DIY: Mesh & Play

Differential Shimming Basics

Differential Shimming Basics

Gear spacing in a drivetrain can be a make or break situation, so it’s important to at least know the basics. RC differentials spin at such high speeds and are subjected to such high loads of stress, so it’s really important these parts move freely with minimal unnecessary movement to avoid damage. To get a truly smooth and reliable drivetrain there will always be some small adjustments required to the differential gear spacing. In order to make these small adjustments we employ the use of drivetrain shims. Drivetrain shims are extremely thin washers used to adjust the spacing between drivetrain components, like differentials and gearing.

Let’s take a deeper look at how we can use shims to dial in our RC differentials and get the most out of their drivetrains.

WHERE & WHY TO USE SHIMS

First off, let’s talk a little about the “where” and the “why” of using shims in an RC drivetrain. We use shims on many major RC drivetrain components, most notably inside gear differentials, on the outdrives of a differential inside gearbox housings, and behind differential bevel gears.

The main reason we use shims is to adjust the “mesh” and “play” between RC differential gears and components. “Mesh” is the term we use to describe how the teeth of two gears fit together with one another. “Play” is the term we use when referring to any type of back-and-forth spacing of a component. If you can wiggle a part back and forth, then it has some play.

How we decide to use shims to adjust mesh and play will vary depending on the type of vehicle and which part of the vehicle we’re working on. Design and manufacturing tolerances can also vary in drivetrain components, so shims will almost always be necessary to get the correct placement of drivetrain components.

Manufacturer recommendations on shim sizes, number of shims, and placement of shims can be a good ballpark estimate, but precise drivetrain shimming ultimately comes down to how the components feel when assembled. Some extra fine tuning beyond manufacturer recommendations may be required. Ultimately, our goal is to have a smooth, freely spinning drivetrain with minimal play.

SHIMMING DIFF RING AND BEVEL GEAR MESH

One of the most common adjustment points in an RC vehicle’s drivetrain is the mesh between the differential ring and bevel gears. These two gears will determine the sound, feel, and performance of your RC vehicle’s gearbox. When improperly adjusted they can also be the most common cause of negative symptoms from a drivetrain and its components, such as loud noises and in worst cases complete gearbox failure. The methods of adjustment listed below apply to both geared differentials as well as axle spools with standard diff ring gears.

The first adjustment point to look at is the side-to-side play of the differential inside the gearbox housing. With the bevel gear in place, install the differential and outdrive bearings into the housing. You should notice a little side-to-side slack between the diff and outdrive bearings. This side-to-side play is what we will be using to adjust the gear mesh. Ideally, we want the diff ring and the bevel gears to contact each other with as much surface area as possible, without there being any noticeable binding or tightness.

To adjust this spacing we will place shims on either side of the differential as well as behind the bevel gear. Placing shims on the gear side of the diff will push the ring gear closer to the bevel gear, while placing shims on the non-gear side will move it further away. Placing shims behind the bevel gear will push it closer to the ring gear. My go-to method to determine the initial number of shims required and where to put them is to look at both the ring and bevel gears in the housing to first determine if there is enough contact between them. If there is not enough contact, simply place a shim behind the bevel gear and reassemble, repeating the process as necessary until the bevel depth is just right.

After the bevel gear depth has been set, rotate the bevel gear input shaft to feel the tightness of the gear mesh. While rotating the input shaft, slowly move the diff side-to-side and judge where the gear mesh feels the smoothest. Add shims to the sides of the diff as needed to set it in just the right position. Depending on the model of vehicle, this may be done by placing shims inside or

outside of the diff outdrive bearings. Use enough shims to take up as much of the slack as possible without it feeling too tight. Both the diff and bevel gear should fully contact each other, spin freely and smoothly, and not feel tight or crunchy.

SHIMMING INTERNAL DIFF GEARS

Shimming the internal gears within a gear differential is a straightforward process. Most standard 4WD gear differentials feature an internal gear set comprised of two larger outer gears, known as side gears, and anywhere between two to four smaller inner gears, or spider gears. Generally, you will always want to place a thin shim behind each side gear. This shim goes around the outdrive shaft, between the gear locating pin and the diff housing. These shims are meant to slightly space the side gear away from the diff housing to eliminate any possible drag on the diff housing. Depending on the design, some manufacturers call for thin shims behind each spider gear as well. This is meant for a similar purpose as the shims behind the side gears but can also be used as a tuning aid to tighten a differential’s gear mesh. The best method to tune this portion of the gear mesh is to place the same sized shims behind each spider gear, assemble the differential dry without diff fluid and spin the outdrives. Just like with the ring and bevel mesh, we are looking for a smooth feeling when spinning the outdrives, with no binding of the internal gears.

WILL DIFFERENTIAL SHIMMING LAST FOREVER?

Simply put, no. Shimming unfortunately doesn’t last forever. Any time you replace a differential component, whether it be an outdrive bearing, diff housing or any of the gearing it’s best to recheck the shim spacing. This will ensure your shims are set just right, making your drivetrain perfectly smooth, and helping it last longer to provide optimal performance. With those simple basics in mind, you too should now be able to properly shim and space your RC vehicle’s differential gearing for performance and longevity.