Do bicycles have ground wires and if so where are they I am trying to put a horn on mines and it’s kinda hard....it’s a bicycle any help would be very appreciated thanks.

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    Most bicycles don't have any kind of electronics – ojs Aug 26 at 7:45
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    Depends on the bicycle. E.g. back in the days when most bikes had steel frames, it wasn't uncommon for 'city' bikes i.e. non-race, non-mtb, to use the frame itself as ground and one of the dynamo's poles would be wired to the frame directly so the front and rear lights also connected to the frame and only one other wire was needed. These days things are usually different though. – stijn Aug 26 at 9:40
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    Even in those days, there were no ground connectors, the dynamo clamp would just have a spike that punctures the paint and then you'd hope that the connection is good enough. – ojs Aug 26 at 9:59
  • @ojs usually, but not always. I've had at least one bike with no such direct clamp but manually wired to a seperate screw on the frame – stijn Aug 26 at 10:14
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    A metal frame is usable as a common ground. Even a metal fork is ground but only if the headset doesn't have plastic components that cut the galvanic link. (bearing races embedded in elastomers) – Carel Aug 27 at 10:50

Just run two wires, a positive and a negative lead. Both leads in the same casing will make for a more robust wire, less susceptible to vibration damage.

If you expect to run one of the leads in the metal of your frame, first make sure there is conductivity between the points you expect to run the wire. Carbon fibre is not conductive. Steel is great for this, and aluminium conducts but tends to have galvanic reactions at points it touches other metals.

Use a multimeter in Beep mode and then measure the resistance in Ohms between the two points. Then compare that reading with the resistance of a length of wire that you'll be using for the positive run - I bet the frame will have a lower resistance, but when you cross things like the fork bearings it could add a lot, or it could be fine. Only measurement will tell. Do note that Paint is generally non-conductive so you'll need to scratch through that to get a measurement.

To attach wires to a frame, the common method was a hose clamp with the bare wire under it. This is ugly, catches and holds muck and water, and means you've got a bare area of steel underneath.

You can also crimp a small ring or fork connector onto the end of your wire, and then trap it under a convenient bolt like a carrier/mudguard stay, or under the light or horn fitting itself.

The last resort is to drill a small hole into the frame, and then tap it for a bolt and trap your cable that way. Good side of this is you can do this in a convenient place. Downside, drilling holes into a frame.

Personally I use an airhorn that is powered by a small canister that sits in a water bottle cage. Works perfectly and is audible from inside most modern vehicles including trucks.

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    I strongly advise against drilling holes into any kind of frame. – Carel Aug 26 at 16:06
  • @Carel Absolutely needs to be considered on a case by case basis. I drilled bottle cage mounts onto an early 80s steel frame, which worked perfectly. Fully documented in bicycles.stackexchange.com/questions/52207 but drilling holes always has the potential to damage stuff. – Criggie Aug 26 at 19:37

Bicycles don't have any wiring installed, which is why it's so important to add it yourself! Doing this opens up a whole new world of 12V motorcycle accessories which will give your bike that custom look.

The frame is covered in paint which is non-conductive, so don't bother using that as a ground (I checked). Instead just run two thin wires for the 12v lights in the front and two more for a horn switch. The best way to do this is using black zip ties or velcro straps. Don't use electrical tape, it turns into a sticky disaster. If your bike has internal cabling like mine does, there may be an additional hole available to run the cables, however doing so is difficult without special tools.

I wanted all of the wires self contained in a nice black sheath, so I'm using 24 AWG stranded twisted pair cable (commonly used for ethernet patch cables) to carry the current and then using a 15V Lithium battery to supply the voltage. For my low wattage lights, it works fine, but be careful how much power you drive over the wires without upsizing. Calculator here. The horn requires about 4 amps or so for mine, but it's a momentary load meaning the wiring works well enough for brief usage. At 4 amps, the 24 AWG wire acts like the perfectly sized resistor stepping down the 15V to 13V for the horn. It's important to test the amperage (current) yourself, as the listed amperage on the retailer sites seems to be a maximum demand, not the continuous draw. You will need to calculate the amps, voltage draw and wiring necessary for your custom job of course.

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    I also made a few minor tweaks to your wording, like adding "stranded" to the cable spec and minor copyediting things. Hopefully haven't changed your meaning or intent significantly, if so feel free to edit further. There may also be a whole new question here about "how to add electrical wires to an existing frame" – Criggie Aug 31 at 8:37
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    @Criggie Thanks, I'm happy to answer any questions. I've been having more fun with this hobby project and so far I've gotten nothing but compliments. Turns out everyone loves the idea of a bike with a bull horn on it (even the guy getting honked at said "nice horn") – SurpriseDog Aug 31 at 14:03

Don't try to use your bike's frame as ground. That has been done for years on bikes some 20-30 years ago, and it was a PITA. I remember modifying a dynamo attachment to be able to connect a "ground" wire to the dynamo, simply because the ground connection through the frame is botchy at best. (The electrical current needs to flow through two clamps and the steering bearings, all of which yield variable conductivity. The clamps may be irritated by vibrations, the bearings are hopefully well-lubricated, and grease is not the best electrical conductor. The connections of a real cable are usually much more stable.)

Modern bike electric parts all use two-lead cables, and are usually insulated against the frame. As the power from the dynamo is alternating current, and as dynamo and lights are insulated from the frame, the two leads are really equal. It doesn't really matter which port of the dynamo you connect to which port of the lights. As long as you do not somehow connect both ports of the dynamo to each other, creating a short circuit, or connect both ports of a light/appliance to the same port of the dynamo, you should be fine.

  • An explanation for the downvote would be nice... – cmaster Sep 2 at 21:45

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