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,
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.
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.
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
SHIM IT UP!
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.