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So now that I built a 12 volt (more like 11v) battery for my bike I would like to accessorize it with some 12v lights to get a unique look. I'm looking at motorcycle tail lights, but they all have two parallel bolts coming out the back 82mm apart. The rails on a bike seat seem to be about 50mm apart.

Is there something I get at the hardware store to jerry-rig a solution?

Source: https://www.ebay.com/itm/12V-Red-LED-Universal-Motorcycle-ATV-Dirt-Bike-Brake-Stop-Running-Tail-Light/133104570821

Back of the bike seat: (two horizontal rails 50mm apart with an angle at the end). Maybe I could zip tie in a ubolt pipe clamp somehow?

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  • Could you add a photo of the rear of your saddle? I'm thinking two P clips, or a backing plate, or mount it to your seat post rotated 90 degrees. This is achievable.
    – Criggie ♦
    Aug 22, 2019 at 9:50
  • @Criggie I'd prefer to attach it to the bike seat rails itself (not the post) because I'm trying to make sure it clears the backpack in the back basket.
    – SurpriseDog
    Aug 22, 2019 at 14:19
  • Just go to a hardware store or home improvement store and browse through the hardware. What one store has another may not, so it's not real effective to call out a specific piece of hardware here. You just need some sort of strappy thing you can adapt, cutting it to length and drilling the required holes.
    – Daniel R Hicks
    Sep 1, 2019 at 1:21

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I would just use a standard electrical mounting 4" crossbar:

electrical box mounting 4" crossbar

You can run the wire through the hole in the middle, and the slots on either side should accommodate both the posts from the light and whatever you use to attach it to the rails. (P clips, as Criggie suggested in a comment, would be the simplest approach for that.)

(You can buy them individually, but if you're like me you have a bin full of random electrical crap with a good dozen of them...)

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Similar lights are often fitted to pannier racks, especially on e-bikes. I've had a plastic part with the right holes that would be easy to modify, but it came with a topeak rack.

It wouldn't be hard to fabricate something. The easiest way is probably to start with a saddle mount for a water bottle cage and make an aluminum plate. Some sellers of offcut sheet metal on ebay will cut to size, then you'd only need to drill the holes, but you can cut it with a hacksaw. I'd probably use 3mm plate.

I assume you've been sensible about light levels and colours and won't dazzle or confuse anyone.

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    @Carel here (UK) the regulations effectively set a minimum. Lights that don't comply with the relevant standard may be sold and fitted, but after dark you're meant to have 1 front and 1 rear that do meet the standard, which is pathetic (not enough to see by), and took a long time to catch up with LEDs. The OP is in the US. Perhaps I need to add a disclaimer: "This answer may not apply to Germany" (and a few other countries that don't allow non-certified lights)
    – Chris H
    Aug 22, 2019 at 8:29
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    @Carel maximum output power is covered by my last paragraph. Please cite "voltage" requirements; I've never come across that despite looking into specifications quite a bit recently.
    – Chris H
    Aug 22, 2019 at 8:33
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    We have a wonderfully vague and useless line of "bicycle lights must not dazzle or distract other road users" as an upper limit.
    – Criggie ♦
    Aug 22, 2019 at 9:51
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    Being a motorbike light it will have running and brake lights, the brake side will be probably too bright, so the OP maybe better just wiring these as running lights. A suggestion of a switch somewhere mounted near the bar grips will also be advisable, no point wasting energy / battery when there's absolutely no need to have lights on in daytime
    – Dan K
    Aug 22, 2019 at 10:14
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    @Criggie vague and useless maybe, but that at least covers misalignment which many don't
    – Chris H
    Aug 22, 2019 at 10:22
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I did this by bolting the light to the plastic part of my carrier rack which already had one hole for a reflector. I just drilled two holes and pushed the mounting bolts through and added some spare washers to protect the plastic. I zip tied the wires to the under of the carrier parcel rack and it works grat for me. I have a 12V battery in the saddlebag and a small switch on the wire. I do not use the brake lights just the running lights when riding at night and in fog.

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