NEWS FLASH - December 2013
We are now selling magneto bearings, insulators, oil seals and armature shims from our on-line shop.

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 critical is the spring rate and spring preload in an ATD?

The spring rate and preload of the springs in a magneto ATD are important, but perhaps not as important as you might initially think. 

With a magneto ATD, the speed dependent advance that the bob-weights are trying to cause is being opposed not only by the ATD springs, but also by the load placed on the ATD by the magneto. The magneto load is also dependent on speed, and in the whole scheme of things, it is quite substantial. It is that load which enables the magneto to generate its electrical output and also produce all its losses (e.g. windage, eddy current losses, bearing losses, friction at the contact breaker and oil seal, etc.). By comparison, in a distributor with centrifugal advance for a coil ignition system, the only load on the auto advance mechanism is the friction of the contact breaker and a little bit of windage around the cam and rotor arm.

The graph shows various advance curves for a Lucas ATD in good condition when driving a Lucas K2F magneto on one of our test rigs using a rotary gap so that the angular positions of the ATD's input member at which the sparks occurred could be readily ascertained. The ATD spring(s) were brand new pattern parts.

For the green line, the springs were adjusted so that they only just became slack in the fully-retarded position of the ATD i.e. 'no preload'. As can be seen, the ATD remained fully retarded up to 750 rpm at the magneto (1500 engine rpm on a four-stroke engine), and then progressively moved to fully advanced by 1160 rpm at the magneto.

The orange line shows the operation with the springs adjusted so that they were '100% preloaded'. By that, we mean that the springs were stretched in the fully-retarded position of the ATD by the same amount that the no-preload springs were stretched in the fully-advanced position of the ATD. At full advance, the tension in the 100%-preloaded spring would therefore be about double that of the no-preload spring.

As can be seen from the orange line, with 100%-preloaded springs, the ATD started advancing at 920 magneto rpm (23% faster than with no preload) and was fully advanced by 1350 magneto rpm (16% faster than with no preload).

To simulate halving the spring rate of the springs, the red line shows the performance with one of the ATD springs removed, and the remaining spring adjusted for no preload. By comparison with the green line, halving the spring rate reduces the magneto speed at which the ATD begins to advance from 750 rpm to 670 rpm (an 11% drop), and reduces the magneto speed at which the ATD becomes fully advanced from 1160 rpm to 950 rpm (an 18% drop).

For completeness, the blue line shows the performance with just one ATD spring 100% preloaded. Also, for a bit of fun, the mauve and turquoise lines show the advance with two bob-weights and with one bob-weight, but with no springs.

In summary, large changes to the spring rate and spring preload produce relatively small changes in the operating characteristic of the ATD. Increasing the spring rate and spring preload increase the speed at which the ATD starts to advance. Increasing the spring rate also reduces the gradient of the advance curve between fully retarded and fully advanced.

 

 

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