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Really BIG alternators for BMW Airhead Motorcycles

Copyright, 2013, R. Fleischer

The Omega (Emerald Island) and EnDuraLast (Euromotoelectrics) alternators provide increased output, and may have other attributes that could be helpful, such as usable output at lower rpm.   They are covered in their own articles on this site.  Refer to Article 15-A and 15-C for information on the Omega and EnDuraLast aftermarket alternators ...and a discussion of the capability of the stock Bosch alternators.   One of the Emerald Island alternators is rated at 600 watts!

The following article covers just a few of the installations of some truly powerful alternators, as installed on BMW Airhead Motorcycles (and on /2 era BMW motorcycles too!).  



There have been installations that attach the alternator to the crash bars.  Generally speaking, that does not work out well over time, unless the crash bars are modified to be beefy and substantially supported.

Below:  Note the bracketry/mounting directly to the engine case!  Note the angled post for the tension adjustment.  Note the cut in engine case for belt passage. Can be left or right side.  Not the nicest installation.

In the above photo, is a statement in Swedish, that I have had much difficulty in translating.  I have used the Google translator, and I also sent the statement to a close friend who grew up in Sweden, and uses Swedish on a daily basis to this day.  Near as I know, the statement has to do with temperature changes on the belt tension due to the pulley design and method of attachment.  Whether or not the writer meant that he overcame the problem, or there remains the problem, or he designed the pulley so it could not have the problem, is undetermined.  If you have a better translation, please let me know.  Contact me at:    E-mail-Image

I have done alternator conversions myself.  The small Japanese alternators are my favorite; and for VERY large alternators, I like the larger more pancake type of Chrysler alternators.   However, many recent cars, particularly larger American cars, have very large alternators.  New cars now appearing have stop-start technology.  These cars engines automatically shut off at stop lights, then, you press the accelerator, and the engine is started.  Some of these engines can have beefy combination starter-alternator units.   I have not yet had my paws on some, but I THINK they may be adaptable to the Airheads, so the stock starter motor will not be needed.  The result might be a more powerful alternator, a substantially powerful starter section in it, and room for, ...shall I say it? ...other nice things could be installed prior areas of the motorcycle, either alternator or starter, depending on the conversion.  ....maybe a supercharger...?

What about installing an oversize alternator (other than the Omega or EnDuraLast)?

There are several ways of doing these types of conversions; here is the basics on how I have done SEVERAL ...on R60/2; R75/5, R90/6, etc.:

1.  Disconnect battery.  Remove fuel tank.  Unplug voltage regulator. Remove the front cover, so you can see the stock alternator.

2.  Remove the stator housing and stator.  Using appropriate HARDENED threaded tool (not a jaw puller!!), remove the rotor.  You will need a rotor, unless you are an excellent machinist at turning precise tapers on a lathe.  Sell your rotor if in good condition, and get one that needs rewinding, as you will need the taper portion (at least).

3.  Remove the diode board (and diode board mounts if the removable type).

4.  Modify the rotor so it has a V-belt PULLEY attached to it.  You can use a toothed belt also.  Be careful not to damage the crankshaft and rotor tapers.  The only thing you really need is that taper, and a pulley affixed VERY squarely and sturdily to it.  I like the rotor pulley to be LARGER in diameter than the new alternator's pulley, so that the engine will run the alternator at a bit faster than engine rpm.  The pulley/rotor-taper modification MUST be done with strength, neatly and very squarely, & I did all of mine on a jig on a drill press or in a lathe or in my mill.  You will have a choice on what type of pulley, and how closely towards the crankshaft it is mounted.  Think about it all before committing yourself.   Take a look at the older Mazda cars power steering pump pulleys in a wrecking yard; think about drilling and tapping holes in the rotor, etc.

5.  Select the proper alternator.   This is actually part of the above step 4.  Try to find an alternator that mounts nicely (and strongly!) to your engine and to whatever mount(s) you make for it.  The alternator can pull a lot of engine power, and the inertia of the rotating alternator parts (including belt, pulley, etc) can be VERY HIGH when YOU shift the transmission, so things must not vibrate nor bend.  I suggest a small diameter alternator, with built-in or built-on cooling fan(s).   Consider the direction of mounting and the direction the alternator fan blows the air in relationship to the direction of your motorcycle's engine rotation!!   You want the oncoming air to work WITH the alternator's fan(s).

6.  Use a large battery if possible. It will better absorb the alternator capability.   For a sidecar rig, a car battery in the trunk of the sidecar is ideal.  Some might want to retain the bike battery too, just for starting; and use the sidecar mounted battery for camping lights, etc.  For  a sidecar rig mounted battery, try to place the battery behind the sidecar seat and to the far side away, all of which is an ADVANTAGE for handling.  Keep in mind that a large alternator can COOK a small motorcycle battery, if that battery is run down much at all. There is a limit to the current flow a small battery can handle short term and long term.    The commonly available 28 or 30 ampere-hour flooded motorcycle batteries will usually work OK, however.

7.  Use a self-contained alternator, of the one terminal type (one additional smaller terminal to drive the dash GEN lamp is fine).  Use heavy gauge wires to the battery and do have a good ground.

AND ...HERE ...for those of you that want something pretty, sturdy, and NOT done as an outboard.  This type of thing, if you were to make it yourself, would require some machinist talents, some design talents, and plenty of time!  This is the Australian-made conversion, is available in the USA. 800 watts, but do not have a link to the manufacturer yet.  Try, they had these in a thread a while back. Here is a link to one, and possibly the only place in the USA for these:  Compare to the Omega or Emerald Island 600 watt alternator.

05/22/2010:  Clean up article
08/31/2010:  Add more photos, permission from Albert Lindberg.  Also add more commentary, edit information #2, 3, 4, 5, 6, 7.
01/07/2011:  Add photo of Greg's 800 watt alternator conversion.
10/13/2012:  Add QR code, add language button, update Google Ad-Sense code.
07/26/2013:  Fix error on pulley size in step 4.
04/27/2013:  Clean up article a bit, add more info on the 800 watt Aussie conversion, including the link.
03/18/2016:  Metacodes, left justification, layout, etc.
10/15/2016:  Metas, scripts, H.L., html cleanup, photo borders.

Copyright, 2013, R. Fleischer

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Last check/edit: Wednesday, January 17, 2018