perhaps thats why they were fitted to Grey Flash
Tired KVF...
- Thread starter Bazlerker
- Start date
This is the reason NOT to grind out the timing slot on the cam ring as it could advance the ring too much, then No1 spark will disappear'
When the armature spins inside the magneto two things take place. The simplest is that since the primary coil is moving through the magnetic field lines this creates a current in the primary coil (as long as the points are closed). If the points always remained closed this current would oscillate in roughly a sinusoidal way as the armature rotated. This primary current would in turn create an oscillating magnetic field in the secondary that would act in concert with the oscillating magnetic field experienced by the secondary because it also was rotating through the field of the Anico magnet cast in the housing.
Instead of remaining closed for 360-deg., if the points open when the current in the primary is maximum the current -- as well as the magnetic field due to it -- would instantaneously drop to zero. This rapid change in magnetic field (or, more properly, magnetic flux which is field times area of the coil) would be experienced by the secondary, creating a high voltage in the secondary due to the large number of windings. While this alone would create a high voltage, it's not high enough for our purposes.
The genius of Robert Bosch was devising the geometry of the pole faces and shape of the armature in ways to generate significantly higher voltages. When you turn an armature by hand it will reach a point where it becomes very much harder to turn further. This is commonly, but incorrectly, called the point of "magnetic zero" or "magnetic neutral" or "zero magnetic flux." There's nothing neutral or zero about it.
The reason for the rapid increase in resistance to turning is the core of the armature has reached the point where there is almost no overlap with the pole face so all the magnetic flux lines from the Alnico magnet are concentrated in a very small area. When the armature is rotated slightly further, instead of bunching even closer together, the flux lines find it energetically advantageous to switch direction through the armature. That is, if they were oriented so they pointed "left-to-right" they now point "right-to-left." This means the magnetic flux in the core of the armature, i.e. threading the primary coil, undergoes a change of magnitude 2x just past the point where the resistance to turning is largest.
Not coincidentally, this happens when the current in the primary already is near its maximum so this significantly boosts that current. So, if the points open when the magnetic flux lines are switching direction the maximum possible voltage in the secondary will be generated. In the case of our magnetos the "E-gap" is a bit of a red herring, because the notch in the armature aligns with the "pip" in the points plate to automatically set the E-gap where Lucas or BTH decided it needed to be. The E-gap is not adjustable in our magnetos unless someone goes out of their way to screw it up using files and hammers.
In the case of a vertical twin the plugs fire 180-deg. from each other, which is ideal for a magneto since it generates maximum current/voltage at the two orientations of the armature 180-deg. apart. A Vincent (or Harley, or Indian, or JAP,...) has to accept something less than ideal for one of its cylinders.
Although a Vincent is a "50-degree V-twin," the HT leads of the magneto don't fire 50-degrees apart. The configuration of this engine is equivalent to a vertical twin (e.g. BSA 650) whose cylinders have been split to be at 50-deg. rather than 0-deg. with respect to each other. As a result, lobes one and two on the points cam need to be 155 degrees apart. The reason for this is, since the magneto operates at half engine speed, 155-deg. corresponds to 310 degrees of rotation of the crankshaft. Thus, if one cylinder fires at 0-deg., the second fires after the engine turns 310-deg. further, i.e. 50-degrees less than another full revolution (360-310=50). It then takes another 50-deg. for the crankshaft to get back to the first cylinder, which is now on its exhaust stroke, plus another full rotation before that cylinder is ready for another spark, which is 410-deg total (50+360=410). So, the angular separation between the second lobe and the first needs to be half that, i.e. 205-deg. Since 155 + 205 = 360, this all works out.
OK, what the previous paragraph means is if the timing is set to be "perfect" on the rear cylinder then for the front cylinder it will be too late with respect to maximum from the magneto by 25-deg. That is, the output of the magneto for the front cylinder will be the same as if it had been retarded by 25-deg. (50-deg. on the engine). While not optimum for producing maximum output from the magneto, note that the max. advance range of ATD units supplied as standard for some other marques was 18-deg. (36-deg. engine) for some models, and as small as 11-deg. for others. This amount of advance wasn't determined by a limitation of the output of the magneto when retarded from optimum, but by the requirements of starting an engine. If a particular engine gave max. h.p. when timed at 38-deg. it would start badly, if at all, if the ATD had a range larger than ~19-deg. resulting in the spark occurring ATDC.
A KVF certainly can supply sufficient voltage to fire the front cylinder at kickover speed. However, reduced magnetism of the Alnico due to the partial demagnetization that is inevitable whenever the armature is removed and replaced means you will be needlessly stressing your right leg if you have someone rebuild your magneto who does not own a proper ~70,000 Amp-turn electromagnet.
Instead of remaining closed for 360-deg., if the points open when the current in the primary is maximum the current -- as well as the magnetic field due to it -- would instantaneously drop to zero. This rapid change in magnetic field (or, more properly, magnetic flux which is field times area of the coil) would be experienced by the secondary, creating a high voltage in the secondary due to the large number of windings. While this alone would create a high voltage, it's not high enough for our purposes.
The genius of Robert Bosch was devising the geometry of the pole faces and shape of the armature in ways to generate significantly higher voltages. When you turn an armature by hand it will reach a point where it becomes very much harder to turn further. This is commonly, but incorrectly, called the point of "magnetic zero" or "magnetic neutral" or "zero magnetic flux." There's nothing neutral or zero about it.
The reason for the rapid increase in resistance to turning is the core of the armature has reached the point where there is almost no overlap with the pole face so all the magnetic flux lines from the Alnico magnet are concentrated in a very small area. When the armature is rotated slightly further, instead of bunching even closer together, the flux lines find it energetically advantageous to switch direction through the armature. That is, if they were oriented so they pointed "left-to-right" they now point "right-to-left." This means the magnetic flux in the core of the armature, i.e. threading the primary coil, undergoes a change of magnitude 2x just past the point where the resistance to turning is largest.
Not coincidentally, this happens when the current in the primary already is near its maximum so this significantly boosts that current. So, if the points open when the magnetic flux lines are switching direction the maximum possible voltage in the secondary will be generated. In the case of our magnetos the "E-gap" is a bit of a red herring, because the notch in the armature aligns with the "pip" in the points plate to automatically set the E-gap where Lucas or BTH decided it needed to be. The E-gap is not adjustable in our magnetos unless someone goes out of their way to screw it up using files and hammers.
In the case of a vertical twin the plugs fire 180-deg. from each other, which is ideal for a magneto since it generates maximum current/voltage at the two orientations of the armature 180-deg. apart. A Vincent (or Harley, or Indian, or JAP,...) has to accept something less than ideal for one of its cylinders.
Although a Vincent is a "50-degree V-twin," the HT leads of the magneto don't fire 50-degrees apart. The configuration of this engine is equivalent to a vertical twin (e.g. BSA 650) whose cylinders have been split to be at 50-deg. rather than 0-deg. with respect to each other. As a result, lobes one and two on the points cam need to be 155 degrees apart. The reason for this is, since the magneto operates at half engine speed, 155-deg. corresponds to 310 degrees of rotation of the crankshaft. Thus, if one cylinder fires at 0-deg., the second fires after the engine turns 310-deg. further, i.e. 50-degrees less than another full revolution (360-310=50). It then takes another 50-deg. for the crankshaft to get back to the first cylinder, which is now on its exhaust stroke, plus another full rotation before that cylinder is ready for another spark, which is 410-deg total (50+360=410). So, the angular separation between the second lobe and the first needs to be half that, i.e. 205-deg. Since 155 + 205 = 360, this all works out.
OK, what the previous paragraph means is if the timing is set to be "perfect" on the rear cylinder then for the front cylinder it will be too late with respect to maximum from the magneto by 25-deg. That is, the output of the magneto for the front cylinder will be the same as if it had been retarded by 25-deg. (50-deg. on the engine). While not optimum for producing maximum output from the magneto, note that the max. advance range of ATD units supplied as standard for some other marques was 18-deg. (36-deg. engine) for some models, and as small as 11-deg. for others. This amount of advance wasn't determined by a limitation of the output of the magneto when retarded from optimum, but by the requirements of starting an engine. If a particular engine gave max. h.p. when timed at 38-deg. it would start badly, if at all, if the ATD had a range larger than ~19-deg. resulting in the spark occurring ATDC.
A KVF certainly can supply sufficient voltage to fire the front cylinder at kickover speed. However, reduced magnetism of the Alnico due to the partial demagnetization that is inevitable whenever the armature is removed and replaced means you will be needlessly stressing your right leg if you have someone rebuild your magneto who does not own a proper ~70,000 Amp-turn electromagnet.
I repeat,do not grind out the A/R slot on a manual magneto cam ring whether K1F ,K2F,or KVF
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As point of interest.....A fellow club member and long time owner, once changed the points assembly in his KV-F to a set out of a K2-F which as we know is the opposite rotation. He swears that it runs better than ever, and never gave him an ounce of trouble since .........Go figure.....
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With one the rubbing block is dragged up the ramp of the cam, while with the other it is pushed. Other than possible difference in how fast the block wears both will open the points and fire the magneto at whatever, um, point they're set. So, it's difficult to know what your friend thinks he means by "runs better." I find it best to take such observations with a grain of Australian red salt.
Well, if the K2F points were out of a Royal Enfield twin, it would be correct rotation.As point of interest.....A fellow club member and long time owner, once changed the points assembly in his KV-F to a set out of a K2-F which as we know is the opposite rotation. He swears that it runs better than ever, and never gave him an ounce of trouble since .........Go figure.....
Hello Greg, The bloke who made mine into a Manual also changed it Number one is now number two !! etc, I had a bit of Fun once, When I took it to a repairer, He was not Happy. Cheers Bill.
Yes Bill I'm not exactly sure why he would have done that ........ My friends mag definitely has a set of counter clockwise K2-F points (contact set) and he keeps asking me as if I automatically know the answer. I have to admit that I did spend over 7 months rebuilding and rewinding magneto's with probably the best man here in Australia up until his passing 4 years ago. I remember him telling me of the so called "Experts" out there, who's magneto repairs where at best woeful. But as he said to me many times ........There was no such thing as a trade or apprenticeship for the rebuild/repair of magneto's, so one had to learn themselves. After spending his life in business as an Auto Electrician, and racing MOV Velo's and later Bultaco's .....He took the time to study and understand what was needed to repair them, just to keep his bikes in good running trim .....Only he found others showing up on his doorstep asking him to repair theirs. This soon became a full time hobby, and after moving from West of Sydney to the Gold Coast here in Queensland, where he set up a small workshop on his acreage property (literally a stones throw from where I live now). He carried on rebuilding mags and genny's 7 days a week pretty much until the day he died. I was actually supposed to take over the business, but sadly I did not have the room to set up at that time, so he sold it to a long time friend north of Sydney who still runs it today. Members on this forum may remember him, his name was Ivan Brown ......A very skilled man who taught me a lot, and I was most grateful to have spent some good learning time from him. Yes I still remember winding the 200 turns of the heavier primary wire, and then followed by the 12,000 turns of secondary wire .......As thin as the hair on your head......!!! This gave a ratio of 60:1 turns of secondary to primary wire, and with the primary giving a good strong 200 volts on the test rig ......The output at the HT end would be around 12,000 volts. Good memories.............. ........Cheers........Greg.
........Cheers........Greg.