Even though their design slightly differs from one RC vehicle to another, differentials are part of all buggies or truggies and their setup is still the same. The center differential is the most important because it ensures the transfer of power between the front and rear differentials, which distribute it then to the left and right wheels.
Besides the transfer of power, diffs are also important when the car is turning, because the outer wheels have to spin faster than the inner ones and, without the front and rear diffs, both wheels will be spinning at the same speed, thus making cornering very hard to do.
What do differentials do actually? Basically, they transfer power to one of the outdrives and the easiest one to spin will be receiving the power, which also means that the wheel with the less traction will receive most of the power. They are filled with different weights of silicone fluid, which locks the diff action. The thicker the fluid in the diffs, the more the diff action will be locked. While some RC cars already have fluid-filled diffs, some do not, so you should check your RC car’s diffs to see if they are properly filled.
Let’s see how each type of diffs work and how you can improve them in order to maximize your RC car’s performance. When accelerating, weight is transferred to the rear of the vehicle and the wheels get more weight. Consequently, they get more traction than the front wheels. The center diff transfers most of its power to the front wheels as they are the easiest to spin and it will make your front tires balloon up. This reaction is called diff unloading and can be fixed by adding thicker fluid in order to prevent the diff to unload during hard acceleration. This is why is so important to find out the exact thickness you might need for your diffs in order to achieve the best acceleration possible. Fluid viscosity varies from 1000wt to 100 000 wt, the bigger the number, the thicker the fluid.
The fluid should be thin enough to be able to transfer more power to the front than the rear of the car when you’re accelerating, but also thin enough not to send too much power to the rear wheels because this will only make your buggy or truggy over-steer when exiting corners. Mainly, the center diff should be filled with a fluid of around 7000wt viscosity for buggies and 10 000 for truggies.
The front diff is also extremely important in steering because changing merely the fluid will influence power steering. Thinner fluid increases cornering and off power steering, while thicker fluid will help you gain power when exiting corners. For the front diff, the average viscosity is 5000wt for a diff from a buggy and 7000wt for a diff in a truggy.
The rear diff is the easiest to tune because it should be free enough not to decrease cornering, but tight enough in order to prevent the inner wheels from ballooning. If you have tuned it with too heavy fluid, your buggy or truggy will act like a fishtail when exiting corners and cause over-steer. You’re good to go if you use a diff fluid of about 3000wt in viscosity.
While there’s no ideal solution for tuning differentials, you have to know which will work best in different circumstances and, of course, it’s a matter of trial and error. You should take your time and analyze each diff independently in order to find out which is best for you.