Let's discuss tyre dynamics, and then the interaction between the tyres and the viscous coupling centre differential.
Let's study the attached diagram.
image upload no resizeSuppose the same car, of the same weight, travelling the same speed is fitted with low grade budget tyres. Diagram on the left.
The driver wants the car to take a turn in the direction of the black arrow.
To generate sufficient side force to move the momentum of the car in the desired direction, the driver needs to input a steering angle to the blue line.
The angle between the direction the tyre is pointed and the direction the tyre moves is called the slip angle.
Now let's consider the same car now fitted with high performance tyres. Diagram on the right.
To make the same manoeuvre to turn the car and change direction, the driver requires a smaller input to the steering wheel.
As the tyre is more grippy, the angle between the direction the tyre is pointed and the direction the tyre moves is less and it has a smaller slip angle.
So, there is a attribute called "slip stiffness". This is a measure of side force generated per angle of slip.
A high performance tyre will generate higher level of side force or turning force per degree of slip, than a budget tyre.
The high performance tyre is said to have greater slip stiffness.
So when you fit those high quality high performance tyres to your car, you notice that the car becomes more responsive, it turns in quicker, less steering angle input is required to turn the vehicle and ultimately higher speeds through corners are achievable compared to the budget tyres. (Higher speeds in turn created higher centripetal forces requiring higher side forces from the tyres)
How does this effect the viscous centre differential?
The tyre has a limit in the frictional force it can excerpt with the road surface. Beyond that, the tyre brakes traction and slide occurs. Once slide occurs the tyre cannot control the direction of the car.
This diagram represents the maximum force the tyre can exert upon the road surface in any direction.
(If the diameter of the circle represents FORCE, then High Performance Tyres have a larger diameter compared to low grade budget tyres and a wet weather circle would be smaller than a dry weather circle)
image hostingDirection A indicates the maximum force that the tyre can provide to propel the car forward. In this position of maximum traction there is no force available to change the sideways direction of the car and turn the car.
Direction B indicates the maximum force that the tyre can provide to slow the car. In this position of maximum braking force there is no force available to change the sideways direction of the car and turn the car.
Direction C & D indicates the maximum turning force that the tyre can provide to change the direction of the car. In this position of maximum side force there is no force available to change the velocity of the car.
Direction E indicates a turn under power. Some force is generated to accelerate the car forward and some force is generated to change the car's direction. The sum of the two forces being at the limit of the contact force between tyre and road surface.
With a very stiff viscous coupling centre differential, and when the tyres rotate at different speeds requiring differential action, say during a turn, force is required between tyres and road to turn the viscous coupling. If this force is too high, because the rating of the diff is very high, or the centre diff is on its way out and failing, then forces required to turn the viscous centre diff can become so large that there is insufficient forces left to change the direction of the car. The force diagram moves to position F.
image upload no compressionThis is one reason why the 20kg.F STI viscous centre differential is renowned for making daily driver road cars "understeer like a pig"
The 20kg.F centre differentials work great when the suspension is especially set up to compensate for them and also on off-road rally surfaces. On Gravel the tyres are continually braking traction with the loose road surface so the static friction diagrams we have been examining no longer apply. Maybe that is a discussion for another day.
So, Stevo.3RB, the thicker silicone fluids exist. Certainly think about tuning the stiffness of the centre differential, however do so understanding the dynamics of the handling of the car that will be affected.