Lots of cars today either have fluid filled gear diffs or at least offer the option to use them. For example my Yokomo cars offer both the ball diff and gear diff options. This is nice because the two diff styles have different benefits and negatives. For now we’re going to focus on the gear diffs and what the fluid weights do for the car handling wise.
In general heavier cars such as 4wd SC or 1/8 buggy start off in the 5/7/3 range. Now you may see this listed in setup sheets or notes so let’s go over that numeric order for a second. Diff oils are always listed front/center/rear in regards to their weights. So if you see a setup sheet that shows 5/7/3 than the car is running 5k CST front, 7k CST center, and 3K CST rear diff oils. In general you will find this is a pretty standard setup on most cars. This setup is at least a good “base” to start on if you’re unsure.
Now if you find yourself on a slippery surface, you’ll generally go to a lighter diff oil setup. 3/5/2 or even 3/3/1 depending on the car, tires, traction, etc. Since I primarily run electric powered cars this maybe a bit too light in the center oil as electric motors provide more torque than nitro powered cars.
A higher grip surface will go to heavier oils such as 7/10/5 or 7/7/5. This slows the diff action of the car which in turn slows the reaction of the car. As a driver you’ll find the car easier to drive as well as more stable on the high grip surface.
Front Diff: Heavier oils in the front diff slow the steering down going into to the turn but can help with on power steering coming out of the corner. Don’t forget there are two ends to your race car here…Keep reading.
Center Diff: Center diff fluids affect your cars acceleration and stability and tracking. Lighter fluids tend to help the car track straighter. There is a down side to this such as the front tires blowing up which slows the cars acceleration rate. This can affect jumping not being able to clear a section. Heavier weights help with acceleration but can also cause a condition where the car wants to swap ends while on the throttle and make the car very hard to drive. There’s always a balance here. Again nitro cars will generally be a bit lower than an electric car. If the nitro buggies are running 5k or 7k, don’t be scared to try 7k or 10k in your E-buggy.
Rear Diff: Rear diff oils affect how the car rotates in the corners. Lighter weights increase the cars rotation speed off power. Too light can make the car inconsistent in the corner giving the driver an unsettled car you can’t trust which is not good for lap times. Heavier fluids can help slow the cars rotation down a bit also and is a good adjustment when the grip is high. Note that the front and rear diffs really must balance the steering and corner speed together. If you’re running 10k in the front you will not likely be running something super light such as 2k in the rear.
Like many other car or suspension adjustments tuning your gear diffs with different weights of oil is all a game of balance. If you talk to the pro drivers not only in r/c racing but any form of racing with 4 wheels they’ll all want the fastest race car they can drive consistently quick around the track. Hero laps only win the race if they’re the last lap while you’re in the lead. If you’re not comfortable driving the car, sit down and work out a plan to make some adjustments and get it where you can drive it consistently quick. Top drivers are often within a .1 of a second in their laps on a clean track which shows you the car is in their control at all times. It may look like they’re pushing and flying around the track, but if their consistently quick…they’re comfortable driving the car at those speeds.