What in the World is Gear Rattle?

Jul 1, 2024 | Tech Help

Gear rattle is a phenomenon that can often occur in modern-day transmissions.  

Today’s transmissions use a much lighter gear oil than, say, your old Muncie 4-speed, to improve gas mileage and drive train efficiency. Older gearboxes typically use heavy 80-90-weight oils to dampen the shock and engagement of the gear sets.  

Current transmissions use much lighter oils and cannot absorb the normal oscillations generated in the driveline. This can lead to gear rattle under light loads.

Typically, this rattle is experienced at lower engine RPMs, between 1200 and 2500, under a light load, or even at idle. In some extreme cases, this rattle or vibration can be felt right at the shifter handle.  

It may also be evident if you drive around a low-speed corner at around 1800 RPM when the throttle is lightly applied. 

Rattle may occur when the car is in the driveway idling as it warms up. Once the drivetrain is fully ‘loaded,’ such as under harder acceleration, or the engine RPM exceeds around 2500, the noise typically goes away.

Trying to Solve the Gear Rattle Problem 

Unfortunately, the manufacturer’s efforts to address this problem proved only to be a ‘band-aid’.  

Dual mass flywheels were introduced. They use a shock-absorbing mechanism built into the flywheel to take up the oscillation in the driveline and help alleviate the noise. The problem with these flywheels is that as they wear, the spring section gets worn and often starts to cause the very same gear noise problem they were designed to address.  

The accelerated wear is worse in high-performance cars that are often driven hard. Couple this with the widespread lack of availability of replacement dual mass flywheels and other alternatives had to be found.

When you convert a dual-mass flywheel application to a solid-mass standard flywheel, such as with our RAM C4 push conversion systems, the shock absorption characteristic of the dual mass flywheel is lost. The only source of dampening at that point is through the center hub of the clutch disc.  

Even the best-sprung hub discs still allow some noise and oscillation to transfer through the drivetrain. It’s made even worse by solid center clutch discs without any spring dampening, as some other manufacturer’s dual disc units are designed.

Another method to address the issue is ‘helix’ or double-dampened clutch disc hubs. In almost any late-model vehicle, you will find helix hub discs utilizing a sprung hub. A second set of small springs is placed near the spline area of the clutch hub to achieve this ‘helix’ effect.  

RAM Muscle Car kits utilize this style disc in some applications, like 86-up Mustang.

Is there a solution?  Not really, but the effect can be minimized.  

One of the best ways to minimize this issue is to use a heavy flywheel. The heavier the flywheel, the more driveline pulses and oscillations it can absorb.  

For C4 Corvette applications, some transmission companies offer a shim that can be replaced in the gearbox to handle some of the slack and reduce the noise.  

If you would like info on this, contact me at [email protected] and I can provide contact info. My last tip is to try a high-quality transmission fluid like Amsoil (this helped in my personal C5 Corvette).