V-Twin Magnetos

Chris Launders

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Hi There,
I wonder if any of you magneto buffs can help me, I'm building a 50 degree manual adv/ret magdyno from scrounged parts and have a couple of things I could use help with.

First. Do I set one cylinder just as the armature poles come from under the magnets and do I set the 2nd cylinder early or late.

Second. Is this the advanced or retarded position.

Chris.
 

Magnetoman

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Timing should be set fully advanced just as the flux in the armature reverses (which is approximately where the poles come from under the magnet) on the rear cylinder. After the #1 plug gets its spark the next spark comes more than 180-deg. afterwards (on the KVF magneto cam) which means the #2 plug gets its spark after the armature has moved significantly past the position of flux reversal.
 

Chris Launders

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Thanks for that, but if #1 plug is the rear, then surely the front one fires less than 180 degrees later ?
That aside it is the conclusion I came to but wanted conformation before cutting the adv/ret etc notches in my cam ring.
I also found out V twin magdynos require special slip rings as the magnets are at 90 degrees to KVFs but the pickups are in the same place !!

Chris.
 

Magnetoman

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It would have been so much easier if those silly Phils had designed a vertical twin instead...

The #1 cylinder is the rear one, and the #1 magneto pickup is the one on the right as viewed from the points end. In a KVF the #1 plug sparks the rear cylinder when the points are forced open by the cam ramp that is near the bottom of the points cavity. The #2 cam ramp comes more than 180-deg later which, since the gearing is 1:2, means it happens when the crankshaft has rotated more than 360-deg. That means the rear cylinder fires, the front one is skipped as the crankshaft rotates, then the rear one is skipped after one full rotation (i.e. 180-deg. of the magneto), and finally the front one fires after the crankshaft has made a full rotation plus another 50 degrees (i.e. 180-deg. + 25 deg. = 205 deg. of the magneto).

Hey, don't blame me for the complexity, blame the Phils.
 

stu spalding

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Has anyone noticed that the points of maximum flux density on a Lucas SR1 rotating magnet magneto are at 155 and 205 degrees. I've noticed one on a Comet and wondered if it would be possible to fit the end cover from an SR2 and get a V twin cam ground. Cheers, Stu.
 

Bob Zwarts

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Has anyone noticed that the points of maximum flux density on a Lucas SR1 rotating magnet magneto are at 155 and 205 degrees. I've noticed one on a Comet and wondered if it would be possible to fit the end cover from an SR2 and get a V twin cam ground. Cheers, Stu.
The 'armature' of the SR1 is assymetrical with regard to its magnetic poles. I have no idea why this is the case but the SR2/4 variants have magnetic poles exactly 180 deg. apart. Both Joe Hunt and Morris make a rotating magnet magneto suitable for a 50deg. Vee twin. They use very strong rare earth magnets and this seems to largely overcome the inherent problems with unequal firing intervals.
 

greg brillus

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The Joe Hunt/Morris 50 degree magnetos are special order and are just a variant of their mag's to suit pre-unit Triumph's. It took me forever to get one done by Joe Hunt and what I should have done was ask for a pre-unit Triumph one and simply modified the cam myself. I had to do that anyway, because the timing split between the cylinders was out by several degrees. They use a wasted spark system for simplicity, and the armatures use NeoDymium magnets (rare earth) and will crank out about 5 amps when spun up. At Philip Island back in 2015 I burnt out a coil, because I was trying to shift using a handlebar kill button which I knew several of the other racers use to do, but they were using earlier Joe Hunt mags with Alnico magnets who's output was not as strong. This proved to overload the coil which failed. Luckily one of the old time racers had a good box full of spares, a new replacement was installed just in time for my first race on Saturday morning. It was the only significant failure I had on the racer, and after choosing to shift normally, it never gave anymore trouble.
 

Chris Launders

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Ha Ha, ignore my last bit about less than 180 degrees, for some reason in my mind the engine was turning backwards, probably because in this case the magdyno is running anti clock.
Chris.
 

Magnetoman

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the armatures use NeoDymium magnets (rare earth) and will crank out about 5 amps when spun up.
People may think that if some (Alnico) is good, more (NdFeB) is better. People who put rare earth magnets magnetized in the right configuration on the outside of their magnetos to help find there is a much better spark. For a while. Until the too-high current burns the points or the internal wiring. Luckily, a long bar magnet almost certainly will be magnetized end-to-end so the effects from the N and S poles largely cancel (it would completely cancel if placed in precisely the center of the armature). Rare earth magnets on a magneto are good only if the wiring in the magneto is designed for them
 

greg brillus

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Question for you on magnetos which we never found an answer for.........On a single mag like a K1-F the polarity makes a big difference to the primary output and this must ultimately affect the secondary output. So on the test rig when spun up to say 1500 RPM at the mag shaft, the primary voltage measured with a peak voltage meter would read 200 volts, whereas if you rotate the end housing 180 degrees thus reversing the polarity and run up again on the test rig, you get about 120 volts. Is there any reason for how or why this is......?........Cheers...........Greg.
 
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