolts go through heat-cycling and thermal stress. So, it is highly recommended to replace them when changing the clutch.
The flywheel has a friction surface with the clutch disk. Hence any issue with the flywheel will affect the clutch disk. Common flywheel issues that lead to clutch slipping:
- Oil or grease on the flywheel
- Improper resurfacing of the flywheel
- Not resurfacing flywheel when changing clutch
- Improper torque on flywheel bolts
- Warped or damaged flywheel with heat cracks and hard spots
Important Note: It is required to resurface or replace the flywheel when changing the clutch. Otherwise, this flywheel will shorten the life of the next clutch significantly, often resulting in slipping. However, it is important to mind the tolerance limit put by the manufacturer. Also, marking the index position before removing the flywheel for servicing is necessary if the engine is externally balanced.
Pressure Plate Faults
Just like the flywheel, the pressure plate also has a friction surface with the clutch disk.
Important Note: It is highly recommended to replace the pressure plate when changing the clutch disk or resurfacing the flywheel. Not doing so will cause your new Action Clutch to fall outside of warranty.
Improper Break-In ( Glazed Clutch / Uneven Wear )
Breaking in is necessary to seat the friction materials properly onto the disc and to create an even friction surface. This requires a lot of street miles (not highway miles) on city streets where one can make a lot of engagements and disengagements. Doing it improperly and inadequately can glaze the disc, making it smooth, resulting in the clutch slipping rather than garbbing the surface in contact. These are the reasons clutch discs can glaze:
Notes: All of our clutches have a recommended break-in distance and procedure and is listed on our website as well as included in the in-box materials shipped with the kit. Following our break-in procedures can avoid glazing the friction material resulting in your new clutch slipping.
Release System Issues
The release system has several components that can fail and ultimately cause clutch slipping. Detecting and replacing these faulty parts is necessary before changing the clutch itself. Some of the common issues are:
Notes: Damaged release bearings can cause tilting in the released shaft. They are hard to inspect and should always be just replaced when the clutch is changed.
Notes: For a fractured diaphragm, servicing and replacement are possible in some cases. However, along with the diaphragm, other components in the release system such as master and slave cylinders, clutch cable, release fork, etc. should also be inspected and corrected. Setting the correct amount of free travel is crucial for diaphragm health.
Stripped Hub on Disc/ Worn Spline
Stripped hub on the disc is not uncommon on clutches. A few things can lead to a stripped hub or a worn spline:
Notes: When fitting, avoid forcing the input shaft into the drive plate as it indicates misalignment. In case of a stripped hub, we can replace the hub itself for a fee, depending on the condition of the kit overall. However, inspecting the input shaft and replacing it if too worn out is also a very important thing to do when installing your new clutch kit.