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Old 06-22-13, 06:30 PM   #1
rperks
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B&M Luxos with Frame Grounded Tail Lights

I have been really enjoying my B&M Luxos U on some early morning rides this last week. As I was getting things ready to wire in the tail light it became apparent that I was going to have to resolve a grounding issue. The Luxos lights have some sort of fault detection sensor for the wired in tail light. There have been a couple of reports of odd and or poor performance, drag and even whining noises coming from the lights when connected and the tail light is shorted to the frame or fender.

I was a little bummed that this was going to be a hassle. Not wanting to give up on the Luxos, I set into sorting out the grounding on my Seculite plus. The process was really quite simple, and I documented the steps here:

http://oceanaircycles.com/2013/06/22...d-tail-lights/

I hope this helps a few of you out. The Luxos really is that great IMO and should not be over looked or passed by for such a simple patch.

Disclaimer: I sell both of these lights, when I can keep them in stock, but this is not meant to be an ad, just a help to others who will be getting this to work. The folks on the touring forum blast the heck out of me on every post for being a spammer, I am trying to be sensitive to that.


Seculite Plus and Luxos U by rperks1, on Flickr
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Old 06-24-13, 01:19 AM   #2
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I'm curious how the light can sense the difference between a wired and frame-path ground? Are both paths simply shorted together, or is there some electronics between them? Is it sensing a drastically different (higher, I would assume) resistance?

That'd be a big problem if you have a metal fender and a grounded axle hub, since you're inadvertently using the frame as a conduction path.
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Old 06-24-13, 06:43 AM   #3
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I got the impression it was mostly a problem when the dyno has a ground path through the frame. This would mean that some of the current would go through the frame and not back through the headlight. The resistance would be lower, but some of the current would be missing. I'm sure the circuit is very cheap and this is an unintended side effect.
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Old 06-24-13, 01:04 PM   #4
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That makes a lot more sense. Probably the part of the circuit that detects the functionality of the taillight is on the ground part of the circuit rather than the "hot" part, and the current reduction through that part of the circuit tells it something is wrong. It's sounds more like a design robustness issue rather than the cost of the circuit itself.

It's a moot point for me, because I have a different headlight and a plastic fender, but my technical side was curious.
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Old 06-24-13, 03:09 PM   #5
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now I want to take my headlight apart and see what's going on. The idea that it's measuring current on the ground side makes a lot of sense. I really need to torture test it now, I'm not really going to put it to good use until early August and I haven't been using it because I was afraid to during peak randonneuring season. There are a smattering of reports of infant mortality, which makes sense given how complex it is.
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Old 07-04-13, 01:45 PM   #6
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Originally Posted by DiegoFrogs View Post
That makes a lot more sense. Probably the part of the circuit that detects the functionality of the taillight is on the ground part of the circuit rather than the "hot" part, and the current reduction through that part of the circuit tells it something is wrong. It's sounds more like a design robustness issue rather than the cost of the circuit itself.

It's a moot point for me, because I have a different headlight and a plastic fender, but my technical side was curious.
It doesn't strike me as necessarily an issue with design robustness, rather that the new headlamps are designed with features that require certain things of the other elements of the system, i.e. the taillight current shall return through, let's call it Terminal A. If this requires two-wire power distribution that's something the dealers and users must be made aware of. In my worlds of automotive, aircraft, and aerospace electronics such constraints are quite common. No component specification is complete without definition of the system interfaces, based on electrical, logical, and mechanical considerations. It's a "this bike is no longer a toy" moment.

The upside is, "you can't just put on the light any way you like" means you get some more features in the lighting system. The downside is, you can't just put on the light any way you like. I suppose in a sense robustness has been traded away to achieve more function.

I plan to go with these lights on both my bike and that of Mrs. Road Fan, so I'm thinking about modularizing the wiring so I can actually remove a fork, cockpit, or fender as a subassembly without having to cut off spade lugs - try to engineer our bikes for electrical serviceability.
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Old 07-04-13, 06:21 PM   #7
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ya, you can't go with just any light, including any of the B and M lights I own. Which is 4 different lights
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