Used in M3 GKN Viscodrive, the world's leading producing company of viscous couplings, announces a new limited slip differential (LSD) that is the first that can be tuned to optimize both traction and handling. The new Visco Lok LSD features an uniquely designed self contained viscous shear pump that provides tremendous freedom in specifying the level of torque transfer that is provided at any particular speed difference. Small torque transfers can be provided at very low speed differences to improve handling while high torque transfer with immediate response reaction is available at higher slip speeds due to its new progressive characteristic. Conventional LSD's are generally capable of providing either traction or handling improvement but not both. The Visco Lok LSD is currently in the advanced prototype stage at two U.S. automotive manufacturers with targeted 1998 model year axle and transfer cases applications.
While the orignal purpose of an LSD was to improve traction, in recent years it has been discovered that equally great benefits apply to handling and stability if the LSD is capable of transferring torque at low speed differences. The Visco Lok can be tuned to provide torque transfer at slip speed of just a few revolutions per minute. The Visco Lok automatically transfers appropriate torque to the wheel with the best traction conditions at any given moment.
The Visco Lok consist of a wet clutch pack, a pump unit, piston with seals, and a counter load belleville spring washer. Viscodrive has developed a highly durable steel clutch pack with plates as thin as 0.6mm which enables large numbers of plates to be packaged as compared with conventional clutch plates. The increased number of plates translates to increased torque capacity.
Hehehe... apparently that MA didn't see the part of the bulletin that said that this information was not supposed to get out, or he/she woul dhave to face the threat of a rabid squirrel chasing them down to pluck out all their nosehair...
Pricing still hasn't been announced though, so stay patient.
Hmmmm...ouch....glad that anonymity is upheld in this circumstance. I didn't know about the lock on the info being provided to others....but, he/she is safe in the hands of our MINI2 community that is grateful for this tidbit of knowledge and is honored that we are so trusted with it.
If it is similar to the description below then it is durable and would be the BOMB on the track.
I just don't know how correct the information is that we are examining from the source
-------------------------------------------------------------------------------------------------------------------------------
M Variable Differential Lock.GKN Viscodrive, BMW M engineers developed a special mechanical limited-slip differential for the M3s.
The principal (and principle) difference between a traditional limited-slip "diff" and this M Variable Differential Lock is that the former senses torque, the latter senses wheel speed (rpm). Under dry to not-quite-dry road conditions, the traditional limited-slip has long enhanced the handling of sporty rear-wheel-drive BMWs; however, under slippery conditions, this differential type has limited ability to improve traction. On all current BMW models, electronic traction control addresses this issue.
The M Variable Differential Lock specifically addresses low- and split-traction situations in a way that reinforces sporty handling, imparting to the M3 a slippery-road ability no previous high-performance, rear-wheel-drive sports car ever had.
Any time a speed difference develops between the two rear (driven) wheels, a shear pump, driven solely by this difference, develops pressure in the silicon viscous fluid in which the lock operates. In turn, this pressure is directed to a multi-disc clutch that transfers driving torque to the wheel with the better road grip ("select high"). The greater the speed difference between the two wheels, the more aggressively the clutch engages. As soon as the difference between the two wheel's speeds begins to diminish, the clutch starts to ease off.
This mechanism is "elegant," in that it achieves sophisticated action by entirely natural means. There is no external pump, no external source of lubrication or operating fluid. The very motion to be controlled – differences in speed between left and right wheels – generates its locking action. Viscous fluid is so-called because it develops internal force (via an increase in viscosity) whenever it is sheared; this is why the relatively small difference between one wheel speed and the other can generate the necessary action.
The Viscodrive info is great...but if we go on what I was told, it is a "clutch type." I assume this is a mechanical lsd as described in the explanation below. If this is the case, then durability might be an issue.
Mechanical
In the mechanical clutch type, differential velocity is detected by some means, such as a centrifugal weighted rotor, and this applies a force to a clutch mechanism which links the two shaft together to a varying extent - the greater the differential velocity, the more force is applied. This forms a negative feedback loop which limits the slip to a preset degree. In some designs, the clutch is self-actuating. Often, small multi-plate clutches are used. One disadvantage of the mechanical type is that the limiting action tends to occur quite rapidly rather than gradually, and this in itself can create unsettling dynamic effects for the vehicle as a whole.
Another type of mechanical LSD is the geared torque-sensing type. This arrangement uses planetary gears to "sense" torque on one shaft. The most famous version is the Torsen differential invented by Vernon Gleasman in 1958, than sold to Gleason Corporation, who started marketing it in 1982. But there are many other types available as well. Geared LSDs are less prone to wear than the clutch type, but their torque distribution characteristics can be less than ideal.
Viscous
The viscous type is generally simpler, and relies on the properties of a dilatant fluid - that is, one whose viscosity varies as the inverse proportion of an applied force. Silicon-based oils are often used. Here, a chamber of fluid rotates with the normal motion of the output shafts, but a differential motion causes paddles or vanes to move through the fluid. The greater the speed of the vanes, the more resistance the fluid will put up to oppose this motion. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, so for the average driver is generally much easier to cope with.
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. However, they are less prone to breakdown as long as the fluid is changed regularly.
Another bit of conversation I found re LSDs....on a MBenz forum:
"Limited slip differentials are rated at "percent limited slip" which is the torque differential allowed before the diff begins to lock up. Street oriented verisons are usually no more than 30 percent, or they can be jerky/chatter in normal driving."
Paul: You may be right. All I see from GNK Driveline is they did the development with BMW for the Visco-Lok. Can't find anything on production.
""Visco-Lok: GKN Viscodrive partnered with BMW to develop Visco-Lok, the variable M differential lock. The new differential is fitted on the BMW M3 model and significantly improves traction, even in difficult driving conditions, as well as vehicle handling and stability. Visco-Lok features a unique self-contained shear pump containing silicone fluid, which generates a pressure proportional to the speed difference across the wheels. The pressure actuates the clutch pack and transmits the torque to the appropriate wheel, without any extra hydraulic fluid being required.""
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.
