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Brightspark Magnetos

NEW ... Take a look at some of the equipment we use in our workshop for magneto servicing and overhauls.

 

 

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FAQs

How does an ATD work?

The Lucas and BTH automatic timing devices (ATDs) each comprise two rotary members which are coupled to each other for rotation about the axis of the magneto shaft with a predetermined amount of backlash between them. One of the rotary members (the input member) is driven by the engine, for example by a chain and sprockets, or by a train of pinions. The other rotary member (the output member) is fixed to the magneto shaft. The amount of backlash defines the range of the ATD between a fully-retarded position and a fully-advanced position. In the fully-advanced position, the magneto generates its spark earlier in the engine cycle than when in the fully-retarded position.

In the Lucas ATD, two bob-weights are each pivotally mounted off-centre on the input member and connected to the output member by a linkage. Each bob-weight can pivot between an innermost position in which the linkages set the two members in their fully-retarded position, and an outermost position in which the two members are set in their fully-advanced position.

By contrast, in the BTH ATD, five loose bob-weights are each constrained for radial movement between two ramped surfaces, one fixed to the input member and the other fixed to the output member. When each bob-weight is in its innermost position and touching  both of its ramps, the two rotary members are in their fully-retarded position. Conversely, when each bob-weight is in its outermost position, the two rotary members are in their fully-advanced position.

In the Lucas and BTH devices, the two rotary members are spring-loaded towards their fully-retarded positions. In the Lucas ATD, this is done by two tension coil springs acting between the linkages and the input rotary member. In the BTH ATD, a spiral clock-spring acts between the two rotary members for this purpose.

In operation a number of forces act in combination to determine whether the ATD assumes its fully-retarded position, its fully-advanced position, or somewhere in between.

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  • As the ATD rotates, centrifugal forces acting on the bob-weights urge them to their outermost positions, and through the linkages (Lucas) or ramped surfaces (BTH) urge the rotary members towards their fully-advanced position. This action is roughly proportional to the square of the speed of rotation. 

  • As the rotary members move towards their fully-advanced position, the spring(s) increase the force with which they oppose that

     movement urging the two members towards their fully-retarded position. This action is dependent on the preload in the spring(s), and the product of the spring rate and the amount of advance.

  • As the magneto shaft is rotated, it places a load on the ATD due to the electrical output from the magneto and the losses in it (e.g. windage, eddy current losses, bearing losses and friction). The relationship between magneto speed and load is probably very complex, but whatever the load is, it urges the rotary members towards their fully-retarded position.

Despite the complexity, these various forces combine to produce, in a normally functioning ATD, an extremely simple result, or 'advance curve' as shown in the graph. Below a particular speed, the ATD stays in its fully-retarded position. Above a slightly higher speed, the ATD assumes its fully-advanced position. Between those two speeds, the amount of advance is approximately linearly related to the speed.

 

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