When most folk talk about limited slip differentials for cars with a BMC
rear end, the minds immediately turn to the "Salisbury". Indeed many think that
word covers any LSD for a BMC. This of course is not so. The Salisbury diff was
designed in the 50's at a time when race cars were not sophisticated, tires were
usually crossply with limited grip due to port compounds, and tracks were more
than a little bumpy. Agriculturally built cars needed diffs built along similar
lines. High static loads were more than common to help compensate for shortfalls
in chassis and tire design - to all intents and purposes the diff was
practically locked up, making vehicle a real beast to drive.
Over the past few years several different types of diffs have evolved, varying greatly in both price an design. Some deem one particular type to be the best, whilst others consider an alternative as the ultimate. Specific design seems to hold sway, as one type will give better results in certain conditions over another. One thing most designs have in common is that their performance can not be altered, or are very limited in their adjustment. Those considering fitting an LSD should be wary of claims by manufacturers that their particular unit is "user friendly". This is an alternative way of saying that the unit is not very efficient in cross-axle torque transfer - the very parameter that an LSD is fitted to control. Some explanation is obviously required as this parameter and the function of an LSD is grossly misunderstood by a majority of people.
Basically it is used to transfer drive from the wheel that has NO grip to the wheel that has grip. An extreme condition is one wheel on tarmac, the other on wet black ice. With a standard diff, the wheel on the ice spins unhindered so no forward motion is obtained. The LSD effectively joins the two wheels together, it's design affection its capability to sense and control the cross-axle torque transfer required to produce drive at the wheel with the most grip.
Driving a vehicle where loose or varying types of surfaces are a feature means that a diff is continually transferring torque across the axle from one side to the other as each wheel searches for grip. This creates the effect of the car snatching one way then the other whilst traveling in a straight line. As a corner is encountered, invariably the inside wheel goes light. Drive is transferred to the outside wheel and the vehicle develops " straight-on" tendencies
Both of these produce the need for a certain degree of physical input to hold on to the steering wheel whilst encouraging the vehicle to go in the direction you require. The amount of physical input required depends on how the diff is set up. This directly affects the overall performance of the vehicle as the ability of the driver and chassis to cope with these situations is paramount. An LSD that is tunable to the chassis, it's usage, and driver ability is an obvious asset.
Ingenious design, meticulous engineering and careful development has culminated in the production of an LSD unit the eclipse most of those currently available. This plate type diff employs high quality materials in its construction, and skillful design ensures a very high strength, reliable unit. It's ingenious layout allows a wide cross-section of settings to suit all applications through road use, to dirt track and rally to road racing.
Static pre-load (often the only "adjustment" available to the various other diff manufacturers) affects the cross-axle torque transfer in a one-wheel-zero-grip situation. The actual amount of pre-load is usually determined by the intended main vehicle usage. As a rough guide, lower settings are used where grip availability is good, higher ones used in poor grip availability. The percentage of torque bias transmitted can be varied by altering the clutch plate configuration. This changes the amount of engine torque actually passed through the diff to the driveshafts before "slip" occurs.
A special feature of the new unit is the facility to change the locking action of the diff on acceleration AND deceleration - a facility non-existent in other designs. Changing the ramp angles allows fine tuning of the diff action on drive and over-run, affection turn-in and power-out characteristics.
The cases are machined from steel billets, and all components manufactured to exact specifications from high grade materials. The cross pins are much more substantial than other types. Their design combined with the employment of an isolated drive case means that they are not subjected to bending stresses, and inherent problem experienced with the Salisbury diff, thereby eliminating cross pin failure.
This, the most modern design of limited slips, is now solely available from Mini Mania for many applications.
This unit works on the plate type principal employed in early designs, but is manufactured using 90's technology. Although once fallen from favor to other design types (pawl, gear, worm drive, etc.) this system is rapidly regaining popularity, being recognized as giving the most efficient and effective performance.
To enhance this, facility is given to the user to alter the diffs performance, not only by changing the static pre-load (common to other plate type diffs)but also percentage of cross-axle torque transfer AND locking action for both acceleration and deceleration.
Static setting are easily changed by replacing one diff plate with a thicker one. A selective plate pack is available separately. Static pre-load is checked by holding unit in a vice and checking slip point with a torque wrench on one of the crown wheel retaining bolts.
Percentage torque bias is changed by interleaving the plate configuration. If this is altered, ensure that there is ALWAYS a lobed plate against this inner cam ring. Interleaving the plates increases the torque bias, but is also dependent on the ramp angle used. Shallower ramp angles give higher percentage torque bias.
The design of the cross pins and the ramps in which they operate allow ramp angle changes to be made. The closer to a 90 degree angle the ramp is made, the less action it has - so a 90 degree ramp effectively unlocks the diff making it behave almost like a standard diff. Conversely, the shallower the angle the greater the locking action.
Changes to cam ring ramp angles can only be done during manufacture. There are three basic types available which are the result of much experimentation and testing. Any other specific ramp angles required can be custom made, but will obviously take a little time to do. The tree available cases are set at 90 degree/55 degree, 90 degree/35 degree, 50 degree/40 degree - quoted as deceleration/acceleration.
The units are available in three basic settings:-
1. 90 degree/55 degree ramps, 25lb ft static, pre-load, torque bias for road us. This makes the diff 'gentle' in use.
degree/40 degree ramps, 25-30lb ft static pre-load, good torque bias for rally/autocross/grass track/ etc.
3. 90 degree/35 degree ramps, 25-30lb ft static pre-load, torque bias for circuit, hill climb, and other race/tarmac/slick uses. The near release of the diff on deceleration greatly enhances turn-in, much reducing under steer.
All the above are easily altered to increase torque bias
if required, as can static pre-load.