revtothelimit - motorbike forum

I've wired up the top box brake light on my CBR250


The bike has an old style incandescent brake light. Three wires, shared earth.

The brake light wires come down from the bike to the OEM 2 pin plug, the red and black wires are an adapter into the OEM brake light, and also back up towards the storage area where there's another 2 pin plug going out to the box. The tail light is the brown one in the double bullet plug, I guess they've done that as a convenient way to split power out to the number plate light.
You have to remove the rack to stretch the rear cowl enough to remove the panel above the rear wheel which covers those wires, so it really does need all those connections unfortunately. But at least the original loom remains untouched so all fully reversible.

How hard would it be to interrupt the tail light wire, and use it to add (guessing) about 8v or so on to the live wire to partly illuminate the top box LEDs as a running light? I'm guessing, and really guessing, that it would take some kind of resistor and presumably a diode such that the 8v didn't keep the bike's brake filament partly-lit all the time.

Or is that fundamentally a bad idea? There must be some reason why car tail lights run LEDs part-bright using duty-cycling.

Givi box + brake light kit = endless problems. DAMHIKT.

The connection to the box is a problem.
The internal wiring in the box is a problem.
Fitting and refitting the light is a PITA
And the lid WILL close on your knuckles as you are trying to fit stuff inside. That effing hurts!

Extra brake light on the top box is a good idea, but IME it will last about 22 weeks. Good idea, absolutely pants interpretation.

I already had it on the Fz6 for a while and the previous owner had it on there most of the time since 2004. The connector on the fz's rack is a bit worn, but it still works and anyway you can replace the bike side harness for £12, which is how come I wired it up to the CBR. I think the problem there is if your box is able to wriggle on its mounting points then it wears at the connection points. Mine isn't able to anymore, as I put a couple of extra foam pads on the bottom of the box.

Also my bikes live a dry garage and there's basically no such thing as winter over here, so it's not corroding in a hurry.

The connector inside the box clamps the wires with two really tiny grub screws. Both had fallen out and were rattling inside the cover, which made the wire loose and the light intermittent. I fixed that easily enough. If I were designing it I'd skip that connector entirely and just run a wire via the hinge something like how the wires run through a corrugated hose on a car hatchback.

Still, couple of minor issues in 20 years is a bit better than 22 weeks.

A_morti wrote: Sat Oct 12, 2024 4:54 pm There must be some reason why car tail lights run LEDs part-bright using duty-cycling.

'cause, as you've probably guessed, it's really hard to ensure the correct 'partial voltages' in automotive wiring (especially when you start thinking about not-brand-new connectors etc) whereas it's quite easy with modern electronics to arrange a duty cycle.

Also means if the system faults it tends to fault into the more simple 'brake lights just work normally and the DRLS stop working' state.

Hmm.
Especially given that resistors allow through x% of voltage and the voltage can vary a fair bit on an older bike with a shunt regulator.
Fair enough, probably not a good idea.
If anything, maybe I'll look at making it light up just a few LEDs as a DRL.
Ta!

Fun (well, not that fun) little side fact popped up today at work by chance.

We obviously use PWM quite a lot to control our motors, which is the same basic tech which is used to control how bright the LED lights are in a lot of cars now.

One of my colleagues mentioned that lights are often PWMd to make them brighter, which I thought was interesting. If you flash LEDs at high frequency with short pulses of high voltage the human brain interprets them as brighter, even though the average power is lower. So they're brighter flashes, but less often. Uses less electricity, things stay cooler, still looks brighter. The DRLs and brake lights are the same average power, they're just PWMd differently.

So there you go, that's the other reason OEMs do duty cycles and not resistors.