Archive for the ‘Solutions’ Category

 

BITRON AND LIMITED SLIP DIFFERENTIALS (LSD)

What is the purpose of a differential?

A LSD is basically a modified differential gear arrangement that allows for differences in the velocity of the wheels, most commonly when turning. As a vehicle turns, the wheels spin at different velocities with the outside wheels spinning faster than the inside wheels. This is due to the wheels having to travel different distances to execute the turn.  Without a differential of some kind, the wheels would be ‘locked ’ together forcing them all to spin at the same speed. This would make turning extremely difficult and hard on the vehicle. Effectively to make the turn, the tyre would need to slip. This requires a large amount of force which puts added strain on other components such as the axle.

TYPES OF LSD

Broadly speaking, there are 2 types of LSD; torque sensitive and speed sensitive.

Torque Sensitive LSD

Also known as mechanical LSD’s because there is interaction between 2 or more mechanical parts such as clutch discs. Mechanical LSD’s include clutch & cone and helical gears. These all respond to torque generated by the drive-shaft of the vehicle. The more torque produced by the drive-shaft the harder the gears are pressed together. As torque increases, the wheels are coupled together more closely.

There are three basic input torque states;

1. No Load where there is little/no coupling. Some LSD’s use a small spring loading mechanism so that when the vehicle is in neutral for instance there is a small amount of torque which keeps the wheels minimally coupled.
2. Load where the coupling is directly proportional to the amount of torque being produced
3. Over-run where the behaviour of the LSD determines whether it is 1 way, 1.5 way or 2 way. 1 way LSD’s behave in a similar way to a conventional diff where when the throttle is lifted the LSD unlocks and there is no additional coupling. 2 way LSD’s increase coupling whether the input torque is in forward or reverse. 1.5 way LSD’s are a compromise between the other two.

Generally speaking 1 way coupling is safer particularly with FWD vehicles whilst 2 way coupling which allow the wheels to continue spinning into a corner regardless of throttle input is for more experienced drivers looking for more high performance handling.

A clutch type LSD makes use stacked clutch discs half of which are coupled to the drive-shaft and the other half to the spider gear carrier. The more torque that is applied the greater the pressure on the clutch discs which results in greater coupling of the wheels. As the vehicle turns , pressure rings adjacent to the clutch discs are forced side-ways which results in more pressure on the clutch discs. Cone LSD’s work in a similar fashion using cones instead of clutch stacks.

Clutch type LSD’s generally have very specific ‘break-in’ periods which allow any imperfections on the metal surfaces to be worked out. Failure to follow break-in procedures can result in permanent damage to the LSD.

Geared LSD’s use worm gears rather than clutch plates or cones and ‘sense the torque speed on one shaft and ‘move’ it to the other rather than actively controlling ‘slip’ like the clutch and cone type LSD’s do. Geared LSD’s are more dependent on the actual torque rather than the difference in speed between the output shaft.

 Speed Sensitive LSD

These are generally simpler as they rely on the hydrodynamic friction produced by high viscosity fluids. They make use of a cylindrical chamber filled with a high viscosity fluid. Perforated discs are stacked within the fluid and alternately coupled to one of the drive-shafts or the differential carrier. They rotate with the output shafts. Differential motion such as turning forces the discs to move through the fluid against each other. In some speed sensitive LSD’s the heat produced from constant friction thins out the fluid causing it to expand. This can lead to non-viscous plate to plate friction causing a noticeable reduction in the speed difference between the drive-shafts.

Speed sensitive LSD’s tend to be less efficient due to this loss of power. Sustained heavy loads cause the fluid to thin out resulting in the permanent loss of differential effect. When this happens, they tend to behave in a similar manner to an open differential. Many of these types of LSD are not serviceable and require replacement as the viscous fluid is usually in a sealed unit.

There are other types of speed sensitive LSD’s such as electronic LSD’s which utilise a computer to monitor the speed of each wheel and in cases where one or more wheels are spinning unusually fast, gently apply braking to these wheels which allows the other wheels to increase in speed. On the whole, these systems which often rely on such things as ABS place much less stress on the drivetrain than mechanical LSD.

BITRON AND LSD

Bitron EP40 can be safely used in all types of LSD. Where the LSD is mechanical it is IMPERATIVE that the manufacturers ‘break-in’ procedure is followed to ensure that the LSD operates correctly and no permanent damage is done.  Bitron EP40 SHOULD NOT be used during this settling in period. Once the ‘break-in’ procedure has been followed as per the manufacturer’s directions, Bitron EP40 should be added at between 5% and 10% of the LSD oil capacity. We recommend that if the vehicle is going to be operated under high load, then 10% should be added. Vehicles not operated under consistent high load should add Bitron EP40 at 5% of the LSD oil volume.

With speed sensitive LSD, Bitron EP40 should be added at 10% of the LSD oil volume. This will ensure less heat induced friction extending the life of the viscous oil – generally silicone is used. Bitron EP40 will not react with high viscosity oils in any adverse way.

Whether the vehicle has a mechanical or speed sensitive LSD, Bitron EP40 will place a protective layer over the clutch discs, cones or perforated discs protecting against heat induced friction. This allows greater torque output with less friction to the discs resulting in increased operating life. Bitron EP40 will not cause excessive slippage within the LSD which can result in less power output.