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
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:

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). 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:

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

Imagine 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 Coating
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.

The Pros
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.

Round One
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 performed.

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.

Simply apply 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.

Round Two
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 runs.
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.

Surprisingly, 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 ticket!