How To Build A Quiet Ford 9-Inch Rear Axle

11/30/2022

How To Build A Quiet Ford 9-Inch Rear Axle

11/30/2022

The Ford 9-inch is the undisputed king of performance rear-ends. With its proven strength, convenient removable third-member, and unrivaled aftermarket support, this tried-and-true design has become the standard for rear-axle assemblies in everything from mild street cruisers to brutally quick Pro Mods.


All that said, the Ford 9-inch isn’t perfect. Offsetting its many positive attributes is one slightly pesky trait — it can be a bit noisy. This tendency comes from one of the 9-inch’s most important design features, its offset pinion gear. Unlike most rear ends, the 9-inch’s pinion gear meshes with the ring gear towards the bottom of it, instead of at the middle as most rear ends do. Locating the pinion in such a position increases the gear-tooth contact, giving the 9-inch vastly more strength than its rivals. However, it also increases friction, making it noisier.


To learn more about the noise factor of Ford 9-inch rear ends and how to control it, we spoke to Lucas Hardin and Rob Knipp of GearFX. The company has a reputation for building some of the quietest 9-inch rear-ends available today, a result of unique methods and techniques they’ve developed over decades. Hardin and Knipp provided loads of insight on the subject, which can benefit anyone looking to make the best choice in a rear-end assembly.

An Elusive Problem

Ford 9-Inch Axle In Car

You can build a 9-inch axle by the book to perfection...noise can still be present. It may not be a noisy axle as much as the vehicle amplifying the noise, however.


There are a lot of variables in how noisy a 9-inch is, how much that noise will be heard, and how annoying it will be. Much of it starts with the owner’s expectations. With car builds now often reaching the refinement of European supercars, blemishes such as gear noise simply stand out more than they used to.


At the same time, the rigid structure and stiff bushings of today’s performance chassis components transmit and amplify resonances that soft and sloppy OEM chassis components used to blot out. “Ford put the 9-inch in original OEM vehicles, so they had to work in Broncos, Mustangs, F-100 pickups, vans — pretty much everything,” says Hardin. “But now the 9-inch is being used in cars with Detroit Speed suspension, subframe connectors, and other really stiff chassis pieces.


“And it’s not necessarily a volume thing,” continues Hardin. “It’s a frequency thing. For example, there was a ‘70 Camaro that came in here, and the rear-end made a high-pitched whine. It had a really stiff chassis, so the frame acted like a tuning fork, amplifying the noise up through the frame rails and making it resonate in the interior. That’s what a lot of people are dealing with now.”


Adding to the problem, the noise of a 9-inch doesn’t always appear in the same situations. It can be drive noise, coast noise, or both.


But these potential issues shouldn’t discourage you from going with a 9-inch for your car. Dollar-for-dollar, pound-for-pound, no other rear-end offers nearly the advantages of this proven design. It just means you have to choose carefully.


Contrary to what many believe, Ford 9-inch rear-ends aren’t a commodity. There are vast differences between them, depending on who assembles and sets them up.

The Art of Rear Axle Set-Up

Quiet 9-in gears

The Ford 9-inch axle's offset pinion is the primary source of the noise that many wish to avoid. The gears mesh at the bottom of the ring gear, instead of in the middle. This increases the gear-tooth contact, which adds to the strength, but also increases friction, the source of the noise.


You may be surprised to learn that setting up a Ford 9-inch correctly isn’t a by-the-book process. Sure, the specs and standards can be a good place to start. But GearFX says that putting a rear-end together without testing, tweaking, and refining that initial setup is a recipe for a poor-performing rear end that disappoints customers.


“You can set up the correct pattern in a rear-end — textbook perfect — and you put it in the car, and it still makes noise,” says Hardin. “We’ve learned that you have to manipulate the gear pattern and adjust things like pinion depth and backlash to quiet it down. That’s because it depends a lot on how the individual gear set was cut and lapped. You have to adjust the pattern to where the gears are happy with each other and mimic that original relationship.”


The craft of setting up an ideal high-performance 9-inch rear end requires the insight and experience of a master practitioner. It’s a form of wizardry one doesn’t acquire overnight. “The guys that do our gears are on a whole other level, because they’ve done it for so long,” said Hardin. “They don’t have to look at a book. They just know by looking at a combination of components what they’ll need to do to quiet it down and get it right.”

Quiet Components

Noisy 9-inch axle quality components

To achieve a quiet rear axle, a combination of quality components and exact synchronicity must be achieved during the build process. If anything is off...even the case itself...you can expect to hear the 9-inch's siren song.


In addition to the expertise and insight needed to build a quiet 9-inch rear-end, quality components make a big difference in the noise factor. Cheap, inferior components generally result in noisy rear ends. In GearFX’s never-ending quest to build the quietest rear-ends possible, they continually test all major brands of bearings, gear sets, and other products to determine the best quality pieces available at any given time.


“We’re not trying to find the cheapest ring and pinion,” says Hardin. “We only use what we know works well. And to find out what’s best, we’ll buy four or five boxes of ring-and-pinion sets in each ratio and test them. We’ve done that with pretty much every brand that’s readily available.”


That said, even within a particular brand, components have their individual quirks because of manufacturing tolerances and other factors. These tiny flaws can make a big difference in how noisy a finished rear end is.


Quiet 9-Inch Quality parts

The importance of high-quality components in a 9-inch axle build can't be stressed enough. If a product doesn't meet GearFX's criteria, it does not get used.


To eliminate these variables, the GearFX team carefully inspects all the components they receive and rejects any that don’t meet their high standards. “We’re very particular about what we’ll actually use,” says GearFX technician Rob Knipp. “We’ve got a whole stack of ring-and-pinions in the back that we won’t use because they’ve got flaws in them. We cherry-pick the very best.”


Included among those components is the case itself. If it isn’t machined correctly, getting the rest of the components to work well together is nearly impossible. “The ring gear, the differential bearings, and the pinion, they’ve got to be 90 degrees to each other,” says Knipp. “If they’re off, it’ll do all kinds of weird stuff.”


Along with these things, the gear ratio can play a part in how noisy a 9-inch is. Although it isn’t a significant enough factor to solely dictate what ratio you should choose, it’s worth noting when assessing the noise potential of a particular rear-end. “It isn’t a hard-and-fast rule, but the lower the number, the quieter the gears tend to be,” says Hardin. “So a 3.00:1 would typically end up quieter than a 4.11:1”

Dyno Testing Makes The Difference

Quiet 9-inch axle dyno testing

To be sure that their axles make little more than a whisper, GearFX uses their in-house differential dyno to check each unit before it is shipped out to the customer. This way, if adjustments need to be made, they can be addressed and the axle re-tested.


One of GearFX’s primary tools for building quiet 9-inch rear ends is the company’s differential dyno. It’s a tremendously effective piece of equipment for this type of work, and GearFX has the only one — they developed it themselves and continue to be the only driveline shop that uses such a machine.


Similar in principle to a conventional engine dyno, the GearFX differential dyno allows technicians to thoroughly test each rear-end before shipping it to the customer. This exacting method can pinpoint component flaws and setup issues with measured precision, allowing the GearFX team to do whatever is necessary to make the setup as quiet as possible.


“Every single one of our rear ends gets tested for noise before it goes out the door,” says Hardin. “That typically includes having to play with pinion shims a little bit. Usually, we’ll have to change the pinion shim once or twice. We put it back on the bench, check it, and then run it again on the dyno to make sure.”

Proper Break-In

Quiet 9-Inch Lapping Gears

Since most of the noise comes from the meshing of the teeth, checking the mesh pattern using gear-marking compound is essential to see if everything will be happy prior to being bolted up into the case.


Like most driveline components, rear ends have specific break-in procedures. These need to be followed carefully to ensure low noise and maximum durability. “Breaking-in gears is very important, because you have to wear the pinion teeth into the ring-gear teeth,” says Hardin. “Even though everything was lapped right when it was manufactured, you’re never going to get it exactly perfect to where it was lapped when you assemble it. So it has to wear its own pattern into itself, to where it’s happy.


“Essentially, break-in is like re-lapping the gears, but without any abrasive material,” continues Hardin. “Once that wear pattern is worn in, you’re fine — it won’t be loud if you beat on it. But if you don’t break it in properly, it will score the gear. It won’t wear properly, and the gears are damaged because the wear pattern is messed up. Then you’ll have to start from square one again.”


Unfortunately, it can be tough for car owners to follow the exact break-in procedure, which typically involves 500 miles of gentle street driving. That sounds simple enough, but many high-horsepower EFI-equipped engines won’t even run well enough to drive on the street with the baseline tune they get from the engine builder. Such engines typically need extensive tuning on a chassis dyno first. And that punishes the rear-end assembly long before its break-in period is finished.

Quiet 9-inch axle break in 1

Even though GearFX sets up their axles to be quiet, a proper break-in procedure must be performed so that the pinion teeth and ring gear teeth wear into each other properly. Usually that means about 500 miles of gentle street driving. If the temptation to go burn rubber is too great, GearFX can perform the break-in procedure on their dyno for a reasonable charge.


Then, of course, there’s the natural temptation for car owners to beat on things. The need to drive a newly-finished car gingerly for so many miles can be hard to handle, resulting in a rear-end that will forever be noisy. Worse yet, most companies won’t cover that under warranty if the correct break-in procedure wasn’t followed.


To prevent these issues, GearFX once again puts their differential dyno to work, offering a break-in service on all their rear ends. For a reasonable charge, they’ll put the completed assembly on the dyno and run it under load for several hours to break in the ring and pinion. That way, it’s ready to go, right out of the crate.


By paying attention to all these critical factors, you can get the proven advantages of the Ford 9-inch, without excessive noise. Like so many other things in the performance world, the secret is in the details. “It’s definitely possible for a 9-inch to be quiet,” says Knipp. “You just have to put in the work to get to that point.”

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