Alternators – Aftermarket

There is nothing wrong with the design of the BMW Airhead motorcycle’s stock Bosch alternators. Like most anything they will age, and/or they may be abused, they may fail, although not in excessive numbers. They are often never maintained until there is a failure. Wattage output was adequate for most riders when the motorcycles were originally sold by BMW.   The modest electric output may be of concern if you have lots of accessories ….such as heated clothing, heated grips, extra headlights, etc. I am not for or against aftermarket alternators. I am reporting only facts and testing information and conclusions. You may well do fine with the stock system. But, if you have special needs, such as large city stop and go commuting; or need more watts ….perhaps you are approaching the stock alternator limits (which decreases reliability) ….there is plenty of information in this article. The /5 Airheads had a 180 watt Bosch alternator with a stator diameter of 105 mm where it fit into the engine case. It can be upgraded easily by using an EARLY /6 alternator stator (if it is 105 mm, and NOT 107 mm), and any pre-1985 (approx.) rotor (2.8 ohm rotor not recommended) and any /6 or later diode board. Reliability can be improved for ANY Airhead alternator, stock or aftermarket, and this includes a stock /5 model, or a /5 upgraded to the 280 watt 105 mm Bosch; OR EVEN AN AFTERMARKET ALTERNATOR;  ….by using the later, better ventilated, front metal engine cover. If you have an RS or RT model, and the front fiber-glass-like cover is NOT louvred, even more reliability will be had by changing to the louvred type, or putting attractive holes into yours.  
Refer to:

This article includes information that is far beyond what advertisements for various alternators provide. Charging a battery in a vehicle like your Airhead is NOT as simple as you may think; nor, is interpreting the alternator ratings, typically simply shown as wattage. This article presents a huge amount of information; and, also presents information that helps you select an aftermarket alternator. I suggest you read this article completely through to get an over-view; then refer back to the areas you are interested in. If confused, please inquire on Snowbum’s favorite forum for inquiries; an E-mailing LIST, the Airheads List, as hosted by Input will help others, rather than my answering individual inquiries, which are discouraged.

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Alternators – Stock

BMW uses 3 phase electricity generators called alternators on all models since 1969, & some before that date. Why 3 phases? 3 Phase is more efficient at converting rotational energy to electricity. 3 phases enables the same or much more output in a smaller, lighter package, even less steel laminations are needed, & possibly more charging at lower rpm & possibly less horsepower drain on the engine. 3 phase is usually better for vehicles with radios & other electronics, as the system is, in effect, using an A.C. generator at higher frequencies than single phase. That means that with the battery acting like a monstrously large capacitor (which it DOES), alternator whine noise & other problems is MUCH reduced in a 3 phase system, compared to single phase. There are various other advantages that 3 phase offers. The 3 phase rotor MAY have less inertia, thus better vehicle acceleration, assuming diameter is kept small & overall inertia similarly, thus there may be a lowering of torsional stresses on the shaft & components. That last item can be critical on some engines where the rotor is affixed to the crankshaft & can be considered an extension of the crankshaft.

A 3 phase alternator is more compact & efficient … up to 1.73 times electrically more efficient.

At the link here, scan down the article, are sketches of the waveforms & some additional detailed information on single and three phase systems. The sketches will help you understand 3-phase.

Ø is the symbol universally accepted in electrical circuits for ‘phase’. A 3Ø system has few drawbacks. One is that the diodes circuitry used for rectification is more complex & there are more diodes; another is complexity & more labor in manufacturing.

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Alternator, Rotor, Stator, Brushes, etc.

Rotors and Stators:

This has been a confusing subject to many. Anton Largiader’s website article with text & photos will explain explain some things; or, some differently. I have minor disagreements with his article, BUT….it does have some useful information & has some photos you should look at. I have the most detailed and pertinent information here in my own article. Read Anton’s article here:

The original /5 BMW Airhead motorcycles had a 180 watt alternator with the end that fits into the motor timing chest cavity being 105 mm in diameter. SOME 1974-1975 /6 bikes, & possibly a few barely into 1976, were made with the 105 mm cavity. These /6 bikes with 105 mm stators had 280 watt alternators, so it is possible to upgrade a /5 from the 180 watt alternator to a 105 mm 280 watt alternator with the appropriate parts changes and the parts WILL FIT. These changes are stator & diode board, at a minimum. If your /5 rotor measures ~7 ohms, which was the original value, then I recommend it be changed to a next version rotor of ~3.4 to 4 ohms; the electronics VR is optional, but recommended. ALL /6 and later stators had a center tap on the stator windings, & some small diodes were added to the diode board. If you do not use the /6 or later diode board, the output will be less. I suggest NOT using the last version of the Airhead rotors, which were ~2.8 ohms. For all other /6 and all later Airhead motorcycles, the cavity and stator were 107mm.

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