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BMC 'A' SERIES AXLE OPTIONS


  BMC 'A' Series Axles
  Crown wheel & pinion
RATIOS
Ratios of 3.72, 3.90, 4.22, 4.55, 4.87, 5.12, 5.37 were fitted to a variety of axles. A ratio of 3.54 was made for competition use. All 'A' series differentials have an aluminium casing.

Usually we are able to rebuild differentials with either a good condition used or reproduction crown wheel and pinion although some ratios are not available.

LIMITED SLIP DIFF
There are two types of these, a plate type and a gear type. Both generate friction to provide the limited slip effect.

PLATE TYPE
  BMC 'A' Series Axles
  Limited slip differential
The lugs on the plates are located internally and the cones are located on internal splines, this eliminates wear on the housing. To create friction in the differential pressure is applied to the plates by a spring, the amount of pressure can be varied by changing the strength of the spring. The pressure applied is determined by the amount of torque required to turn a side gear while the other gear is held static. We can adjust the amount of torque required to rotate a side gear to any setting, generally the range is anywhere between 30-90 LBS/FT and is dependent on what the car is used for.

In addition to this the friction in the plates can be made to increase when the car is moving. This is achieved, depending on the design and make, by the planet gears or the planet gear shafts moving up ramps in the cones
  BMC 'A' Series Axles
  Internal parts
when the car is accelerating or decelerating. By varying the angle of the ramp the friction in the plates can be altered. As an example a ramp angle of 30 degrees will increase the friction in the plates more than an angle of 45 degrees. The characteristics of the LSD can be altered for the type use and driving style. For example a driver may prefer a setting that gives maximum friction in the LSD under acceleration but no extra friction when decelerating. This would require cones with 35 degree ramps for acceleration and 90 degrees for deceleration. Another preference could be cones with a ramp angle of 45 degrees on both acceleration and deceleration. There are many combinations of ramp angles, we can advise on choice according to type of use and specification of car.

A useful feature of the plate type LSD for competition use is if one rear wheel loses contact with the road, as often seen in a racing and rallying during fast cornering, the drive is still transmitted to the other rear wheel to keep car's speed going.

Today's classic cars often have engines with considerably more power and torque than originally fitted. This puts higher stresses into the axle and failure of the teeth on the two planet gears in the standard diff has become a common occurrence. Fitting an LSD eliminates this problem. When operating a plate type LSD can be noisy. When the car is turning a clonking sound is made, this is more apparent at low speed but is a normal characteristic.

  Gear type LSD
  Gear type LSD
GEAR TYPE
The gear type LSD comprises helical teeth gears fitted into pockets inside the housing. When there is a difference in the loaded rotational speed of the halfshafts, friction is created by forcing the gears against the wall of the pockets. Both wheels need good contact with the road when cornering to generate friction in the LSD and increase traction. The differential is silent in operation.


  BMC 'A' Series Axles
  Broken planet and
side gear teeth
  BMC 'A' Series Axles
  Broken crown wheel and pinion teeth
  BMC 'A' Series Axles
  Inside the 4 planet
gear differential
  BMC 'A' Series Axles
  Special halfshafts
4 PLANET GEAR DIFFERENTIAL
This replaces the standard open differential where competition rules prevent the fitting of an LSD. Failures with the standard open differential are common.


One problem is the teeth breaking off the planet gears and the broken pieces then getting jammed between the teeth on the crown wheel and pinion.


Another problem is the weak cast iron carrier fracturing under load, also the spigot holding the side bearing can break off. Any of these failures cause major damage to the complete differential assembly.


The standard differential has only two planet gears and one shaft to transmit the power. Replacing this with our differential that has four planet gears running on two shafts inside a fully enclosed steel carrier will prevent failures.


In addition the flange that the crown wheel is attached to has a much thicker section than the standard item. The increased strength of this carrier keeps the crown wheel into mesh with the pinion under hard acceleration whereas the standard carrier deflects under load leading to increased tooth wear and possible failure.






HALFSHAFTS
The halfshafts in the BMC 'A' series axles are weak and often fail in standard road cars. The breakage usually occurs where the spline exits the side gear in the differential.

Stronger material shafts are essential for a car with increased performance. The shafts are a two piece design to fit the Sprite/Midget. Longer shafts can be supplied to fit other cars.

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