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How much difference does the alternator rotor make?

Discussion in 'Norton Commando Motorcycles (Classic)' started by maylar, Aug 19, 2019.

  1. maylar

    maylar VIP MEMBER

    May 13, 2007
    I put a Sparx 3 phase charging system on my 850 some years ago to keep up with halogen headlamps. The rotor that came in the kit was undersized and didn't fit the crankshaft, so I used the old Lucas rotor. Now I'm wanting to increase the output a bit to better handle my Li battery and I wonder if changing rotors could have any effect. Has anyone noticed a difference between rotors?
    Joe Schlaberdowski likes this.
  2. Triton Thrasher

    Triton Thrasher

    Feb 10, 2009
    If the magnets are stronger you will get more power.

    Do they feel stronger?
  3. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Dec 3, 2012
    Just google Ducati 750GT (I am going to get one of the Road & Race 400w kits but they are around $600)

    Ducati's of that era had 150 watt on the 750 roundcase and went to a 200 watt on the 860/900 squarecase engines.
    The 750 stock was lucky to be charging the battery at 5000rpm with the headlight on so electrical uncertainty was not limited to Lucas.
    One of the first upgrades for the 750 stator was adding the rare earth magnet rotor from a 1998 Ducati ST2 so the rotor has a big part in things.

    I guess there is no perceived market for a modern charging system kit for British bikes with rare earth magnet rotors and mosfet regulators.

  4. DogT


    Jan 20, 2009
    Used to be you could get your rotor re-magnetized, they do get old.
  5. Time Warp

    Time Warp .......back to the 70's. VIP MEMBER

    Dec 3, 2012
    The catch is they still use 1960's magnetism so you get a brand new old part.
  6. oldbeezer

    oldbeezer VIP MEMBER

    Nov 2, 2017
    Don't know about rare earth magnet rotors but here are some modern regulators (I will be putting one of these on my bike shortly)
    Time Warp likes this.
  7. texasSlick

    texasSlick VIP MEMBER

    Jan 2, 2013
    I changed over to LED's, now my old rotor is loafing.

  8. comnoz

    comnoz VIP MEMBER

    Dec 10, 2008
    I tried rare earth magnets in a custom alternator rotor for my Norton one time. The operating temp was too high and the rare earth rotor would not charge after some time at highway speed.
    Rare earth magnets do not handle temps as high as old conventional magnets but they work fine in a water cooled engine.

    When a rotor is stored it should be left inside the stator or the magnets will loose strength. The stator acts like a magnet keeper.

    In the past when you purchased a new Norton rotor it came with a steel clamshell ring around the outside to help retain the magnetism.

  9. Fast Eddie

    Fast Eddie VIP MEMBER

    Oct 4, 2013
    That’s how mine came recently from AN.
  10. alan hodge

    alan hodge

    Oct 10, 2017
    I had the old Lucas rotor on my 68 BSA A65 remagnetized by Joe Hunt Magneto about 6 years ago but it was getting tired again I put LED headlight and tailight bulbs in as an experiment...last ride I checked battery voltage before and after 40 mile ride...it actually went up a couple of tenths
    PS I have read that the new rotors offered are a smidge larger in diameter than old ones so wont fit old stators gotta buy both anyone else hear that?
  11. Danno


    Feb 7, 2010
    This, esp. with a Mk III charging system.
  12. Torontonian


    Dec 28, 2009
    Always a good idea to hang onto those outer steel clamshell rings. Magnetism can bleed out of the rotor if stored next to or touching other pieces of metal without them in place.. Had to learn the hard way.
  13. comnoz

    comnoz VIP MEMBER

    Dec 10, 2008

    The magnet keeper shorts the north and south poles of the magnet together to keep the "magnetism" in the magnet.

    Without the keeper the magnet looses it's strength to the atmosphere.
    The stator serves the same purpose as a magnet keeper, so if the rotor is off the bike during a rebuild you can safely store it inside the stator.
    Any piece of ferrous metal that is in close proximity to both poles would do the same.

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