In this article we explain how the differential works and why many 4WD radio-controlled cars have only front and rear differentials.
How the differential works
No matter the type of construction, the differential transmits and distributes motion to its outdrives, which must be able to rotate at different speeds. If an external action reduces the RPM of one outdrive, the other one increases revolutions by the same amount lost by the first.
Therefore, the rotational speed of the differential is always the same as the average speed of the two outdrives. Everyone experiences this feature every time spins a wheel in one direction and see the opposite wheel moving in the opposite direction.
The wheels speed when cornering
If you look at the wheel routes in curves, you can see that each wheel follows a different circular path.
Since all wheels run through the turn at the same time, the speed of each wheel is different from the speed of the other wheels.
Front, rear and center differential
In a 4WD transmission there must be front and rear differentials to differentiate speed between the left and right wheels.
If we take a closer look at the paths of the wheels, we also notice that the left front wheel goes farther than the left rear wheel, and the same is true for the right ones. This means that the average speed of the front wheels is higher than the average speed of the rear wheels, so the front differential turns more than the rear differential.
To ensure this condition in a 4WD transmission, it is also necessary to include a center differential that transmits motion to the front and rear differentials.
4WD transmissions with only 2 differentials
If 3 differentials are needed to work properly, why do many 4WD RC cars have only front and rear differentials?
The reasons for only 2 differentials
Actually, 1/10 scale cars or smaller do not have a center differential, with rare exceptions.
There is undoubtedly a size-related reason, but more than that, these models are rather light, and the additional stress experienced by the transmission is virtually negligible.
Some larger-scale models are designed without the center differential, but they are usually RC cars with strong plastic transmissions to keep costs down, unsuitable for competition and easy to use.
The dynamics with 2 differentials
2 differentials scheme has no disadvantages as long as the 4WD model car runs on straight track, but as soon as it gets into a turn, the simplified transmission scheme forces the wheels to rotate at a different speed than they should according to the trajectory.
The situation could also be explained analytically, but we prefer not to burden the topic. What really happens is that 3 wheels turn at the right theoretical speed, while 1 wheel is forced to turn at the "wrong" speed compared to the path, slipping.
As can be guessed, the wheels that naturally take turns in this "sacrifice" are always the wheels with less vertical load, i.e., those inside the corner: Under braking and corner entry, it is normally the rear wheel that loses load, which recovers it under acceleration until corner exit, when the front wheel unloads.
When cornering, it is as if the car has only 3 useful wheels for movement; the effect is undetectable because the fourth wheel, the "useless" one, would still contribute so little to the overall dynamics.
Transmissions without differentials
This is a very special situation that only concerns scaler/crawler models, which have a transmission without differentials to avoid wheel slip and overcome any obstacle.
This feature greatly worsens the ability to steer for these models, but there are technical solutions that partially compensate the absence of differentials. We discuss them in detail in this article.
Conclusions
In very general terms, we have seen what the differentials of a RC car are for and how they work.
In next articles we will discuss in more detail the types of construction, the role of all three differentials and the effects obtained by adjusting each of them.
Stay tuned!