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| NickG:
Thanks Mr fluffy, think I'll be purchasing a cheap engine from somewhere to study to see what I can make of it! That scooter takes me back - had one almost identical minus the engine though - I would have loved that! :lol: Nick |
| MrFluffy:
I feel I need to stress this again after talking with someone, although I'm sure you have understood it already Nick, so this is for their benefit rather than yours and I prefer to post in public so people can correct me if I'm wrong or comment to develop the idea further. Thats what forum's are about. Those two components (magnet in flywheel, and coil) are ALL it needs. There is absolutely nothing else required to make a spark at the correct time. You do not need to even make a advance mechanism because all that is built into the coil and taken care of already. If you want to make the timing adjustable, make a slotted mount for the coil on its bracket with the slots arranged radially round the flywheel, and just slide it round to alter it. The only tolerance or adjustment on it originally is the gap between magnet and coil. Probably a coil bracket which can slide is easier giving you gap adjustment on the coil bracket itself and radial timing on the bracket mounting. We're good at ingenuity and making things out of metal here, this is the easy bit. In fact its all the easy bit :headbang: Theres also nothing stopping you putting the coil in a housing, and arranging a spinning magnet on the end of a shaft to emulate a "real" magneto in appearance and configuration. And hanging it off the end of a drive chain, or gear train for appearance. But Im a form follows function guy and prefer simplicity, so I prefer the idea of a magnet on the flywheel triggering it directly without all that in the way. Max rpm of the donor engine in this case was 12000 rpm. I doubt the ignition setup is the limitation for more rpm but sooner or later it will run out of time (dwell angle) to generate enough power. In fact it will work at a higher max rpm than a setup using points as there is no points bounce or variance which occurs at higher engine speeds. And there is nothing stopping you either using it with a distributer and firing multiple cylinders taking note that it needs some tiny amount of dwell angle just like any other ignition setup, or just mounting more than one coil up, I can't think why you couldnt have 4 or 8 or 28 of them if you so desired, or two with a pair of mechanical distributers, or any combination of such. I cleaned the swarf from the flywheel and put fuel supply to that motor in my picture and had it running clamped in the vice today, started up and ran fine after all this time. The only extra components were a little bottle of 50:1 premixed petroil and a tube going to the carb from it. The scooter was retired for personal safety reasons and the fact that we found out someone's 11 year old lad could do amazing things on it making us look like stiff old men when we went on it :) |
| NickG:
Thanks for that info fluffy. I was wondering whether it could be adapted to run off a smaller shaft / disc, guess it'd just be a little more fiddley to time as there won't be much distance between the magnet and coil - the bigger the radius, the more accurately you'll be to time it. Nick |
| MrFluffy:
The shape of the laminate exposed on the lower edge of the coil changes its curvature to match the radius of the rotated magnet of course, and ditto the shape of the magnet to maintain a even air gap. But I have in the past massaged a coil from a larger sit n ride size 12hp briggs with my dremmel to fit on a unknown brand motor with a much smaller flywheel (on my cement mixer, smaller because it didn't require a charging generator and ring gear for the starter) and maintain the brigg's air gap from the rotating magnets. It ran fine afterwards despite being attacked in this way. I could have been lucky with my combo and Id rather have changed the magnets in from the donor too (or even bought the correct coil *gasp*), but we were rushing to get a reinforcing pillar filled up inside a house when the little petrol mixer threw a fit. Its still like that today mind, theres no fix so permanent as a temporary one :wack: |
| madjackghengis:
Hi guys, I managed to forget this part of the forum existed, but a bright sunny day, some personal issues worked out, and my mind is back on track, so I thought I'd chime in. That string trimmer setup is the basic modern maggie, as good as it gets, the laminated core is set up so there are two poles, the magnet embedded in the flywheel has two chunks of lams to bring its two poles out to the edge, and they line up almost perfectly with the two poles of the maggie along the way, which creates a large pulse in the coil when they are exactly lined up, ramping up, and then the inductance of the coil prevents the pulse from ramping down, as the magnets poles move away from the coil poles, until they come to the edge, and at that point the polarity starts to switch, magnetically, which is the trigger pulse which fires the SCR or transistor, opening the circuit giving an "avalanche" effect to the voltage drop in the coil, and providing the spark. It is critical to have two poles magnetically for both the flywheel and the coil, to get the polarity swap, and the strength of the magnet is probably calculated to a pretty fine line, but I believe they are at full advance fixed, at least on the small two stroke engines, but I will put a timing light on one today, hopefully, and check that. While the surface feet per minute is part of the energy formula for the maggie, I suspect having the two poles, and the speed at which the polarity changes is of greater consequence, as we tend to use smaller gaps on plugs, so the maggie could well be made stand alone as long as the magnet was strong enough, and the poles set up for that almost instant change at the edge of the lams of both the coil and the magnet. I took out the laminations from one of these maggies, with great care hopefully doing no damage, and found the lams through the coil are ten mm by eleven mm, the size of the coil by its self is almost as small as the minimag coil, and I see good opportunity to follow Bogs build on the minimag but using self made lams to go through the coil, and cut out lams similar to those in the minimag, and build the same or similar aluminum frame around it. I haven't tested this yet, still in the beginning stages, but I expect the spark will generate in such a setup at the same point John found by trial, when setting the points break in the minimag, in answering my question about that aspect, based on its internal electronics trigger. I have a couple of magnetos built for four cylinder engines, one from about 1910, and one from the thirties or forties. Both use a two lobe cam, run a one to two ratio gear from the cam to a phenolic distributer case, with the maggie running at cam speed. The one from about 1910 has an impulse coupler and goes on a morris mini sized stationary engine, so it is probably set up at full advance, since they were mostly governed, fixed speed engines, the other was in a fixed, adjustable mount setup, to allow retarding it for start up, and manually advancing once started. I don't know what it came off of, but it's a Fairbanks Morse, and it has an odd magnet arangement inside providing four pulses per revolution by segmenting the magnet, I need to take pictures to properly show it, it's the only one I've seen exactly like this, and it has a four lobe cam and I believe was driven at half engine speed. John S. covers the issue of dwell pretty well, and that is the significant issue I will have to deal with regarding nine cylinders and the radial engine, however that engine runs with a redline of about three thousand, so dwell should not be a problem, particularly if the electronic trigger works for my intended setup. On the many Harleys, Triumphs, Nortons, BSAs and an assortment of other bikes I don't remember, they all run full advance all the time, although I have worked on an early Triumph maggie which had centrifugal advance and went on a late fifties or early sixties 650 cc bike. I believe some early Nortons also did this, because they don't get full power in the lower range when at full advance so auto-advance is a considerable advantage in bikes. All the Harley mags I've worked with have been either full advance all the time, manual retard for starting, and advance for running, with a few aftermarket ones having an impulse coupling, and run fine but require the idle set up to prevent low speed misfires, excepting the one which is in the engine I have on my bench waiting a frame being built, which has the centrifugal advance unit out of a sprint car maggie, modified to drive from a chain off the cam of the engine, set up to advance the magneto in concert with the electronic "distributer" which will be firing a second set of plugs providing two independent ignitions. That engine is the one with the cam cover off which I have as my mad modder icon, I believe its called. That was done specifically because my experience on the road is electronic ignitions have a set lifespan, and always die when there is no place for parts, and having a redundant ignition means not having to take out the electronics, dig in my tool bag and pull out a points plate and cam, and replace the electronics with points to get two thousand miles back home, as I've done at least half a dozen times, usually in the rain. I've run that magneto setup with my lathe to drive it, to test the auto advance and ensure it provides the same thirty degrees of advance as the distributor, and the only untested part is the chain drive in from the cam, which should be fine, according to the tensile strength numbers, but chains are strange things and act according to their own set of rules. I normally would start such an engine and break it in on the bike I've got, but that engine has a five inch stroke with big bore cylinders, 3.625 plus forty over, and that stroke cuts engine life in half while providing a full 103 cubic inches, and was built that size because it was the only "stroker flywheels" I had at the time, having been given them in payment of a debt, however I subsequently found some custom flywheels which don't have a crank pin hole, so I can set them up with any stroke I want, and I have the taper reamers to ream the crankpin holes right and proper now, so I intend to tear the engine down without running it, once I've bored and reamed holes in a set of flywheels and got them balanced, and I intend to run a four and five eighths stroke instead of five inches, which puts piston speed and engine life back close to stock, and still ends up with 98 cubic inches. I have been off that project for some time because of my pursuit of the radial engine, the flame suckers and a steam engine, along with a whole list of other projects which keep intruding in my world. I hope I'm not boring anyone with my long posts, but I'm trying to get the most out of this forum, as its been a great boon to my engine modelling, and I keep getting up to my ears in other people's project custom engine rebuilds, which puts me off getting hot on my own. I must say I am very pleased there are so many others interested in the maggie business, and are helping me fill out my knowledge and understanding of them, as I pursue one for the radial. I will get some pictures of some various magnetos I'm using for information gathering, as electronics has substantially changed the field of maggies in the past couple decades. :nrocks: :headbang: jack |
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