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Old 11-27-2012, 11:47 PM   #167
FlyingFinn
Beastly Adventurer
 
Joined: Jul 2005
Location: Los Gatos, CA
Oddometer: 1,112
Like it's been discussed before, the the pre-08 FD units aren't interchangeable with later ones. The reason is the rear wheel speed sensor in the FD. Mechanically the final drives are 100% interchangeable.

The newer FD units have several improvements over the earlier ones:
- Larger, more robust pinion bearing
- Added vent to equalize the pressure, should help the seals
- New units also come with the steel wheel flange (the aluminum ones are prone to crack)
And a newer used one is simply likely to have less miles on it.

But problem is that BMW also changed the wheel speed sensor and the ring gear the sensor reads. To use a 08+ FD in a older bike, there are two fundamental problems that need to resolved.

1) The sensors are mechanically different size so it's not possible to simply use the old sensor in the new FD.
2) The sensors use different output signal level. The computer on an older bike can't even tell the rear wheel is turning based on the signal from a new sensor.
3) The ring gear in the 08+ FD has 48 slots and the older one has 78. Even if the sensor signal levels were compatible, the new FD would make the speedometer read 38.5% too low, and on a ABS equipped bike the ABS computer would throw an error.

The wheel speed sensors used in the r1200 bikes are 2-wire Hall effect sensors with binary current output. The two current levels of the older sensor are 4mA and 10mA, the new sensor uses 7mA and 14mA.
That's bad news since all the electronic speedometer calibration devices out there support only other types of wheel speed sensors. Either open-drain outputs or old inductive sensors that produced an AC voltage that was relative to the wheel speed.
All Japanese bikes, most other European ones and most cars use open-drain output sensors (also called "ground switching" sensors).
So none of the SpeedoHealer or other comparable products work with our bikes.
If they did, it would be trivial to plug in one of those, dial in the correction factor and be done with it.

What I've done is first studied these sensors to figure out the interfaces they use and designed an electric circuit that a) converts the current signal from the sensor to a format that a "SpeedoHealer" can use as its input, and b) takes the open-drain output of the "SpeedoHealer" and converts that back to current signal that is compatible with the BMW computer input.

I have all the measurement data from old and new type of sensors, how they interact with the computer and simulation results (used a SPICE simulator) of the signal conversion circuits.

Next step will be to order some samples, build prototypes of those circuits and plug it in with a standard SpeedoHealer. This will take bit of time but I'm quite confident it will work. I have all e data I need, the simulation uses real component models and results are good.

Mikko
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