Clutch Chatter
Bill, Pensacola, FL; I have a '67 Camaro with a ZZ4 crate motor and a four-speed. The car chatters and shakes all the way to the rear wheels at low speed, while pulling in and out of parking spaces, and during normal takeoffs. I don't feel it when taking off hard and shifting hard. The problem also exists during downshifting like from Fourth to Third; as I release the clutch, the car chatters. Shifting between gears is smooth, no problems. The problem seems to be only at low rpm. I have already replaced the clutch, pressure plate, and flywheel.
Jeff Smith: While this seems like such a simple situation, there are literally dozens of things that can contribute to clutch chatter. We'll go through some of the more common problems, but we offer no guarantees. This is really a can of worms. I expect we'll get additional letters with other solutions.
Since a new clutch and pressure plate did not solve the problem, let's start there.A new clutch also requires a fresh flywheel surface. We'll assume the new flywheel was properly cleaned before installation, since most flywheel suppliers coat the flywheels with oil or another oxidation-resistant coating to prevent rust. If the flywheel and pressure plate were both properly cleaned, that would mean the cause is not directly related to the clutch itself.
We really need to think in terms of the entire vehicle with regard to clutch chatter. Worn U-joints, a poor driveshaft angle, or lame leaf-spring or trailing-arm bushings can all contribute to clutch chatter. We had a clutch manufacturer once tell us he discovered a too-quick advance curve caused the clutch to chatter, where the ignition curve began as the rpm came up and then went back down as the rpm dropped as the clutch was engaged. This was enough to cause the chatter. All he did was put stiffer springs in the mechanical advance curve and the problem disappeared. Does the clutch linkage deflect or move around as the clutch is engaged? We've also seen a poorly adjusted linkage cause weird symptoms, where all the parts are correct but the clutch is not engaging at the proper point in the linkage movement. We've also seen firewalls deflect as much as 11/48 inch, which causes all kinds of linkage difficulties.
We're also assuming you're using an organic clutch facing material. Several street-clutch companies, including Centerforce, use a metallic facing on one side with an organic facing on the opposite side. Generally, the Centerforce Dual Friction clutches we've used work well with minimal chatter. This is where a good dialogue with the clutch manufacturer can be useful. If your clutch disc uses a race-oriented friction material, some clutch chatter should be expected.
Clutches operate on a similar principal to brakes. The three interrelated variables that dictate performance are clamping force, surface area, and the coefficient of friction between the friction surfaces of the disc and the flywheel and pressure plate. For example, the greater the surface area (as with a larger-diameter clutch), the less clamping force is required to withstand a given amount of power through the clutch. Generally, race clutches use metallic facings that offer a higher coefficient of friction, which increases the holding power of the clutch without resorting to higher pressure-plate loads. Organic compounds intended for street use generate a lower coefficient of friction but offer smoother engagement for a more pleasant driving experience.
All street clutches also include what is called a marcel: a stamped, wave-formed spring located between the two friction surfaces on a street clutch. This marcel compresses as the clutch is engaged with the flywheel, which softens the engagement. It can be seen by looking carefully at the cross-section of the clutch disc straight in from the side.
You didn't mention what type of pressure plate you are using. The best street pressure plates are the diaphragm-style, where the spring covers the entire 360-degree area of the pressure plate. Conversely, the Borg & Beck or Long-style pressure plates generally employ three pockets of coil springs spaced around the plate. We've seen slightly warped, three-finger pressure plates that "skip" across the clutch disc on light engagement until fully loaded. This occurs because the coil-spring packages on a three-finger clutch deflect the pressure plate slightly.
There's tons more information than we can deliver here. We've condensed paragraphs of ideas into a short list that may help you solve your dilemma.
Causes of Clutch Chatter
*Flywheel has excessive run-out.
*Flywheel was not resurfaced or improperly resurfaced.
*Bad U-joints in driveshaft.
*Excessive backlash.
*Excessive driveline angle.
*Worn leaf springs or bushings.
*Bad bushings in trailing arms on coil-spring cars.
*A race-type metallic friction material.
*No marcel in clutch disc.
*Solid hub clutch disc with no springs.
*Clutch facing contaminated with oil or grease.
*Bent pressure-plate assembly or disc.
*Worn or broken motor or transmission mounts.
*Worn pilot bushing.
*Improperly matched clutch components.
*Clutch disc hangs up on the input shaft-possibly caused by worn input-shaft splines.
*Linkage adjustment and incorrect angle of clutch linkage during engagement.
*Poor bellhousing alignment.
*Clutch linkage binding.
*Possible weak or broken clutch hub springs-allows hub to strike travel stops in disc.