For many of us,
differentials in 1/8-scale vehicles are among the most abused and
neglected parts on the kit. But for many they are the #1 tuning
option when they hit the track. All 1/8-scale buggies and truggies
have three of them: one for the front, one for the center, and one
for the rear of the car. As a racer, I usually search the hot
combination, but rarely use it as a tuning aid. The effects that the
diffs have in the handling of the car are huge; they can drastically
alter the way a car handles bumps, exits a corner, and accelerates.
We decided to talk to one of the pro drivers who has been
experimenting with different combinations for years. Chad Bradley
recently switched to Jammin' where he's been tearing up the track
with both the X1 CR and the CRT. Most of today's differentials are
tuned in with silicone oil, and the most common weights used range
from 1,000wt to 50,000wt. Understanding what each differential has
to offer, and how to tune them properly, can give you an edge over
the track conditions as well as the competition.
Words: Derek Buono
Chad Bradley on
Diffs
Front Differential
Buggy and truggy front (Steering): Depending on how your buggy/truggy
is handling, you can alter the amount of steering by altering the
weights in the front diff. If you want more steering entering the
corner try running lighter oil (3,000wt buggy, 7,000wt truck) This
will give the vehicle more off-power steering, which is better on
tight tracks. Heavier oils (7,000-10,000wt buggy, 10,000-20,000wt
truck) will give your car better on-power steering and will help on
higher speed tracks.
Why Have
Differentials
Hopefully it's no surprise why we have differentials, but if you
were just born here's a quick explanation: During a turn the inside
wheel travels a shorter distance, and therefore can spin at a slower
speed than the outside wheel, which has to go farther. Since both
are on the same car, it would be difficult for that to happen
without the invention of the differential. A diff allows one wheel
to spin faster than the other, which improves handling in corners.
In four-wheel-drive vehicles a third diff is used in the center, to
allow the front and rear wheels to travel at different speeds as
well.
Without any friction with the ground, the system would transfer
power to the wheel with the least resistance, and result in a loss
of speed in a corner and over bumps. We've all seen Subaru
commercials explaining that having power to the wheels that grip
(have the most resistance) improves handling. The way that is
controlled in RC vehicles is with silicone diff fluid (and in some
cars thick grease). This acts as a "limited slip"
differential, and allows the wheels to spin at different speeds -
but maintain power delivery to the wheels with more traction. The
thicker the oil, the closer to a solid axle the diff acts, and the
thinner the closer to an open differential it is.
It is by controlling the diffs with fluid viscosity that we alter
the handling characteristics of a car for faster acceleration or
more steering. Chad went through the ranges for his vehicles, but
they hold true as starting points for any brand, and how you drive
may alter what you end up with.
Quick Oil Reference: what should be in your diffs:
OIL CHANGE
Fluids wear out. Even though you really cannot judge the effectiveness
of any oil by the color, it is a good indicator that it's time for a
change. Chad usually changes his diff oil every three to four club
races, and rebuilds his diffs before any major event, and if he's
comfortable with the setup he generally doesn't change it. Regular
rebuilds are a cheap way to make sure the differentials stay
consistent.
Buggy and truggy front (Bumps): You also have to consider how the diff
affects the handling through bumps. Lighter oils (3,000wt buggy,
7,000wt truggy) are better for bumpy conditions. If the oil is too
heavy it will make the car want to change direction or oversteer in
the rough or rhythm sections.
Center Differential
Buggy and truggy: Lighter center oils help the buggy or truck track
straight while accelerating (3,000-5,000wt buggy; 7,000-10,000wt
truck). Running lighter oils in the center has a negative effect on
the acceleration. The lighter oil allows the power to be directed
toward the front of the car and hamper acceleration out of corners. If
you're having trouble clearing jumps that are out of a slow corner,
that could be a sign that the center diff is too light, but for really
bumpy sections having lighter oil in the center will allow the car to
accelerate better and straighter.
Rear Differential
Buggy and truggy: I don't adjust the rear too often, and if I do it's
usually only in 1000wt increments. Lighter oil in the rear diff gives
the vehicle more off-power steering, but can make the car or truck
feel inconsistent around the track, especially in long main events.
The majority of the time I will use 2,000 and 3,000 in the rear diff
in both buggy and truck.
Differential Tuning With Chad Bradley
We ask these questions all the time when we're racing, and have heard
them asked at least seven times while at the track... and we're sure
that as you're reading this you may be asking "What do I do
when...?"
...the track is
rutty:
Buggy: Try changing the center to a lighter setting first
(3,000-5,000wt); if the car is changing directions out of a bumpy
corner, try making the front lighter (3,000-4000wt).
Truck: Same as buggy but the diff setting to try for the front
(7,000wt) and for the center 7,000-15,000wt.
...the track is loose and smooth:
Buggy and truggy: The best way I've found to increase traction with
the diffs is to increase the oil weight in the rear diff slightly
-3,000-5,000wt.
...if the track is loose and rutty:
Buggy: A "square" diff setup here helps -5,000wt front,
5,000wt center and 3,000-5,000wt rear.
Truck: Lighter center and front -7,000wt front, 10,000wt center.
...the track is blue groove:
Buggy: Most of the time I run between 5,000-7,000wt in the front,
and 7,000-10,000wt center.
Truck: Most of the time on blue groove the cars and trucks on
throttle turn-in good because of the weight transfer to front tires,
but you lose some steering coming out of the turn. So to sacrifice
some turn in for out of the corner steering and acceleration, a
heavier front and center is better suited majority of the time
-20,000wt front, 30,000-50,000wt center.
...the track is blue groove and rutty:
Buggy: I would normally just go lighter in the center, but not too
light (5,000wt)
Truck: Since most trucks handle the bumps I wouldn't change the diff
settings from the smooth blue groove setup.
... I want more steering entering the turn:
Buggy: Lighter front oil (3,000wt) and rear (1,000wt)
Truck: Lighter front oil (5,000-7,000wt) and rear (1,000-2,000wt)
...I want more steering exiting:
Buggy: Thicker front oil -5,000-10,000wt
Truck: Thicker front oil-10,000-20,000wt
...I want more acceleration out of a turn:
Buggy: Thicker center oil - 7,000-10,000
Truck: Thicker center oil - 20,000-50,000wt
...I want it to go better through a rough section:
Buggy: Use thinner oil for the center (3,000-4000wt) and thinner oil
for the front as well (3,000wt).
Truck: Use thinner oil for the center (7,000-10,000wt) and thinner
oil for the front as well (5,000-7,000wt).
...it pulls around too much through ruts:
Buggy and truck: Usually this happens because the oil in the front
and center differentials is too thick; reduce weights.
Starting Point
All this is worth nothing if you don't have a starting point. Chad
gave us some starting weights for a variety of conditions that will be
good at most tracks. You can apply what we've gone over in the article
to fine tune your vehicle:
Conclusion
Once you understand the principal of how a diff works and how
different weight oils slow the action you will be a master tuner in a
few short days. This doesn't mean that dropping 1,000wt in the front
will make you a better driver. Learning how to drive is something that
has to be done before you can fine tune to get the car to behave like
you want.
