Back in the day--well, my day, anyway--you only had one diff to worry about. Once you became an expert at tuning your single diff, the industry threw a curve ball your way. Enter: 1/8-scale off-road and 4WD on-road [also 1:10 off-road]. In 1/8-scale, you're now dealing with three diffs instead of one, and anything that's shaft driven--including TC--has bevel gears in the equation. The one common factor ...

Words: Michael Wortel

Issue 140 (July 2007)

... in anything 1/8-scale or shaft propelled is the introduction of a new concept: diff shimming. Shimming diff gears essentially involves using thin, washer-like shims as spacers that can alter the mesh between the bevel and diff gears. If there is too much play in the gears, they will wear excessively and you'll lose efficiency. If there isn't enough play, gears can bind--placing stress on your internals and leaving you with a tight drivetrain that heats up. To go fast and ensure a long life for your gears, read on.

Horizontal diff shimming

The most common place that you'll shim a diff is on the outdrives, between the bearing and the inside of the diff housing. This essentially moves the ring gear toward or away from the bevel gear, which lies perpendicular. To tighten the mesh, add shims to the right side of the assembly. A properly set mesh will feature just a bit of play, but not as much as you'd be looking for on the clutch bell and spur gear, for example. If the mesh is too tight, it will put unwanted pressure on the bevel gear, driveshaft and bearings on the bevel housing. If it's too loose, it will be noisy and wear the gears at a much higher rate.

Shimming inside the diff

The spider gears that are in your internal diff assembly don't need much attention, but if you ever start stripping them, there's a good chance you'll need to add a shim or two behind the outdrive gear so that it pushes down on the spider gears, preventing them from slipping. Be careful not to add too many, because if they push together under too much pressure, they will bind. Adding shims can also have the effect of locking the diff a bit.

Shimming bevel gears

This applies to anything that uses a shaft-driven 4WD drivetrain. The bevel gears in the front and rear part of the driveshaft have the ability be to set either closer or further to the ring gear. Since bevel gears are conical, moving them toward or away from the ring gear has the effect of matching up the surface area of both gears. You want 100% of the ring gear to be in contact with the bevel, but you don't want it so close that the bevel is pushing forward too hard on the ring gear. Adjust this setting by adding or removing shims from inside of the bevel housing, behind the gear itself. You're looking for close to zero play. The bevel gear will wear quickly if the setting isn't correct.

Helical gears

Over the last few years, there has probably been as much R&D done on 1/8-scale off-road as the last 10 years. This is evident in the new helical-style gears that are finding their way into more and more kits. The curvature of the teeth, as opposed to standard straight gears, allows for a smoother, quieter gear action. This new mesh comes at a price. Shimming correctly, with as little play as possible, is paramount, because the new gears are thinner and more susceptible to stripping. Shim helical diffs with even less play than standard gears, and you'll make your helicals last through the longest mains.

After break-in

It's inevitable that over time, no matter how correctly your diffs are shimmed, the gears themselves will wear. Diffs take on more heat and abuse than most other places on your car, and even though they are designed to be durable, they will inevitably wear. It is important to monitor the wear of your gears and shim accordingly. As your gears get older, you'll have to add shims to retighten and align the mesh, because worn gears are smaller. Every few race days, scope out the wear and determine if you need to make any changes.


You'll find out that shimming diff gears is sort of a trial and error process. It's also one of those RC adjustments that you simply have to "feel for," because there isn't a single, standardized setting from car to car. And even when you finally have your shims in order, time will wear the gears a bit, and you'll have to readjust them again. It's a good idea to have various shims on hand at all times and keep the sizes separate, because their thickness may vary only slightly. In time you'll learn what a perfectly shimmed diff feels like, so don't be afraid to experiment a little. RC is a "learn by your mistakes" hobby, but coming in armed with a how-to can save you time and money in the long run.