Inside the Serfas TL-STP Taillight-
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  1. #1
    mtbr member
    Join Date
    Mar 2013

    Inside the Serfas TL-STP Taillight

    I needed a rear taillight and the Serfas TL-STP seemed like it might work reasonably well for my purposes.

    The stock rubber strap mount with it's quick release was worthless to me. I wanted to permanently mount the taillight underneath my rear rack to keep it out of harms way and to prevent any load I was carrying on the rack from blocking the taillight, as well as making it theft resistant.

    Serfas supposedly made a rack mount for the light, but they didn't show any photos of it on their website and nobody seemed to be selling it. I think it was also a quick release mount which would not have been any use to me as it would have been difficult to make it theft resistant.

    The only photo of the light on their website did not show me any of the rear detail of the light that I needed to see. Again, typical piss poor marketing by an aftermarket bike parts company. What are these people thinking? They have a website and a perfect opportunity to show they're wares, yet they only include one photo. Companies like this deserve to fail!

    Unfortunately, not a single Serfas dealer in town stocked this taillight. So much for supporting the local neighborhood bike shop. Worthless as usual.

    So I took a chance and bought one online for $24 shipping included.

    This light is bright enough to be seen a block away on a sunny day. It is also fairly visible from side angles. The light uses one bright 35 lumen light in the center with a lens to direct the light. There are 8 additional low intensity LEDs arranged in an X pattern that aid in visibility from the sides that add about 5 lumens of light. That X pattern unfortunately makes the light look somewhat like a Confederate flag.

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-lens-inside-side.jpg
    Inside the Serfas TL-STP Taillight-serfas-tl-stp-lens-inside.jpg

    It has four modes. Flashing all 9 LEDs together, continuous where only the bright center LED is on, and then two very moronic gimmicky flashy modes.

    The selling point of the additional smaller LEDs was supposed to be visibility from greater angles. They do actually improve visibility for the sides. So it doesn't make sense NOT to turn them on when the 35 lumen center LED is running continuously.

    The lens cover also doubles as a reflector and has enough area to meet most bicycle ordinances requiring reflectors.

    But it has more than a few problems. When the battery voltage drops to about 2.60 VDC (1.30 VDC per NiMH cell), the intensity of the middle 35 Lumen light is less than the 8 low intensity LEDs. That's probably less than 1/4 of the battery charge before light intensity is minimal and nothing close to it's advertised light output. It boasts a 40 hour run time with all LEDs flashing. All LEDs might be flashing, but they aren't putting out anywhere near their rated lumen output when they do. That is misleading to the point of being fraudulent. The LED driver efficiency is less than 60%.

    This light is only sufficiently bright for a short period of time before the AA cells need changing.

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-pwb.jpg

    There was black epoxy covering the mode switching circuit. This was the best schematic that I could produce.

    Inside the Serfas TL-STP Taillight-tl-stp.jpg

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-battery.jpg

    When I opened the light to install the AA cells I noticed a few problems. The lens assembly also holds the AA cells. This assembly is way too heavy to be held into the case by four plastic clips. Plus, if you need to remove the lens assembly to replace the AA cells in subzero weather, the plastic clips are likely to crack. I've already had problems with a different Serfas taillight having the lens cover pop off after hitting a bump in the road and having it's AAA cells spill out onto the ground the very first time I used the light.

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-water.jpg

    Next problem. The rubber cover of the switch was NOT sealed!!! The light isn't even close to being water resistant in real world conditions. It's a fair weather light! Now for the proof that this light is not weather resistant. From a distance of only 1 foot, I poured a gallon of water over the light. See how much water infiltrated the light past the switch dustcover.

    In a thunderstorm, large rain droplets would be traveling at a far greater velocity and could easily infiltrate the light.

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-gasket.jpg

    Add to that, there is an overcomplicated and not very compliant gasket. After a number of times opening the light to replace the batteries, the plastic clips that hold the lens assembly to the rear case are not likely to hold the lens sufficiently tight against the gasket and water infiltration is likely to occur.

    To make matters even worse, the gasket is so narrow that it only seals the rear case against the battery holder and the lens cover doesn't actually have any seal between it, the rear case or the battery holder. Water can infiltrate between the lens cover and the battery holder

    The fix is simple, but a PITA. Wrap the edges where the lens cover meets the rear case with 3M Super 88 Electrical tape. It keeps the water out and I can still actuate the switch. However, the switch is very difficult to actuate while wearing heavy gloves or mitts, even without the electrical tape covering it. The electrical tape also keeps the lens and AA cell assembly from falling off. But every time I need to replace the AA cells I need to undo the electrical tape and then rewrap the light after installation.

    BTW, I highly recommend 3M Super 88 electrical tape. Not only is it great for insulating electrical connections, it also makes a handy patch for vinyl saddles and a host of other repairs. The adhesive is better than other brands and the vinyl holds up over time, instead of getting brittle and nasty. It is also slightly thicker than 3M Super 33 Electrical tape.

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-mounted.jpg
    Inside the Serfas TL-STP Taillight-serfas-tl-stp-rear-mount-bracket.jpg

    It wasn't too terribly difficult to make a bracket to hold the light underneath the rear rack out of harm's way. To make it reasonably theft resistant, all that is necessary is to fill the screw heads with epoxy. You can always remove the epoxy with a Dremel grinder if you need to.

    Now for some quality issues.

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-cold-solder.jpg

    An obvious cold solder joint.

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-crooked-leds.jpg

    The low intensity LEDs are mounted in breadboard fashion, are crooked, and not even mounted as well as many hobbyists would do. The LEDs are NOT supported and are subject to failure caused by vibration. This is NOT a production worthy design!

    Inside the Serfas TL-STP Taillight-serfas-tl-stp-poor-joint.jpg

    Note that the solder did NOT completely wet around the LED lead. That could mean that the LED leads were dirty, the flux was old, the soldering temperature was not high enough, or the soldering iron tip had insufficient thermal mass, just to name a few possible reasons. It's not a good quality solder joint. These are tack solder joints which makes the solder joint quality even more important.

    Ultimately I want my taillight permanently mounted, wired, connected to a single battery pack for the front and rear lights, and also switchable from either the handlebars, stem, or top tube.

    I'm not sure if I'm going to try to correct the LED driver issue or replace it with another. Converting it to an 8.4 VDC system would be much easier than trying to make this 2.4 VDC system work well.

    It should be noted that this light appears to have recently been discontinued. But it gives you an idea of the crap that Serfas is willing to sell you. REI is closing them out for $16.73.

    Scott Novak
    Last edited by Scott Novak; 02-09-2018 at 12:00 PM.

  2. #2
    mtbr member
    Join Date
    May 2005
    Sad to see!

    I've had my hands on a couple of the Serfas headlights while designing GoPro adapters for them and they looked nicely designed and well built. Didn't tear them apart to give the insides a close look though.
    GoPro adapters for bike lights

  3. #3
    mtbr member
    Join Date
    Mar 2013
    BTW, for anyone interested, that is a JANDD rack on the back.

    They make a nice rack, EXCEPT that they do NOT properly prep the aluminum before they paint it and the paint starts chipping off before you've even finished the installation.

    If you want the rack to stay looking nice, you need to strip off the original paint, file any sharp edges, give it a light sanding, dip it in Alumaprep or any other phosophoric acid solution designed for aluminum, rinse it thoroughly, dry it, then prime it and paint it, or have it powdercoated.

    Scott Novak

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