Issue 130 (September 2006) - Words: Derek Buono
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.
Chad Bradley on Diffs
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.
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
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:
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.
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
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
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.
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...?"
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
Truck: Same as buggy but the diff setting to try for
the front (7,000wt) and for the center 7,000-15,000wt.
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.
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,
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.
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
... I want
more steering entering the turn:
Buggy: Lighter front oil (3,000wt) and rear (1,000wt)
Lighter front oil (5,000-7,000wt) and rear
more steering exiting:
Buggy: Thicker front oil -5,000-10,000wt
Truck: Thicker front oil-10,000-20,000wt
more acceleration out of a turn:
Buggy: Thicker center oil - 7,000-10,000
Truck: Thicker center oil - 20,000-50,000wt
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).
pulls around too much through ruts:
Buggy and truck: Usually this happens because the oil
in the front and center differentials is too thick;
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
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.
Issue 132 (November 2006) - Words: Jeff Huneycutt
yourself behind the scenes in a NASCAR Nextel Cup
garage. You may be quite capable with everything when it
comes to RC, from treating foam tires to tuning a nitro
engine, but here no one can do everything. You are a
gear specialist, and your life is consumed with finding
ways to cut friction in the drivetrain. You demand that
any gearset that will go into your race car be fully
polished. You pull the gears and rebuild the
differential after every race, and sometimes even
between practice sessions, rather than risk a breakdown.
You use a pump that not only pushes the gear oil through
a cooler, but also sprays the oil directly onto the
gears to maximize lubrication. You are constantly trying
different lubricants to see which is best at cutting
friction and improving the gear's service life. And now,
within the last few years you have begun experimenting
with coatings which have just been developed and promise
to cut friction more than ever before.
Of Course, in
the world of RC racing we don't have the nuclear-powered
budgets those guys do. Most of us don't have the luxury of
specialists working on our cars--either you do it yourself
or it doesn't get done. But that doesn't mean we cannot
piggyback on the advancements the NASCAR guys are making and
claim some of the benefit too.
Although most RC cars do a very good job of reducing
friction by using bearings throughout, quality materials,
and even CVD axles in some cases, there is still more that
can be done. The diffs are lubricated by either grease or
silicone fluid, but the ring-and-pinion and spur gears are
often left to run dry. This is one area where it might be
possible to steal a trick the Nextel Cup teams have
developed and cut a little power-robbing friction in our own
cars and trucks by coating the gears.
Gear coatings are nothing new, but they have advanced quite
a bit in the last few years. There is a coating for
everything from blocking heat, to increasing a material's
surface hardness, to cutting friction. For our application,
friction is our only concern, so that is what we will
concentrate on. Anti-friction coatings generally work in one
of two ways (or both): They can use a naturally slick
material like graphite or Teflon to coat the material and
create a smoother surface, or they can use a compound that
attracts and encourages oil to cling to it. A lubricant will
stick to anything coated with this compound and provide
better lubrication longer than it would with bare metal.
Because chemistry wasn't my time to shine in high school, I
turned to a professional when it came time to determine what
type of anti-friction coating might work best on an RC car.
Tech Line Coatings is one of the biggest manufacturers of
advanced coatings for everything from aerospace to Nextel
Cup and Indy Car racing. When I spoke with Leonard Warren,
the owner of Tech Line, he took particular interest in the
small tooth sizes and tight clearances that these cars
typically run and recommended Tech Line's WSX Powerkote dry
lubricant. The WSX is buffed onto the material and bonds
with heat. The advantage here is that this is a dry powder
and does not build up on the material like a typical wet
lubricant that is sprayed on and then baked to form a bond
with the metal.
The WSX powder was originally designed to be used inside the
cylinder and lifter bores of full-size race engines, so it
can handle extreme temperatures and pressures. When properly
applied the coating is less than 0.000020 of one inch thick
(that's 20 millionths!) and can bear up to 350,000 psi of
pressure without breaking down. It is both naturally slick
on its own and also bonds with liquid lubricants to help
improve lubricity (fancy talk for "slickness"),
which makes it perfect for RC applications.
Now that we had the proper coating for our needs, we needed
to devise a test to see whether it actually works. The trick
is to eliminate as many variables as possible. For a test
rig we chose Kyosho's FW 05T chassis. It is a nitro on-road
car with shaft drive, a two-speed transmission, and ball
bearings throughout. Once the car was completely broken in
so that additional driving wouldn't affect speed, we planned
to do a series of timed runs with the car as it arrived
stock then apply the coating to the gears and repeat the
test. We specified that once the first set of tests was
complete, nothing on the car could be changed--including the
engine settings or transmission shift points,--until the
coatings had been applied and the second set of timed runs
After thoroughly breaking in the car, it was disassembled,
checked for any necessary repairs, and cleaned. This was to
ensure that everything was in proper working order for the
first set of timed runs. A straight course 300 feet long was
laid out on a stretch of flat, smooth asphalt, and the car
was run for several passes during the early evening (because
the air is calmer at that time). The air filter was cleaned
just before the runs, and engine temps were kept between
250- and 275-degrees F. The car began each run from a dead
stop, so within the span of the 300-foot pass would have to
work through the friction in the gears both from
acceleration and maintaining maximum speed.
It turns out the biggest variable we had to fight was the driver (yours truly)
who often seemed to find a pebble that was missed when the
course was swept. This was enough to upset the car and
change the times, so those passes were thrown out. We also
threw out any passes when the engine temp was greater than
275 because they were too fast. After about a dozen passes
most of the times for the 300-foot run hovered right around
the average time, 6.91 seconds, so we were confident we had
a good baseline to work from.
After the first set of runs the car was torn down again and
checked for any damage that could potentially slow the car
on the second set of runs. There was none, so I moved on to
the next step--applying the anti-friction coating. The
coating was applied to both the front and rear
ring-and-pinion gears as well as the spur and pinion gears
on the two-speed transmission. Interestingly, the spur gear
on the Kyosho is plastic, but Warren assured me that the WSX
powder works on plastic as well as steel and aluminum.
Fortunately, applying the WSX powder is neither complicated
nor very labor intensive. First, however, the gear to be
coated has to be prepared. If you are applying the powder to
metal it must first be etched by blasting it with a mild
abrasive like aluminum oxide or fine sand. To do that I used
a hobbyist's mini-sandblasting kit from Northern Tool and
Equipment that even came with a small container of aluminum
oxide. It won't knock the paint off of your grandpa's
tractor, but it is good enough for this job. All that is
needed is to lightly etch the metal.
After that all
parts must be thoroughly cleaned. Nitro cleaning spray is
good for this job and should already be handy. From this
point forward you will need to wear latex or rubber gloves
whenever you handle the pieces to prevent the oils from your
skin from contaminating the pieces.
It turns out
that the trickiest part when it comes to getting the WSX
powder on the gears is getting the powder on the gears. That
may sound stupid, but it's the truth. The powder is
extremely fine and absolutely dry. Also, it bonds to the
metal through pressure and heat, so it requires a little
more than just sprinkling it on and hoping for the best.
My first attempt was with a small buffing wheel attached to
a Dremel tool, but the powder would not stick to the cloth
buffing wheel and I ended up slinging the stuff everywhere.
After a couple different tries, I found the best method for
applying the WSX powder is with a toothbrush. Pretty low
tech, but it works.
powder to the gear and then rub it in with the toothbrush.
The bristles will also quickly load up with the powder,
which will help get it on there. To keep from wasting any
powder I worked with the gears inside a small box lined with
plastic. That way any powder that fell off wouldn't get
dirty and could be recycled. It's also easy to tell if the
coating is sticking, as the part you are applying it to will
slowly begin to take on the same deep glossy gray sheen as
the WSX powder.
After I applied powder to the eight different gears (two
sets of ring and pinions, two sets of pinion and spur gears
for the two-speed transmission), I re-assembled the car
exactly as before and prepared for the second round of speed
For the second round the time of day and location was the
same, but the temperature was approximately 10-degrees
warmer. I felt the extra air temp was acceptable, however,
because if the car turned out to be faster the reduced air
density would be a good counter-balance against any unknown
variables that might have artificially added speed.
the first few runs with the car were significantly slower
than the first round of tests. Excluding a warm-up run, the
first four runs over the 300-foot course hovered around the
7.30 mark--almost four-tenths of a second worse!
At fist I was
worried that the only thing this test had managed to
accomplish was slowing down a perfectly good car, but on
additional runs we noticed the times dropping. It turns out
the powder just needs a few high-speed runs to buff in.
After several more runs the times leveled out around the
6.65-second range. This marked an improvement of 0.26 (twenty-six
one thousandths) of a second over a 300 foot run from a
standing start. In terms of miles-per-hour we are talking
about improving from an average speed of 29.60 to 30.76.
(This is average speed, not top speed.)
An improvement of a little over one mile-per-hour average
speed isn't exactly going to set the world on fire, but
considering that an ounce of WSX Powerkote retails for
$24.95 (and we only used a fraction of it on the Kyosho), it
is a relatively easy and inexpensive way to add speed. After
completing the second set of test runs, we ran several more
tanks of fuel through the Kyosho before pulling the gears
for one final inspection. So far there are no signs that the
anti-friction coating is wearing off any of the gears, so it
seems to be relatively durable. I hope to be able to test
other vehicles, like a monster truck which puts even greater
stress on its gears, in the future, and if we see gains in
any other areas we'll be sure to post them in future issues
of XRC. Until then, for all you speed freaks looking to get
a little bit more out of your cars, this may be just the