Different Differential oils provide different performance characteristics. These characteristics can be controlled by the different weights of oil placed in to the differential. The weight of a differential fluid is determined by the viscosity of the fluid. A higher viscosity represents a higher weight oil. Weightings can range anywhere from the low thousands to several hundred thousands. The diff fluid represented below has a weighting of 10,000.
The differential operates differently with different weighted oils. An open differential or one that has no viscous fluid in it will allow all motor or engine power to be routed to the tire with the least resistance. This means all the power potential you have will travel to the wrong wheel. In most RC applications, this is not desired.
There is however a way to provide more bias towards the slipping wheel so that the non-slipping wheel will be provided with more power. The way to do this is tune the proper weight oil for your application. There are either 2 or 3 differentials in the typical RC vehicle. Each one must be tuned with the proper weighted oils. It is recommended to use the weights that the manufacture suggests. A baseline is needed to begin the tuning. A factory configuration would be best for this. First it will be necessary to understand what increasing or decreasing the weight will do in each differential for an RC car.
A very good starting point for most buggies and truggies out there is to use 5000 weight oil in the front diff, 8000 in the center and 3000 in the rear.
Differential Performance – Front Differential
Increasing the weight of oil will provide you with more on power steering. This will provide an advantage to those who drive your buggy hard out of a corner. On the flip side under off power steering, your buggy would be more prone to understeer through the turns increasing the difficulty for off power steering.
Decreasing the weight of differential oil will increase off power steering into corners. This is excellent if you prefer to drive harder into a corner. On the downside under power, your buggy is more prone to understeer through corners while under power.
Differential Performance – Center Differential
A heavier oil will increase acceleration potential but will decrease off power steering capability. Under heavy acceleration there is potential for the rear end to swing out. A lighter oil will provide more on/off power steering, but decrease acceleration potential. Decreasing acceleration potential will make it easier to get out of the hole with no throttle control as the front axle performs majority of the acceleration.
Differential Performance – Rear Differential
Lighter oil will decrease rear end traction. This may provide more predictable steering at a loss of straighline acceleration. Heavier oils will increase traction, but may decrease a positive feel for steering.
In order to achieve the best setup for your application, it will take some trial and error in order to match your cars performance with your driving preferences. If you already have an idea for this, it will be easier to achieve the best setup more quickly. In general all choices will be a trade off. You will select a specific weight of oil in order to gain a performance characteristic, however you are always going to be sacrificing something.
Also, it is important to consider that higher weighted oils will generally be more difficult to handle. It may provide excellent acceleration performance, but with a higher power setup and tight turns, experience and skills will be needed to keep it all under control with throttle, brake and steering management.