The great thing about differentials is that they allow the two outputs to rotate at different speeds which is important when cornering. The downside of differentials is that they allow the two outputs to rotate at different speeds which can be a problem when you try to accelerate straight ahead. In particular, if one of the outputs has less grip than the other output (or, more accurately, if one output exceeds the available grip before the other output), the engine's torque will gravitate to the output with less grip, following the path of least resistance. The result is a lot of blue tire smoke and not much acceleration.
To combat this, one adds a limited-slip device to the differential. There are three ways that this is done in DSMs.
The LSD that comes stock in the center of AWD DSMs (as well as the rear of many AWDs) is a viscous coupling (aka vicious coupling). These have two sets of plates in close proximity but separated by a silicone fluid. One set of plates is connected to one of the outputs. The other set of plates is either connected to the other output or the carrier getting the input. The key point is that the plates rotate relative to each other when the differential is allowing one output to rotate faster (or slower) than the other output.
When the plates within a VC spin at different speeds, a resisting torque is produced by the viscous fluid between the plates. In a simple world, the larger the difference in the speeds between the two sets of plates, the more resisting torque is produced. However, the function relating the speed difference to the amount of torque is sub-linear and almost as bad as a square-root function.
Fortunately, the silicone juice gains viscosity as it is heated, which adds more resisting torque. Even more, it also expands as it is heated. At first, this expansion causes more of the plates to be in the fluid, so these two changes both increase the resisting torque. However, at some point, the expansion of the fluid will cause direct metal-to-metal contact within the VC (which is why half the plates have holes and half do not), often producing enough grip to lock it solid. This happens rather suddenly and it is often referred to as the "hump phenomenon." When this occurs, all relative motion between the plates usually stops and the fluid begins to cool and, at some point, the VC will release.
The main problems with this type of LSD are that it is outrageously non-linear and also subject to the effects of initial conditions (e.g., the previous temp of the fluid and diff housing), so it is not what you want in a competition car because it is very harc to predict. It is also relatively slow to react, given that heating must occur before the hump phenomenon locks the diff. That is why the new Evo doesn't have a VC. Nor does the STi, for that matter.
The second main type of LSD that you find in DSMs is the modified clutch-pack (which I believe is also called a Salisbury but is more often just called a Kaaz amoung DSMers because Kaaz makes a good one for our cars). These also have two sets of plates, but they are not separated by a silicone fluid. Instead, they are in direct contact and are therefore more like the clutch between the flywheel and transmission. And also like a "normal" clutch, the pressure between the plates changes. But unlike a VC, which reacts to differences in output speeds, the modified clutch-pack reacts to the amount of torque being transmitted through the differential. To explain this requires another stolen picture:
To combat this, one adds a limited-slip device to the differential. There are three ways that this is done in DSMs.
The LSD that comes stock in the center of AWD DSMs (as well as the rear of many AWDs) is a viscous coupling (aka vicious coupling). These have two sets of plates in close proximity but separated by a silicone fluid. One set of plates is connected to one of the outputs. The other set of plates is either connected to the other output or the carrier getting the input. The key point is that the plates rotate relative to each other when the differential is allowing one output to rotate faster (or slower) than the other output.
When the plates within a VC spin at different speeds, a resisting torque is produced by the viscous fluid between the plates. In a simple world, the larger the difference in the speeds between the two sets of plates, the more resisting torque is produced. However, the function relating the speed difference to the amount of torque is sub-linear and almost as bad as a square-root function.
Fortunately, the silicone juice gains viscosity as it is heated, which adds more resisting torque. Even more, it also expands as it is heated. At first, this expansion causes more of the plates to be in the fluid, so these two changes both increase the resisting torque. However, at some point, the expansion of the fluid will cause direct metal-to-metal contact within the VC (which is why half the plates have holes and half do not), often producing enough grip to lock it solid. This happens rather suddenly and it is often referred to as the "hump phenomenon." When this occurs, all relative motion between the plates usually stops and the fluid begins to cool and, at some point, the VC will release.
The main problems with this type of LSD are that it is outrageously non-linear and also subject to the effects of initial conditions (e.g., the previous temp of the fluid and diff housing), so it is not what you want in a competition car because it is very harc to predict. It is also relatively slow to react, given that heating must occur before the hump phenomenon locks the diff. That is why the new Evo doesn't have a VC. Nor does the STi, for that matter.
The second main type of LSD that you find in DSMs is the modified clutch-pack (which I believe is also called a Salisbury but is more often just called a Kaaz amoung DSMers because Kaaz makes a good one for our cars). These also have two sets of plates, but they are not separated by a silicone fluid. Instead, they are in direct contact and are therefore more like the clutch between the flywheel and transmission. And also like a "normal" clutch, the pressure between the plates changes. But unlike a VC, which reacts to differences in output speeds, the modified clutch-pack reacts to the amount of torque being transmitted through the differential. To explain this requires another stolen picture: