Auto advance spring tension

[Skullza]

Re - my 1959 A10GF. Last time I replaced my magneto auto advance springs was about 15 years ago. Suspecting some issues with the unit, I noted while it would return to the normal retarded position at rest, it was pretty lazy doing that - i.e., pretty low spring tension. I removed it, gave it a good cleaning with mineral spirits (I don't think in the 41 years I've owned it that it's ever had a good clean), and then replaced with new springs from my friends at British Cycle Supply in Canada. Wow! Serious weight tension here - takes more effort to deploy the weights and they really snap back into position. Question is (before I close everything back up) is it possible that the new springs are TOO strong?

[duTch]

 Hi there Skulzia- I can't honestly answer your question, but the guys who may will likely be along when the sun rises....and then the  crew will want to know all about you  

[Skullza]

Thanks for the welcome!

[a10 gf]

Welcome & hint:
http://www.a7a10.net/forum/index.php?board=13.0
http://www.a7a10.net/forum/index.php?topic=4457.0

Centrifugal force at 1000's of rpm is ....strong. Presumably some spring could be wrong, but IMO unlikely if they are sourced from a reputable dealer who (hopefully) should know and have experience with the supplied parts.

[groily]

Not sure what the correct strength of the springs is meant to be - but I do know that the preload etc makes less difference than you might think, in operation. The thing needs to slip back to 'retarded' at low speed, obviously (which with strong springs it will). But, bear in mind that the magnet, the friction of the points against the camring etc affect the recoil - the springs ain't that strong that in all position of the armature the thing will flick back quickly. This sometimes makes people think their units are tired, when in fact they aren't.

For what it's worth, we did some experiments a while back, to try to understand whether there were risks in pattern springs vs originals, whether variations in pattern parts mattered much, what happens if the springs go soggy (or fall off), etc.

And the graphs here:
 http://www.brightsparkmagnetos.com/faqs/FAQs%20about%20ATDs/How%20critical%20is%20the%20spring%20rate%20and%20preload%20in%20an%20ATD.htm
show what we observed.
As a result, I haven't worried overmuch about the exact state of my own atds in this regard, as long as they operate OK - ie the weights don't stick, the pivots aren't fubarred, etc.

When talking to the good people who make the modern B-TH units, I think I recall being told that full advance occurs around 3000rpm (engine) or 1500 mag on the K2F lookalike - which says they take longer to get to full advance than an ATD. And they are reckoned to be good units.
Fully-preloaded, a typical Lucas atd gets to full advance about 2600rpm (engine), we found - and that was the latest, ie highest speed, we could get for full advance. In all other circs it occurred earlier.

[RichardL]

I made my own springs a while back and posted the story and a little video. I did not calculate "K" in F=Kx  and could not say at what RPM maximum advance is achieved, however, my bike seems to run very well with the results.  I recall it being said that a gradual change in advance with increasing RPM is not critical, but my gut tells me it would be better than getting to full advance at quite low RPM. If true, springs a little stronger than absolutely necessary would be preferred versus weaker (I think). I would say Groily and Beezermacc are the trusted souces for the tech spec and gurufic opinion on this.

Here's my post about the springs.

http://www.a7a10.net/forum/index.php?topic=937.msg6301#msg6301

Richard L.

[beezermacc]

Here's an extract from my website............ www.priorymagnetos.co.uk
A bit of ATD theory!

ATD's can be a bit baffling! Occasionally customers phone me, whilst fitting their magneto, to ask if their ATD is faulty. The conversation goes like this... "I'm fitting the magneto and just about to fit the ATD. When I hold the ATD in my hand it flicks back OK but as soon as I start to tighten it onto the magneto shaft it doesn't seem to flick back properly. I can move it backwards and forwards by hand but it seems to have gone stiff and won't flick back on its own. I've taken it off again but I can't find anything wrong with it. What's going on?!"

Think of the ATD as a component that works in three stages....  1) the fixed gear picks up drive from the engine then.....  2) the drive is passed on to a flexible connection made of springs and bob weights, then... 3) the drive is collected from the flexible mechanism by a nut fixed onto the magneto armature shaft. Remember that the gear is fixed because it is meshed to the camshaft gear, so the springs are trying to pull the magneto armature back to the 'at rest' position whilst one end of each spring is attached to the gear and the other end of each spring is attached to the armature shaft. Now here's the critical bit.... (and the reason why the ATD doesn't flick back when stationary) ... the magneto armature is not free-floating because there is friction and magnetism in the magneto which prevents the armature responding fully to the pull of the springs. The friction/resistance is caused by the pickup brushes, earth brush, heel of the points, drag in the bearings and the attraction of the magnets. As the engine is turned over very slowly the friction is reduced because the parts are moving - sliding over each other, and the springs are able to pull the mechanism to its 'at rest' position. You can test this by turning the bike over slowly on the kick start and watching the ATD return to its fully retarded position - Hey Presto!

ATD units are not very sophisticated but they do need to work properly. The springs should be strong enough to return the unit to 'fully retarded' as the engine comes to rest - this will enable you to start the bike again without getting launched over the handlebars. If the springs are too strong they will prevent the magneto advancing soon enough which may cause overheating and sluggish running. Springs which are too slack will advance the spark too soon causing advancement at kick start speed (kick-back) and spitting in the carb when blipping the throttle.

[duTch]

 Actually, the one thing I will add regarding spring tension, is that the tension of Coil springs is determined by #1; the thickness of the wire, #2; diameter of the coil, #3; number of winds in the spring over #4; the spring length. As far as I can tell (at least with 'standard' springs), there isn't generally a variation in the materials used in that it's either spring steel or not, the one exception I know is that Stainless Steel is ~15% softer.

 No doubt someone with more metallurgical knowledge will set me right, and maybe add that over time and many periods of heating and cooling springs will lose their tension, and I know enough to be dangerous from researching coil springs to have some made for my Plungers suspension (in Stainless ), but I think I got that reasonably ok.