Another burnt stator - plus a cooling hack - BMW F800 Riders Forum & Registry



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  1. #1
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    Usual saga - looks like this is at least the 3rd stator for my 2007 ST, so averaging ~30k km per unit. Quality BMW design, yeah right!

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    Managed to source an ElectroSport replacement - which unless its an Asian knock-off (eg AliExpress BS, tho the ElectroSport box does say manufactured in China) - seems to have the identical design to the dud unit. Not interested in the OEM BMW part, especially when I'm paying for a replacement rotor that should have been a free recall part anyway!!

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    This failure also comes with some background:
    - I've had the bike about 13000kms
    - when I got it it was mentioned that the stator had 'been done', but no exact timeframe
    - because of this, for the last 12 months / approx 10000kms its been running a Shendigen 775BA series (NOT shunt) rect-reg (not a knock off, sourced new from a Polaris dealer)
    - still running the original less-holey rotor - not many chances to pick up used parts here in NZ (even the cover gasket is apparently 6 weeks+ ex Germany via BMW NZ - obviously not interested in anything out-of-warranty)

    So again not impressed with yet another BMW design fault (I've had lots of history with BMW flaws across bikes & cars ...). From this and other online resources I've also bitten the bullet & gotten out the cutty-type tools & added an oil cooling jet to the center of the stator, as per these examples

    https://advrider.com/f/threads/f800g...ystem.1415644/

    or the T12 commercial kit for Aprillas: https://t12tech.com/fitting-instruct...or-oil-kit.pdf

    Bascally add an oil jet into/onto the center of the flywheel rotor & let that get centrifuged out, cooling the inside of the stator core, then having to overflow out past the coils to get back into the cover then into the sump, hopefully taking some heat with it!!

    On the F800, the main oil gallery is capped by the right side engine cover, so I tapped a take-off (aiming for a safety backtrack of just putting a blanking bolt back in if I need it) for a brakeline fitting & hose I already had hanging around

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    The bore on the adapter is the flow restriction - 2mm
    The other end is just another hose fitting (cut down from a banjo bolt, its steel not brass) ... where it screws into the cover is just sealed with thread tape, its only at crankcase pressure, only oil pressure in the hose

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    Whole shebang looks like

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    And with the fairing on it's 99% invisible

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    (the white is extra thread tape, just me being untidy )

    Will have to see how this works; I've also got the series regulator helping (hopefully) but obviously that didn't help with this last stator in the end... I'm also going to get the old stator rewound locally, so should be interesting to see what gauge wire they recommend & sealings etc
    Last edited by maxfnz; 3 Weeks Ago at 06:25 AM. Reason: atrocious spelling

  2. Thanks Runmyownlife, MGD109, IanA thanked for this post
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  4. #2
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    Do you have a lot of electrical items drawing power from the bike?
    BMW F800ST, 2010, Black, Fuzeblock, Sat Nav, BMW Tank Bag, Givi Touring Screen, Denali Spotlights, Denali CANSmart, Wunderlich Crash Bars, 55K Miles & counting 

  5. #3
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    No, almost zero in terms of watts drawn. Have a GIVI USB charger in the topbox with nothing connected. Nothing else non-standard - HID lowbeam, but that's 55w like the std halogen, although it's on all the time so maybe an extra 55w comapred to a euro-spec bike ('lights on' is part of the Aust/NZ design regulations so there is no way to turn the lowbeam off). LED stops but that's not going to toast anything

    No sign of prior hacking for extra farkles by former owner(s) either...

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  7. #4
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    Congratulations on your hack. Brave man drilling into the expensive pieces! I'm appreciative of your plan B -using a bolt. I like to think projects through to other outcomes like that too.

    Great post. Thank you for sharing the details and pictures.
    Last edited by Runmyownlife; 1 Week Ago at 12:29 AM.
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  8. #5
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    Lights are on all the time on any ST as there is no way to turn them off on any model. So that's not an undesigned draw/issue.

    I would guess its the heat in your part of the world which is not helping the lack of cooling, causing the premature failure.

    I'm not sure the oil cooling is the best idea - especially if it gets contaminated and could short out the windings. I'm guessing that feed is off an oil return so the oil will also be hot from running around the engine already before feeding back down, a potential to exacerbate the already hot stator.

    I would have probably gone down a cool/cold air install (with a filter) to introduce fresh air, and potentially an exhaust outlet too in order to aid venting.
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  9. #6
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    Feed is main gallery post-filter, post heat exchanger. Cooled, clean - it has to be, its feeding all the main bearings

    Attachment 372193

    Hot weather ... maybe? Hottest in last year maybe 26degC? Nothing compared to the 110+degC my contactless IR was reporting off the housing before the 775. At least with the oil as coolant with the Rotax design it does get cooled to 'engine average' with the heat exchanger ... and air cooling, I don't think trying a 5mm hole compares to cutting out experimntal inlet/exit ports in a major casting!!? ... If it gets that bad I'll go buy a K75; my old k1100 had 200k kms on its original unserviced alternator, why in less than a generation BMW expect owners to accept 30-50k I can't fathom.

    Happy to take the hit on oil pressure with a 2mm 'jet' ... less work for the oil pressure bypass was my thought, given the wide pressure range across running RPMs.

    Side observation ... the oil galleries do really drain back, mine were 100% dry about 1/2 hr after running, s no wonder the camchain tensioner clatters on startup, its waiting for any hydraulic pressure at all!!

  10. #7
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    Another member has had multiple stator failures.
    I don’t think he lives in an abnormally hot climate.
    There must be something that causes it.
    And I don’t have a clue what it is.
    I did have a regulator rectifier fail @ 25,000miles.
    Good luck
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  11. #8
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    The K75 / K1100 series has an external air-cooled alternator, not an internal oil-bath stator, so it’s apples and oranges.
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  12. #9
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    Quote Originally Posted by emlevy View Post
    The K75 / K1100 series has an external air-cooled alternator, not an internal oil-bath stator, so it’s apples and oranges.
    Yeah, that's 100% obvious. The point was they're both BMWs, marketed & sold as premium bikes, both designed to BMW specs - whole bike, engine & running gear! - no one else. One lasts & lasts, the other continually burns out.

  13. #10
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    Quote Originally Posted by maxfnz View Post
    Yeah, that's 100% obvious. The point was they're both BMWs, marketed & sold as premium bikes, both designed to BMW specs - whole bike, engine & running gear! - no one else. One lasts & lasts, the other continually burns out.
    I understand your frustration though doubt the problem relates to external ambient temperatures, here in Queensland Australia we have temps considerably warmer - not that I ride in extreme temps but if I waited for sub, 30 degrees I would not get much riding in summer.
    I ride a 2008 ST with only 43,000 kms and when the R/R failed, replaced the original with a different type - MOSFIT regulator/rectifier which are supposed to make the stator run cooler
    I was advised on this forum to replace the stator at the same time but being a pensioner, I tested it and as all 3 phases were giving the same output decided to keep what I had to save some money
    Replaced R/R myself and it is running perfectly though in fairness I have probably only travelled about 3000 km's since the change over

    The problem you are having is not a new one, and there are a number of threads that relate but as to why - there are many answers to that, my personal thinking is there is a deeper issue that has not yet been exposed
    Wished that I could be more help, good luck with tracking down the cause

  14. #11
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    Got an ST three years ago, second hand, 2009 model, 14k km.
    Since I came from an F650GS, I perfectly knew about the burning stator matter (4 burned up!). So, just the time to look for an SH775 and I replaced the original rectifier with this one.
    Result: with the original one 115°C on the stator carter, with the SH 95°C. 20°C less.
    Measurements with a IR reader, average measurement in 10 shots each, same sample point, same conditions, same rides.
    I rode 10k km with this configuration, charging value 14.1 +/- 0.3 V, so far so good.

  15. #12
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    Quote Originally Posted by DJ123 View Post
    Lights are on all the time on any ST as there is no way to turn them off on any model. So that's not an undesigned draw/issue.
    Stator and regulator/rectifier will get hotter if you could turn off the headlights.

    The stator+rotor alternator design runs at 100% output for given RPM all the time. The R/R is a shunt which shorts the excess current. Running your headlights consumes power the R/R would otherwise have to turn into heat.

    I understand how a MOSFET R/R can run cooler (there is less voltage drop across a MOSFET than an SCR) but not how it can keep the stator cooler.
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  16. #13
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    Quote Originally Posted by N4HHE View Post
    Stator and regulator/rectifier will get hotter if you could turn off the headlights.

    The stator+rotor alternator design runs at 100% output for given RPM all the time. The R/R is a shunt which shorts the excess current. Running your headlights consumes power the R/R would otherwise have to turn into heat.

    I understand how a MOSFET R/R can run cooler (there is less voltage drop across a MOSFET than an SCR) but not how it can keep the stator cooler.
    It isn't the MOSFET vs SCR but rather the series vs shunt regulation. You can use a MOSFET in either scheme.

    In the standard shunt scheme the regulator effectively shorts the output so as to draw down the voltage. The resulting current draw is said to contribute to stator heating.

    In the series scheme the regulator opens the circuit to drop the voltage but being an open circuit the current draw from the stator is less which is said to reduce stator heating.



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    Quote Originally Posted by bmwroadsterca View Post
    It isn't the MOSFET vs SCR but rather the series vs shunt regulation. You can use a MOSFET in either scheme.

    In the standard shunt scheme the regulator effectively shorts the output so as to draw down the voltage. The resulting current draw is said to contribute to stator heating.

    In the series scheme the regulator opens the circuit to drop the voltage but being an open circuit the current draw from the stator is less which is said to reduce stator heating.
    I remember being lost in "back EMF" and other things studying electric motor theory and practice in engineering school. Majoring in mechanical engineering I just remembered the basics. Years later I had to deal with a surplus aircraft servo working with a real EE who designed a motor controller with many unexpected (by me) parameters. Showed me a lot of magic current flow on the oscilloscope, and current vs voltage out of phase due to the motor windings no matter we were DC.

    I brought motorcycle stators into the discussion with the EE over 30 years ago. Forgot what issue I had in mind but knew I had 70VAC open circuit and wondered why we couldn't just take what we need from that for greater efficiency. In layman's terms he said the stator is an inductor so once the current is flowing you have to keep it flowing else you have to counter its flow to make it stop, then start again, and something about "back EMF", and that results in huge spike currents and voltages.

    This source of retrofit motorcycle MOSFET R/R says it is still a shunt regulator. Says nothing about cooler operation of the stator.
    Sindengen Moset Regulator
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  18. #15
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    Yes the MOSFETs in a series regulator have to be able to withstand the back EMF which may be considerable.

    Generally the manufacturers do not cite stator cooling as a benefit of series regulation.

    That being said there is a significant number of F800 owners that claim the the stators last longer or do not fail when a series regulator is installed.

    There are examples over on ADVrider.

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    Quote Originally Posted by bmwroadsterca View Post
    Yes the MOSFETs in a series regulator have to be able to withstand the back EMF which may be considerable.
    You misunderstood. Is the insulation in the stator that has to be stout enough.

    Generally the manufacturers do not cite stator cooling as a benefit of series regulation.

    That being said there is a significant number of F800 owners that claim the the stators last longer or do not fail when a series regulator is installed.

    There are examples over on ADVrider.
    Been there. Didn't see any convincing evidence anyone understood what they were talking about.

    There is a lot of urban folklore on the internet.

    Riddle me this: If there is no current flowing in the stator if a series regulator is "off" then why is force still required to spin the magnets around the stator?
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  20. #17
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    Quote Originally Posted by N4HHE View Post
    You misunderstood. Is the insulation in the stator that has to be stout enough.


    Been there. Didn't see any convincing evidence anyone understood what they were talking about.

    There is a lot of urban folklore on the internet.

    Riddle me this: If there is no current flowing in the stator if a series regulator is "off" then why is force still required to spin the magnets around the stator?
    The metal core is more magetically permeable than air. The magnetic field resists the core's poles moving out of alignment with the direction of the field. Any kid that has turned the shaft on a DC motor knows that.

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    Quote Originally Posted by bmwroadsterca View Post
    The metal core is more magetically permeable than air. The magnetic field resists the core's poles moving out of alignment with the direction of the field. Any kid that has turned the shaft on a DC motor knows that.

    Sent from my SM-T813 using Tapatalk
    OK' I'm no Electrical engineer, and further, claim no specific knowledge of different rectifier / regulators, but aren't most countries including NZ mandated in the use of headlights at all times whilst riding?
    Certainly the case here in Aussie, so that combined with my not being a "revhead" may be the difference in longevity of my stator - who knows???
    My average riding would see revs up to about 4000rpm (apart from the odd squirt) - certainly don't exploit the engines potential but fuel consumption is low - and no burned out stators!
    Or am I just lucky?

  22. #19
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    Quote Originally Posted by bmwroadsterca View Post
    The metal core is more magetically permeable than air. The magnetic field resists the core's poles moving out of alignment with the direction of the field. Any kid that has turned the shaft on a DC motor knows that.
    Yes. That was my point. Moving magnets over the cores is creating and collapsing fields. Windings around the coils can not help but energize, de-energize, as magnet moves from core to core, which means things are happening in the the wires.

    What I know is that there is a lot to electricity and engineering which is counterintuitive. When faced with the desire for more power out of an existing motorcycle magnet/stator design I brought the problem to a couple genius electrical engineer friends with whom our complementary skills have done some amazing things. They confirmed some things I suspected already, that unless the magnet flywheel and the existing stator are mismatched changing one or the other is not going to improve output very much. Maybe 105% of what you had before. The other thing that came out of it was how total loss shunt regulation was a good thing and less risky for the stator coils than a series regulator. The simplified explanation said the alternator was operating like an oscillator, currents are building and falling, happily out of phase with voltage. Selective series regulation disturbs that balance. That when voltage is greater than the battery it goes to the battery and vehicle load but the instant it drops it gets shunted to ground. The current keeps flowing without having to stop. The starting and stopping of current was what my friends were concerned about. Is not easy, and it is hard on wires and insulation.
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    Quote Originally Posted by N4HHE View Post
    Yes. That was my point. Moving magnets over the cores is creating and collapsing fields. Windings around the coils can not help but energize, de-energize, as magnet moves from core to core, which means things are happening in the the wires.

    What I know is that there is a lot to electricity and engineering which is counterintuitive. When faced with the desire for more power out of an existing motorcycle magnet/stator design I brought the problem to a couple genius electrical engineer friends with whom our complementary skills have done some amazing things. They confirmed some things I suspected already, that unless the magnet flywheel and the existing stator are mismatched changing one or the other is not going to improve output very much. Maybe 105% of what you had before. The other thing that came out of it was how total loss shunt regulation was a good thing and less risky for the stator coils than a series regulator. The simplified explanation said the alternator was operating like an oscillator, currents are building and falling, happily out of phase with voltage. Selective series regulation disturbs that balance. That when voltage is greater than the battery it goes to the battery and vehicle load but the instant it drops it gets shunted to ground. The current keeps flowing without having to stop. The starting and stopping of current was what my friends were concerned about. Is not easy, and it is hard on wires and insulation.
    Ask your friends if the back EMF concern when a series regulator cuts off the current is mitigated if the regulator has a zero crossing detector such that it switches off at zero or low current.

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    My brain hurts!

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