A SERIES EVO L.S.D - 50's DESIGN. 90's ENGINEERING

When most folk talk about limited slip differentials for the Mini, minds immediately turn to the name "Salisbury". Indeed many think that word covers any LSD for the Mini. This, of course, is not so. The Salisbury diff was designed in the 50's at a time when race cars were not sophisticated, tyres were usually cross-ply with limiting grip due to poor compounds, and tracks were more than "a little bumpy". Agriculturally built cars needed a diff built along similar lines. High static pre-loads were more than common to help compensate for short falls In chassis and tyre design - to all intents and purposes the diff was practically locked up, making the vehicle a real beast to drive.

Over the past few years several different types of diff 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 design seems to hold sway, as one type will give better results in certain conditions over another. One thing most of the 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 affecting 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 the 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 cars steering pulling one way then the other whilst travelling in a straight line. As a corner is encountered, invariably the inside wheel goes light. Drive is transferred to the outside wheel, consequently steering input by the driver is increased as he has to overcome the vehicles new inbuilt tendency for straight line travel created by the diff.

Both of these produce the need for a certain degree of physical input to hold on to the steering wheel 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 tuneable to the chassis, it's usage, and driver ability is an obvious asset. This is readily achieved at an affordable price with the EVO unit. Ingenious design, meticulous engineering and careful development by Transmission Services of Redditch has culminated in the production of an LSD unit that eclipses most of those currently available. This plate type diff employs high quality materials in its construction, and skilful design ensures a very high strength, reliable unit. Its ingenious layout allows a wide cross-section of settings to suit all applications through standard road use, grass-track and rallying to circuit racing .  It is worth mentioning that nearly all the competitive, factory run, front wheel drive cars in all forms of motorsport are turning away from gear or viscous type diffs. They are returning to well designed plate type units to acf)ieve the level of performance required to match the sophistication and power of the vehicles. Assistance is given to the driver in the form of power - assisted steering to overcome the high loadings transmitted by the diff to the driver via the steering. Back to back tests in rally-cross carried out by EVO against other non-plate type diffs have resulted in greatly reduced lap times - sometimes by as much as 3 seconds. In fact "off-pace" cars have become front runners. Static preload affects the amount of cross - axle torque transfer in a one-wheel-zero-grip situation, and is also responsible for initiating the movement of the cross pins up their respective drive ramps. The actual amount of preload is usually determined by the intended main vehicle usage. As a rough guide, lower settings are used where grip availability is good such as on tarmac, higher ones used in poor grip situations such as rally-cross. 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. Commitment and experimentation is the ultimate way to get what is right for you and your vehicle. A special feature of this unit is the facility to change the locking action of the diff on acceleration AND deceleration - a facility not previously commercially available. Changing the ramp angles allows fine tuning of the diff action on drive and over-run, affecting turn-in and power-out characteristics. 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 made during manufacture. There are three basic settings available with the first figure being the deceleration and the later figure acceleration.

1. 90/55 degrees ramps, 25 Ib ft static preload, torque bias for the road use. This makes the diff 'gentle'
in use.
2. 45/45 degrees ramps, 25-30 Ib ft static preload, good torque bias for rally/autocross/grass track etc.
3. 90/35 degrees ramps, 25-30 Ib ft static preload, torque bias for circuit, hill climb, and other race/tarmac/slick
uses. The near release of the diff on deceleration greatly enchances turn-in much reducing understeer.

SEE PART 2 OF THIS ARTICLE

SEE EVO ATRICLE

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