Published by Forza Magazine
A “rubbing” noise has just started at the rear of my 599 GTB?Fiorano. My family has owned the car since new. It has 14,000 miles and receives the factory-recommended service/oil change/inspection every year. The noise occurs at low to medium speeds when the steering wheel is turned away from center, but it doesn’t sound like a worn bearing or CV joint, a tire rubbing, or anything I’ve heard before. Performance seems unaffected, although I’m not going to attempt any burnouts or try to hit top speed until the issue is figured out.
I?applaud your decision to drive cautiously and investigate this noise!?It truly is better safe than sorry with these cars, as undiagnosed issues that get ignored can quickly turn into very expensive, potentially dangerous problems.
I suspect I know what’s causing the noise, which is a fairly common issue, but rather than jump the gun let’s start with the basics. First, is anything about the noise related to traction??For example, does the noise occur when you turn the wheel and stand on the throttle, and the worn rear tires scrabble for grip??I’m guessing not, because you’re likely very familiar with the car’s behavior after 14,000 miles, but it’s important to address the most obvious possibilities. Second, during its yearly visits to your dealer or mechanic, has the car received its most recent biannual gearbox fluid change??Ferrari calls for changing the fluid every two years with Shell/Pennzoil TF1055, a special blend that’s much different from a generic, Brand X gear oil. Assuming the noise isn’t related to traction and the gearbox is current on fluid changes, the issue likely relates to the internal workings of your car’s transaxle. It’s probably not a serious problem, at all, and it provides the perfect opportunity to do a little digging into a piece of performance hardware most people have heard of but few understand.
Most readers of a certain age will have heard of Posi-Traction, an American brand name of the humble limited-slip differential. Many readers may also have learned how these devices work thanks to the courtroom scene in My Cousin Vinny, starring Joe Pesci and Marisa Tomei, but for those who haven’t seen one of my favorite films, here’s the basic theory. The right wheels of a car making a right turn are covering far less ground than the wheels on the left side. At the same time, the engine and transmission continue to drive the car forward at a constant speed. In order for the car to turn smoothly, without jerking or skidding, the driven wheel on the left needs to be able to turn at a different speed from the one on the right—and the device that allows this differential in speed is called, suitably enough, the differential. In a front-transmission, rear-drive car, the differential is housed in the rear end. In cars with transaxles, like your 599, the differential is integrated into the transmission casing. However it’s packaged, a differential contains a small gear set that allows the constant flow of power from the pinion to the ring gear to be split through two axles spinning at different speeds, and then to the wheels. This “open” differential design works perfectly well in many situations, but there’s a catch:?It only supplies torque to the wheel with the least amount of traction. This means that if one wheel has no traction, the car may not be able to move at all, as one wheel spins endlessly and the other just sits there. The solution to this problem was to add internal clutches to the differential assembly that allow power to be transferred to both wheels, which can keep a car from getting stuck in place and deliver more torque to the tires for improved performance.
Over time, those internal clutches have been augmented and enhanced with springs, pumps, and/or electronic forks (as found in Ferrari’s later E-diff electronic differentials), although the basic function remains unchanged. That brings up one final piece of the puzzle:?If these clutches were only able to be fully open or fully closed, the car couldn’t turn smoothly without the jerking and skidding the differential was invented to prevent. As a result, they have to be allowed to slip—a limited bit—to allow for smooth cornering. To do this slipping, the clutches require a special type of lubrication or friction modifier. On cars with a separate differential, this friction modifier just gets poured into the differential housing along with the differential oil. On a car with a transaxle, the same fluid lubricates both the gearbox and the differential, so changing that fluid is key to maintaining the limited-slip clutches. That’s why Ferrari specifies the TF1055 fluid, which already has the proper friction modifiers. This brings us all the way back to your issue. When limited-slip clutches get “dry,” they will let you know by a groaning or rubbing noise on turns. In severe cases, you will feel a chatter around tight turns. Sound familiar? This issue usually occurs on cars with low transaxle fluid levels, but every once in a while a correctly filled 599 (or, much more often, F430)?transaxle will create the rubbing noise. In these relatively rare cases, a bottle of limited-slip friction modifier—we use BG Products’ MGC 328CC here at San Francisco Motorsports—should take care of those around-town groans.