Author Topic: Advance retard springs  (Read 410 times)

Offline Rudgeman

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Advance retard springs
« on: 31.08. 2019 13:07 »

I've now stripped the advance retard unit and fixed the wobbly post. I've played around with it a lot and I haven't been completely convinced that the wobbly post and proud rivet heads would have caused the low engine speed pinking so consistently.

So I ordered some new springs and rivets from Draganfly and prepared to rebuild it. I measured the length of the old and new springs - both the same at 19.7mm. They're the same coil diameter and number of coils. Then I measured the spring wire diameter. The old one is 0.65mm, the new one 0.78mm. That's a whopping 20% increase and since torsional stiffness is proportional to diameter to the power of 4, it means the new spring is twice as stiff. I did a quick extension test with a weight and confirmed this.

I'm assuming my new springs are correct - Draganfly have been very reliable in all the other parts I've bought from them. Perhaps a previous owner fitted springs from a different ATD, or perhaps there are some dodgy springs being sold - I don't know.

I'm tempted to model the advance vs engine speed characteristic to compare the impact of these different spring rates. If I do I'll add the results to this thread.

Something to check if you have pinking problems.

R

Online JulianS

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Re: Advance retard springs
« Reply #1 on: 31.08. 2019 13:56 »
Two photo of a new old stock ATD, unfortunately cannot measure the wire thickness as it is now in the A10.

Online RichardL

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Re: Advance retard springs
« Reply #2 on: 01.09. 2019 05:44 »
I made my own springs a while back and reported the effort in this post: https://www.a7a10.net/forum/index.php?topic=937.msg6301#msg6301

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Online duTch

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Re: Advance retard springs
« Reply #3 on: 01.09. 2019 07:26 »

 May be useless and irrelevant information, but my Gutzzi and probably many Gutzzi twins use two different ( non identical) springs in the advance unit
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Offline BSA_54A10

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Re: Advance retard springs
« Reply #4 on: 01.09. 2019 09:28 »
There is a lot more to a spring than the diameter of the wire.
Your rule of thumb is only correct  if
IT IS THE SAME ALLOY AND HEAT TREATED THE SAME.
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Trevor

Online Greybeard

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Re: Advance retard springs
« Reply #5 on: 01.09. 2019 11:28 »
There is a lot more to a spring than the diameter of the wire.
Your rule of thumb is only correct  if
IT IS THE SAME ALLOY AND HEAT TREATED THE SAME.
If one were to hang a weight on various springs to measure and compare the extension, would that be useful?

Offline Rudgeman

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Re: Advance retard springs
« Reply #6 on: 01.09. 2019 17:27 »
Hi - yes I did a deflection test with a weight and it confirmed the new spring has twice the rate of the old one.

I've had a go at mathematically modelling the atd this afternoon. My initial findings are that with the old spring it should go to max advance at only 1500rpm. The new spring will only hit max advance at around 2300rpm. I was expecting it to be more like 3000rpm +, based on gut feel alone. I need to go over a few aspects again,  but I don't think I'm way out.

Does anyone know what the advance vs speed characteristic is supposed to be ?

R

Online RDfella

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Re: Advance retard springs
« Reply #7 on: 01.09. 2019 17:44 »
Does anyone know what the advance vs speed characteristic is supposed to be ?

Was interested in that myself. Surely someone has an advance curve for these engines? When I changed my Jag to electronic ignition, there were half a dozen profiles to choose from, but most were 'all in' around 3,000rpm (similar hemi-head design).
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Offline Rudgeman

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Re: Advance retard springs
« Reply #8 on: 03.09. 2019 16:57 »
I've charted the charcateristic that I modelled. Hope it pastes OK. R

Online JulianS

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Re: Advance retard springs
« Reply #9 on: 03.09. 2019 17:14 »
Interesting graph from Brightsparks website.

http://www.brightsparkmagnetos.com/faqs/FAQs%20about%20ATDs/Typical%20Lucas%20ATD.pdf

Engine speed twice mag speed.

mod edit: pic inserted.

Online groily

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Re: Advance retard springs
« Reply #10 on: 03.09. 2019 18:02 »
It's interesting how modest a difference variables like springs and preload make on this. 
Typically, when I test people's ATDs in action, I see them starting to advance by about 1600 engine rpm, and reaching full advance in the low to mid 2000s.
This other link looks at the issue and shows a few measurements in various circs, not all of them by any means typical!
http://www.brightsparkmagnetos.com/faqs/FAQs%20about%20ATDs/How%20critical%20is%20the%20spring%20rate%20and%20preload%20in%20an%20ATD.htm
and
http://www.brightsparkmagnetos.com/faqs/FAQs%20about%20ATDs/Lucas%20ATD%20modified.pdf
Some folk may also find the video on ATD Range under this link interesting: http://www.brightsparkmagnetos.com/workshop/DBRig/index.htm

No idea unfortunately what any maker intended as the rpm threshold for full advance, the only certainty seems to be the different ° of movement to be found on different ATDs, managed by how the 'stops' are placed. That seems to vary from the 11° shown in the graph Julian referred, to as much as 18° on some others.

In practice, for standard engines, timed reasonably accurately with the ATD fully open, I don't think it matters a lot what the fully retarded setting is, as long as it's enough to allow a smooth tickover, so I never care much how much actual movement there is on, say, my A or my Notruns. On my manual mag machines, I often leave them at full advance all the time except for prolonged tickover, and in 2 cases I've been running fixed timing for ages with no serious ill effects except setting the tickover a fraction higher than I would otherwise. In fact, with good effects mostly, because the magnetos in question are blessed with accurate 180:180° firing across the rpm range, which to me is about the most important thing - and the hardest thing to achieve sometimes.
Bill

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Re: Advance retard springs
« Reply #11 on: 03.09. 2019 20:25 »
Have to say those figures surprise me. A lot of hemi-head car engines seems to share similar ignition curve requirements, including long-stroke ones such as the XK Jaguar. And they like around 34 - 36* of advance, starting around 1500rpm and all in at 3,500rpm. I would have expected our engines to at least be similar given their similar characteristics, but instead they have less advance, come in much later and reach full advance much earlier. It almost seems as if the system was designed to be correct at around 2,500 to 3,000 (the sort of revs most would cruise at) and them simply retard it for starting, without any consideration regarding how advance affects economy, power and general running throughout the rev range.
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Re: Advance retard springs
« Reply #12 on: 03.09. 2019 20:57 »
G'day Rudgeman.
Your graph shows the new curve. How and why?
I often thought to combat pinging (hard acceleration from low revs) advance needed to come in a little later but reach full at about the same time.
Cheers

ps: I fitted a Dynatek 2000ip to a mates HD where I can dial in my own advance curve. Wish I could make it fit the A10
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Online groily

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Re: Advance retard springs
« Reply #13 on: 03.09. 2019 21:15 »
Don't forget though, RD, that pretty well all distributors have vacuum advance as well, which has the big advantage of retarding things when you stick your foot on the pedal under load. Quite a lot if needs be. So the curve can be longer, and the full advance position can be a bit further out in terms of engine rpm - Maybe?
In fact, 34-36° isn't so different from an ATD with say 16 or 18° of movement, as you have to double up for crank degrees obviously, so total advance isn't all that far off is it?

Presumably, the makers of things like Boyer and Rita systems and more modern equivalents sussed all this out and made things which might suit better than an ATD - but as an old fart I don't know. When I've had electronic ignition on things I've usually ditched it in favour of a magneto (as I have done on my P11), cos that's what I understand best!

ATDs are pretty basic, and because they are controlled only by rpm, are not really a very smart way to manage a motor in any sort of state of tune. But hey, by the standards of the day, when the choice was a sloppy camring, sub-optimal sparking when the ignition was in any position other than full advance  . . . not a bad thing overall.

I have often wondered whether there is enough inlet manifold depression to allow a vacuum advance system to operate on a typical vertical twin. Had a  half-formed idea to (try to) make something with a stock ATD on the shaft, plus a manual camring controlled by the vacuum if there was enough variation in the manifold to actuate it. However, I have no idea how big a diaphragm might have to be, what complications there would be getting anything to integrate with a mag, miss the carburettor, etc etc etc. So it never got done. Part of me thinks that folk like Lucas, who after all made most UK car distributors at one point, must have thought about it too, as they were much smarter people than critics sometimes allow (- and much much smarter than me  . . .) The fact that they didn't go there suggests perhaps that the gains weren't deeemed worth the hassle  . . . As folk like Bosch and BTH didn't either (afaik), although the technology was there in their time, seems to say the same thing. And no doubt penny-pinching cost people who wanted their magnetos for 25/- apiece (old GB money for one and a quarter pounds for our younger and/or overseas friends!) wouldn't have liked the idea at all!

Bill

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Re: Advance retard springs
« Reply #14 on: 03.09. 2019 23:08 »
It almost seems as if the system was designed to be correct at around 2,500 to 3,000 (the sort of revs most would cruise at) and them simply retard it for starting, without any consideration regarding how advance affects economy, power and general running throughout the rev range.
I reckon you've got that right.