• Two 12v automotive horns: one high-note and one low-note
• Four female spade connectors
• A pair of Molex locking connectors (I used 3-pin, but we're only using 2 wires. I just couldn't find a 2-pin connector)
• 2 male pins of the same size as the Molex connectors -- I used .093"
• 2 female pins that match the above -- again, I used .093"
• About 2 feet of ~18 gauge wire to use for ground (I used black)
• About 2 feet of ~18 gauge wire to use for positive (I used red)
• Wire cutter/stripper
• Soldering iron/solder
• Electrical tape
• 10mm socket
• Appropriate sockets for mounting the horns wherever you choose. To mount them on the fork brace, as I did, you'll need:
  • 12mm socket
  • Drill and 12mm drill bit (5/16" works too)

The voltage drop across the connectors is probably in the millivolts and is therefore irrelevent. Adding a relay might shorten the overall length of wiring, as the wire then goes from battery->relay->horn (with a separate wire from the handlebar button ->relay to control the relay) instead of battery->handlebar->horns...let's see how much of a difference that really makes, though.

Looking at http://www.stealth316.com/2-wire-resistance.htm -- 20 ga wire = 10 ohms/1000'. If you consider 2 feet of wire and 10 amps, which is overestimating the length savings from adding a relay, and overestimating the horn current, you get 0.02 ohms added, and 0.2V lost. Meaning if the horns get 12v with a relay, they probably get 11.8V without one. Even 1v isn't going to matter at all, since the typical range of the battery is +/- 3v. Summary: if you want your horns to be louder, charge your battery instead of buying a relay.

background.
I was washing the bike last weekend when I noticed a wire hanging down by the radiator. "Huh, wire," thought I, and started to walk away. About three feet later, my brain kicked in and I crouched back down to look. "Oh, poo," thought I again. "That's where the horn was."

Yes, somehow I managed to lose a part the size of a saucer off of my motorcycle without noticing. Hooray. So I needed a new horn. Since I commute during rush hour and travel a lot, I figured I'd upgrade to automotive horns. A new stock SVS horn costs $40 (not counting the bolts and mounting bracket, which were also long gone); a pair of new FIAMM horns from the local auto parts store cost $30. So there ya go.

pre-procedure.
I didn't know where I wanted to mount the new horns, so I figured that the easiest thing to do would be to remove the fairings. You can probably do this whole thing without removing them, but I'm a big proponent of "get shit out of the way so you can see what you're doing".

Incidentally, if you have the naked SV model, you probably don't have to move anything. Lucky stiffs.

procedure.
The first thing that I did was figure out where I wanted to mount the horn. That way I could set everything up and measure the wiring correctly.

It was unfortunate that I also lost the mounting bracket for the stock horn, since that's both a good place to mount a horn and also a tight area that requires a more-or-less custom bracket. If you've still got yours, consider mounting one of your horns there (though, honestly, I don't know if that would leave enough room for the second horn).

After some putzing around, I decided to mount one horn to each lower fork brace.

I used the little mounting tab that came with the horn -- you just unscrew the nut on the back of each horn (10mm socket) and attach it. Since the fork brace bolts are 12mm, the existing hole on the mounting tab was too small. I had to drill it out using a 5/16" drill bit.

You'll notice that nothing's hooked up at all -- I'm just making sure that the horns will work in this position. After I was happy with how they looked in place, I held the center fairing up to make sure they wouldn't interfere. Also, be sure to sit on the bike and turn the steering from right lock to left lock. The first time I tried mounting the horns, they were angled up too far, and they hit the fairings at full lock. Obviously, this is only an issue if you choose to mount the horns on the forks, as I did.

Once you know where the horns will go, it's time to do the wiring. You're going to make two Y-shaped wiring thingies; one for ground and one for positive. One prong of the Y will go to each horn, and the main branch will lead to the wiring connector. Let's call that main branch "wire 1" and the two prongs can be wires 2 and 3.

With your horns still in place, hold your wire up and measure the distance between the two horns. Add about two inches of slack to that measurement, then divide it in half. That'll be the length of wires 2 and 3. Mine ended up being about 7" each. Now measure from the existing horn connector to a midway point between the horns. Again, add some slack, since mistakes do happen. This'll be the length of wire 1 -- mine was about 9". Once you think the wires are the right length, go ahead and cut them. You should have three pieces of black wire now, and three pieces of red wire.

The horns come with male spade connectors, so the first thing we'll want to do is hook up some female spade connectors to wires 2 and 3. This is easy to do: strip the ends of wires 2 and 3 and slide them into the female connectors. Crimp the connector with a pair of pliers or a wire crimper. That's it. Even I can do it.

Here's a picture of the horns with wires 2 and 3 attached (you don't need to attach them yet; this is just for illustrative purposes).

Now it's time to make the Y out of wires 1, 2, and 3. Strip the end of all three wires, lay them down in their Y formation, and twist the stripped ends together. A small bit of solder over the wire twists will ensure a good electrical connection. Do this with both your red and black wires.

Once that's set, strip the end of wire #1. Attach a male pin by sliding it over the stripped wire, then clamping it in place with pliers or a wire crimper. The male and female pins are difficult to tell apart at a glance; make sure the tip of the pin is closed, not open (the latter is female).

Your two Y wire thingies are now done! Hooray! I wrapped my solder joint in electrical tape, btw. This is mostly because I really like playing with electrical tape. You'll notice that two of the female spade connectors are wrapped in black electrical tape; this is just because the greenish insulator things fell off while I was crimping them. Yes, I bought cheap spade connectors.

Okedoke. Now, go find your Molex locking connectors. You'll need the female one now; that's the one on the right in my picture. Insert the male pins from each Y wire thingie into the holes in the female Molex connector. They'll click in place when they're seated. If your connectors have more than two holes for pins (mine, for example, have three), it doesn't matter which two holes you use.

Now it's time to move on to the other connector. Leave your Y wires and female connector for a minute and head over to your bike. Snip the existing connector off of the horn wiring; make sure to cut down close to the connector to leave yourself as much room to work as possible. Cut back some of that protective rubber there and strip the wire ends.

Go grab the female pins that came with your Molex connector. Attach them onto the horn wiring just as you did the male pins -- slide them over the stripped wire and crimp them in place with pliers or a wire crimper.

Again, I put a dab of solder onto the pin/wires once they were in place. Snap them into the male Molex connector -- again, if your connectors have more than two holes for pins, make sure to use the same holes that you used for the female connector.

You're almost done! At this point, I tested everything by snapping the two Molex connectors together, turning on the bike, and hitting the horn. Obviously, if it doesn't sound, you'll have to troubleshoot the wiring. If you give yourself a heart attack with your new loud horn, you're all good and can go ahead and mount the horns.

This should be easy, since you already know where you're going to mount the horns. Simply bolt them down wherever you decided to put 'em in step one -- make sure to double-check the clearance again by turning the handlebars. Once you're happy, torque down any bolts you use (the fork mounting bolts there, btw, are 23 newton-meters).

Your last step is just to zip tie the loose wiring out of the way. There's enough wiring and crap in that section of the motorcycle that you won't have trouble finding something to zip tie to.

Ta da! All finished! Now you have a nice new pair of horns which will hopefully deafen the next asshat who tries to cut you off on the freeway. Here's a top view of the horns:

Peter's shirt matched the SVS so well today that I asked him to demo the new horn. Click here to hear it..