Ratbag
Administrator
- Joined
- Jan 1, 2012
- Messages
- 7,476
- Location
- Bayside, Melbourne, Vic
- Car Year
- MY06, MY10
- Car Model
- Forester SG & SH
- Transmission
- 5MT/DR & 4EAT Sports
Gidday KF
The actual vLSD is a sealed unit inside the diff housing. It contains a non-Newtonian fluid - i.e. instead of becoming thinner as it gets hotter, it gets thicker and stiffer.
Inside this sealed unit, there are many asymmetrical plates that are about 5-8 thou thick. These are in pairs, but always with an odd number in the stack.
One member of each pair is connected to each of the output shafts.
The 'stack' is tightly packed, the drive plates only being separated by a very thin layer of the fluid.
As the fluid heats up from the different rotation rates of the two shafts, it locks the plates together, giving the limited slip effect. As the two wheels start to rotate at the same speed and loading, the fluid returns to its static state - i.e. one of no resistance to the now non-existent shear forces.
If the vLSD is abused by being worked too hard, the internal plates can come into physical contact with each other, grinding against each other instead of the fluid causing them to lock. The result of this is to shed very fine metal particles into the non-Newtonian fluid, turning it into a Newtonian fluid - i.e. its ability to behave as a LSD is destroyed.
The same can occur with the centre diff when it's a vLSD one.
I have seen one of these diffs disassembled, and have photos of them in both new and buggered condition. The damaged diff "oil" is disgusting black muck. A freshly re-conditioned vLSD has fluid that is almost clear (just cloudy). It is clear enough that one can see well into the stack of asymmetrical plates.
These diffs are not really all that different from a "conventional" LSD. These latter can also be destroyed by abuse (but will withstand more abuse than a vLSD). They also require adjustment and maintenance, neither of which is necessary with a vLSD.
IOW, a vLSD is an ideal device for many applications. It has none of the vices of an AWD vehicle with "conventional" LSDs. It requires no special driving techniques. If it is not abused, it will last for a very long time. As an example of this, the centre vLSD in my Impreza gave no symptoms of degradation after nearly 18 years of normal use. I think I would have noticed if it had gradually turned into a FWD car!
:rotfl:.
This is how failure of the centre vLSD diff presents. The front output shaft is fixed, and the rear output shaft is locked/unlocked as required (as I understand it! Someone please correct me if my description is wrong or flawed ... ). In most of the MT cars, the diff is set to 50:50 F:R as the default condition. It can vary between 40:60 and 60:40 as required (assuming that the centre vLSD diff is working as designed).
The range of distribution between F:R varies a little from model to model. The above is true for my old '93 Impreza, and for Foresters (both with MTs).
The ECU manages the F:R torque distribution by some arcane magic that involves the angle of the car, along with other things ...
The rear vLSD is entirely 'mechanical'. It is not controlled by the ECU or TCU.
Hope this is both accurate :iconwink:, and of some benefit to you and others.
Without power through it, and a differential power balance, a vLSD will behave exactly as if it doesn't exist ... :iconwink: :lol:. Just like an open diff, as you have said.
The actual vLSD is a sealed unit inside the diff housing. It contains a non-Newtonian fluid - i.e. instead of becoming thinner as it gets hotter, it gets thicker and stiffer.
Inside this sealed unit, there are many asymmetrical plates that are about 5-8 thou thick. These are in pairs, but always with an odd number in the stack.
One member of each pair is connected to each of the output shafts.
The 'stack' is tightly packed, the drive plates only being separated by a very thin layer of the fluid.
As the fluid heats up from the different rotation rates of the two shafts, it locks the plates together, giving the limited slip effect. As the two wheels start to rotate at the same speed and loading, the fluid returns to its static state - i.e. one of no resistance to the now non-existent shear forces.
If the vLSD is abused by being worked too hard, the internal plates can come into physical contact with each other, grinding against each other instead of the fluid causing them to lock. The result of this is to shed very fine metal particles into the non-Newtonian fluid, turning it into a Newtonian fluid - i.e. its ability to behave as a LSD is destroyed.
The same can occur with the centre diff when it's a vLSD one.
I have seen one of these diffs disassembled, and have photos of them in both new and buggered condition. The damaged diff "oil" is disgusting black muck. A freshly re-conditioned vLSD has fluid that is almost clear (just cloudy). It is clear enough that one can see well into the stack of asymmetrical plates.
These diffs are not really all that different from a "conventional" LSD. These latter can also be destroyed by abuse (but will withstand more abuse than a vLSD). They also require adjustment and maintenance, neither of which is necessary with a vLSD.
IOW, a vLSD is an ideal device for many applications. It has none of the vices of an AWD vehicle with "conventional" LSDs. It requires no special driving techniques. If it is not abused, it will last for a very long time. As an example of this, the centre vLSD in my Impreza gave no symptoms of degradation after nearly 18 years of normal use. I think I would have noticed if it had gradually turned into a FWD car!
:rotfl:. This is how failure of the centre vLSD diff presents. The front output shaft is fixed, and the rear output shaft is locked/unlocked as required (as I understand it! Someone please correct me if my description is wrong or flawed ... ). In most of the MT cars, the diff is set to 50:50 F:R as the default condition. It can vary between 40:60 and 60:40 as required (assuming that the centre vLSD diff is working as designed).
The range of distribution between F:R varies a little from model to model. The above is true for my old '93 Impreza, and for Foresters (both with MTs).
The ECU manages the F:R torque distribution by some arcane magic that involves the angle of the car, along with other things ...
The rear vLSD is entirely 'mechanical'. It is not controlled by the ECU or TCU.
Hope this is both accurate :iconwink:, and of some benefit to you and others.
