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
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.
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.
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.
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.