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Tran-Ex Limited Slip Differential (LSD) for Austin Healey Sprites & MG MidgetsThe most modern design of limited slips, is now solely available from Spridget Mania HERE
When most folks talk about limited slip differentials for cars with a BMC rear end, their minds immediately turn to the "Salisbury". Indeed many think that word covers any LSD for a BMC. Of course , this is not so. The Salisbury differential was designed in the 1950's at a time when race cars were not sophisticated, tires were usually cross-ply with limited grip due to port compounds, and tracks were more than a little bumpy. Agriculturally built cars needed differentials 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 differential was practically locked up, making the vehicle a real beast to drive.
Over the past few years several different types of differentials have evolved, varying greatly in both price and design. Some deem one particular type to be the best, whilst others consider an alternative as the ultimate. Specific designs 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, a limited slip differential (LSD) 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 differential, the wheel on the ice spins unhindered so no forward motion is obtained. The LSD effectively joins the two wheels together, its design affects 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 differential 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 differential 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 that eclipses most of those currently available. This plate type differential employs high -quality materials in its construction, and skillful design ensures a very high strength, reliable unit. Its 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 differential 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 differential to the driveshafts before "slip" occurs.
A special feature of the new unit is the facility to change the locking action of the differential on acceleration AND deceleration - a facility non-existent in other designs. Changing the ramp angles allows fine tuning of the differential's action on drive and over-run, affection turn-in and power-out characteristics.
The cases are machined from steel billets, and all components are manufactured to exact specifications from high grade materials. The cross pins are much more substantial than other types. The design combined with the employment of an isolated drive case means that they are not subjected to bending stresses thereby eliminating cross pin failure, an inherent problem experienced with the Salisbury differential.
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 differential's 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 differential plate with a thicker one. A selective plate pack is available separately. Static pre-load is checked by holding the unit in a vice and checking the 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 differential making it behave almost like a standard differential. Conversely, the shallower the angle the greater the locking action.
Changes to cam ring ramp angles can only be done during manufacturing. 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. The three 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 use. This makes the differential 'gentle' in use.
2. 50 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 differential on deceleration greatly enhances turn-in, much reducing under steer.
All of the above are easily altered to increase torque bias if required, as can static pre-load.