Additional Diode board earthing
Just thought I'd share my thoughts and the results of some
experimentation on the earthing systems of 2V Boxers.
First of all, why?
Earthing problems on ‘R’ series airhead boxers, seem to be a recurring topic of posts on BBS’s and Forums (fora?). A look at the setup as used by BMW shows that it is far from perfect. To my way of thinking, there are two areas (at least) which could use some improvement.
For the charge circuit it is most important that there virtually no resistance in the electrical path between the diode board and the battery.
On the face of it, there is a big Earth strap to the metal mass of the engine/gearbox unit to the battery and the diode board is connected direct to the engine front cover... what could be better?
Closer examination, however, shows that the earth strap actually connects to the rear cover of the gearbox and the current has to flow from this, to the gearbox housing, then to the crankcase and finally into the front cover. Three joints between the metal masses. In addition, two of these joints have gaskets which are almost certainly good insulators. This means that current has to flow from alloy casing into steel bolts, then back into alloy casing at two of these joints. Steel and alloy is notorious for corroding and becoming 'high resistance'.
In addition, there is the problem with the black painted front
covers, where connection between the diode board and the metal mass of the
cover is reduced by the paint on the cover.
In practice, therefore, it is quite possible for a 'high' resistance to develop in the earth path, even though all of the accessible connectors are clean and tight.
The second major area of concern is to the earth side of the motorcycle circuits.
The main earthing point for the motorcycle circuits is at (or near) the coil mount on the top of the frame. There is, however, no direct frame Earth to the battery. As a result, the earth current to the bike circuits must first flow from the battery to the gearbox rear cover, thence to the gearbox and into the crankcase before flowing into the frame at the engine mounting studs. (Anyone who has ever taken the engine out of an old boxer will know just what a state these studs can be in) It then has to flow through the frame to the earthing point. Again, a path which almost seems designed to attract high resistance joints.
The normal solution to earth problems is to provide an extra earth cable from the diode board to the engine front cover. It is my contention that this is, at best, an incomplete solution.
So what is my solution???
On two bikes now, I have added an additional earth loom. This starts (or ends depending on perspective) at the diode board where a ring connector is trapped under one of the earth bolts that hold the diode board to the front cover.
The main cable is in 30 Amp wire and runs through into the starter cavity before exiting through the same rubber grommet that seals thick starter lead between front cover and starter cover. It then runs direct along the spine of the bike to the negative pole of the battery. This provides a direct ground from diode board to battery.
In addition, a second wire connects to the first by a solder joint just under the frame tube of the bike. This is 15Amp wire and runs to the main Earth point near the coil.
The result of this is to 'tie together' electrically, all of the major ground points in the charge circuit with the minimum of high resistance joints.
The first machine I tried this on was my R60 restoration. Since there was no apparent charging problem with the machine and all joints were clean when I reassembled it, all I can say is that it has worked perfectly since it was rebuilt.
The next 'test' was on a 1987 R80RT. This had been charging perfectly but had started showing signs of charging problems with the battery running flat. I was resigned to buying a new battery but the faults were intermittent enough to suggest that there was another problem.
After fitting the auxiliary earth (and, incidentally, cleaning up all charge circuit connections) I noticed the following.
1) All symptoms of poor charging disappeared. I have not replace the battery and it appears to be standing up well despite the intermittent use of auxiliary lamps (110 Watts) and heated grips (OK. so I'm getting old!).
2) The voltmeter indicates that charging starts earlier and reaches the limit point earlier than it ever has done before.
3) The voltmeter also indicates that the charge circuit is more resilient and can stand up better to the full drain (as shown above).
3) The lights seem visibly brighter (but that's probably wishful thinking).
I appreciate that the results are possibly subjective and not at all 'scientific' but it is my belief that this is a mod that was well worth doing and the results are very satisfactory.