Obviously, as a reader of our blog, you have noticed that we are talking a lot about the friction number i.e. the oil friction factor of the automatic transmission. This is because this is a very important indicator of the oil’s health. Let’s take a closer look at why it is like that.
The lifetime of the clutches and the maximum capability to transfer the force from the engine to the wheels depend mainly on the friction characteristic of the discs. Thus, the clutch engagement is critical and one of the most important features of the transmission. How well or poorly the clutch discs interact with each other when there is a thin layer of automatic transmission oil indicates the friction coefficient.
In fact, the static friction number and the dynamic friction number are also distinguished. The dynamic friction number indicates the friction resistance between the sliding clutch discs. The static friction number indicates the friction strength of the clutch discs between the stationary clutch discs!
When do the clutch discs slip?
The clutch discs slide when the gear is changed. This is the moment when the clutch discs are pushed together increasingly tightly. The higher the dynamic friction number of the oil, the better the clutch discs interact, or the faster the speeds converge between them – the gear moves in.
The lower the dynamic friction number, the longer the clutch discs slip, and gear goes in for a very long time. Obviously, the gear switching must not be too short or too long. In the first case, you feel a strong knuckle, in the second case, too many clutch discs are heated and worn.
The static friction number is the oil feature to hold stationary clutch discs together. It may seem strange, but static and dynamic friction numbers are often not the same and should not be the same!
Let’s take one example of everyday life, where the static friction is greater than dynamic. Take and put a finger on the wet surface of the wet sink and slip it – you probably hear a creak. There will be a creak because the dynamic friction factor between your finger and the sink is less than the static friction coefficient – the sliding finger slips more easily than if the standing finger would slip. No one wants a vehicle that swings when driving, or gear shifting. Therefore, the static and dynamic friction number of the transmission oil is attempted to be “as accurate as possible”. Some chemicals raise and others lower the friction number.
What does it mean if transmission’s static friction number is large?
The clutch discs slide at the moment of the shift (the dynamic friction number plays a role). Immediately before the rotation of the clutch discs is aligned (locked), the static friction number takes over. If the latter is larger than the dynamic, the clutch discs are braked suddenly and with a yank. You can feel it from the yank of the shifting gears.
Thus, it is best to have a static and dynamic friction number relatively similar (static slightly smaller) and a dynamic friction number to increase as the slip increases. The latter means that the faster the discs rotate, the more they will be braked.
Various additives can be used to change one or the other size. Unfortunately, over time, the properties of the oil change and the static friction number (especially at high temperatures for long periods of time) changes. One common feature of old and worn oil is the vibration or color of the clutch discs. This is precisely because of the change in oil and clutch discs over time.
What matters is that the friction number does not depend solely on the oil, but also on the material of the clutch discs and the transmission software. That is why it is extremely important that the transmission is equipped with the right oil, the characteristics of which are in line with the transmission’s own materials and control systems. The same transmission that is under BMW or Audi uses, for example, a different oil and the reason lies in the software that controls the transmission. In one case, it is considered that the ultra-thin oil is used, in another case it is not.
We hope that you have a better understanding of the mind of the oil of the automatic transmission and therefore will not use universal oils in the transmission. Transmission oil is a high-tech product and therefore only licensed oils should be used.