another brake light/undertail question

[btwice]

here is a picture of my undertail.



i was wondering since the leds are out, if i could replace them with these if i got the right size.
Optronics MC57RBP - Marker/Clearance Light | O'Reilly Auto Parts

just looking for a little input

[earz]

I think they are just glued in place on the undertail, do if you find one the right size, I'm sure you could just replace them. I'd do them as a set though, even if one side still works properly.

Every undertail that I've seen like that has junk ass tail lights.

[mike93hatch]

brandon, no!

that trailer light is just a marker light. so it would not have brake light.. and you also wont have any turn signals.

just pay the $40 for new ones from the manufactuer of the undertail. then you will have a reciept as well and their lights are lifetime warrenty.

[earz]

Originally Posted by mike93hatch

brandon, no!

that trailer light is just a marker light. so it would not have brake light.. and you also wont have any turn signals.

just pay the $40 for new ones from the manufactuer of the undertail. then you will have a reciept as well and their lights are lifetime warrenty.

Where there's a will, there's a way. Didn't feel like retyping it all, but here's how to make a single filament bulb (or LED) to act like a dual filament. I posted this on a different forum.

I've seen it done with LED's but not with an incandescent. It will probably work the same though. I didn't see the article in The Horse, but this is how I've always done it.

You will have to put a resistor inline with the taillight wire to cut down the voltage going to the light. You'll have to try a couple different resistor values to get the brightness that you're looking for.

Route both the taillight wire (with resistor) and the brake light wire to the positive wire of the light (single filament).

The taillight wire voltage will be reduced by the resistor from 12V to say 9V, resulting in a slighly dimmed light. Once you hit the brakes, the full 12V goes to the bulb and it gets brighter

they sell LED replacement bulbs for most light sockets. Of course the LED will require a different resistance value than an incandescent bulb to achieve the same results.

Here's a diagram to help illustrate this (sorry it's sideways)




K, I'm bored at work and figured I'd do some math. The incandescent bulb will require much less resistance to dim it than an LED will - due to the amount of resistance already present in the circuit (the bulb).*

I figured a normal 1156 single filament bulb. The specs needed were found here :
Voltage = 12.8V
Amps = 2.1A
Filament resistance = 6.1ohm
Watts = 26.9W
lumens = 402

We'll start with the brake light wire. The voltage will be battery voltage so we'll put it at 12.8v. The bulb has a 6.1ohm resistance so using ohm's law, the wattage of the bulb is 26.9.

Now we want to figure how much resistance is required to get 75% output of the bulb.

First figure lumens per watt (LPW)
Lumens/watts = LPW
402/26.9= 14.9 LPW

Now take 75% of 402 = 301.5 lumens
This is our goal for light output from the tail light wire.*

14.9 LPW x ______ watts = 301.5 lumens

A little algebra and we end up with*
301.5/14.9 = 20.23 watts

Going back to Ohms law
Resistance = voltage squared/watts
12.8 volts squared = 163.84
163.84/20.23 watts = 8.09 ohm

Since our bulb has 6.1 ohms of resistance and the resistor in our tail light wire will be in series - you subtract the known resistance from the calculated resistance to determine the needed resistance.*
8.09-6.1 = 1.99 ohms

You will need a 2 ohm resistor in the tail light wire to dim the bulb 25%. You can use this same process to calculate resistors needed for different bulbs or different % output. Just get the required info off the package, or google the bulb type. **

Here's the original post(s)
Making Tail light running light into Brake Light - Page 2 - American Bikes & Tech


Of course, it's not the best course of action in this circumstance, but it is an option.