Installing Air Horns on the BMW R1100RT

By Gaylord R McCurdy 

Trying to install another set of louder of horns on my RT has been arduous.
The first attempt to install snail-type Fiamms (high and low note) presented issues with the low amperage on-board horn relay and interference problems with the brake line (stock location). Moving on to the flatter disk-type Fiamms presented a new problem: striking the Tele-lever on serious bumps and still current problems with the stock relay.

Some on Cary's RT BBS site had reported luck with air horns. However, the people having the luck had room near the front of the glove box area for one horn and the other horn on the opposite side. On RTL models (i.e. the ones with the radio installed) the amount of wires coming out of the center front of the box leave no room around that area to install a horn. Add to that area the bike's air intake horn and  nothing is left.

My only option was to install both 7" and 9" horns on the left side. With a lot of fitment trials, here is what I ended up doing.

First the parts list for the brackets:

This is the bracket kit hardware that I used with the the Pep Boys 9" and 7" air horn kit with compressor and 30 amp included relay:

1. 5/16" N.C. x 3/4" bolts and self-locking nuts (need 2 each)
2. 5/16" internal-toothed lock washers (need 2)
3. #8-32 x 1 1/2" machine screw, lock washer, and nut to mount the horn's relay to the back of Bracket D described below.
4. A 3/4" piece of tubing (or washers) to make a standoff to hold the relay off Bracket D for clearance of the compressor bolt and nut. Uses the above 8-32 screw.
5. 9/16" vinyl covered metal conduit harness to clamp to .6" diameter metal front tubular stay near right side of oil cooler. (Need 2 from Ace Hardware or electrical supply shop).
6. 1/8" x 1 1/4" x 2' aluminum flat stock from hardware store used for the following brackets.

The brackets that need to be made:

A. Cut one piece 2 7/8" long. Drill 5/16" hole at one end and 1/4" hole at other. Distance is 1 7/8" apart (centerline). Used to hold rear 9" long trumpet. 5/16" hole bolts to one of the harness clamps above, which is installed, between the metal support for the oil cooler hose on the right stay. Mount it with the bolt cinching tabs pointing up and on the outside of the bike. Mount it close to the gas tank "between" the oil cooler hose support ties welded to the stay.

B. Cut another bracket 3 7/8" long. Drill one end with a 1/4" hole and the other a 5/16" hole. Distance between holes is 2 3/4". Centerline both.

C. Cut another piece of aluminum flat 3 1/2" long. Bend it 90 degrees (vise and hammer) about 1 3/4" from one end. Drill a 5/16" hole 1" from the outside of the bend in the short section of the L. Drill a 1/4" hole 1 3/4" from the outside of the bend in the longer section of the L. Centerline on both. You may want to cut the ends just to finish the piece once the horn's location is determined (not necessary).

D. Cut another piece 3 7/8" long. Centerline the entire length. From the end of the piece drill holes as such: drill one 5/16" hole (for mounting to the bolt from which the old horn was removed from) 1/2" from the end. Drill another 5/16" hole (compressor mounting bolt) 1 5/8" from the end. Drill the last 3/16" smaller hole (for the relay) 3 1/4" from the end.

Here are the brackets that were described previously.

Installation of the horn mounting brackets:
Assemble the brackets as follows: Bracket A above goes on the outside of the 9/16" vinyl covered metal clamp wrapped around the RT's stay. Attach with a 5/16" bolt, flat washers, and locking nut. The angle is tilted to that of the front slope of the gas tank (you need to remove the right fairing) as viewed from the right side (i.e. throttle side) of the bike.
From the front of the bike the clamp is secured when the Bracket A is angled with an imaginary line from the center of the front axle.
Bracket B (1/4" hole) goes from the top of Bracket A forward to another 9/16" vinyl covered clamp to the stay (mounted also tabs up and on the outside of the bike's stay).
On the outside of Bracket B and the clamp above, install the 90 degree Bracket C facing rearward and to the outside of the bike. The angle of the bend is also parallel to the front of the gas tank. Secure with the 5/16" bolt, external-toothed lock washer, flat washer and self-locking nut.
At each of the 1/4" holes goes the air horn's flat square bolt and nut. The longer trumpet is towards the rear; the shorter to the front. Both are angled forward and downward and "also" slightly tilted to the inside of the bike towards the forks. The rear trumpet will clear the fork by about 1/2" when the handlebars are rotated. Tighten the horns up. This bracket assembly is best viewed as a triangle from the side to help rigidity.

When the fairing is installed, you should have space all around: gas tank, forks, and fairing.

9/16" vinyl covered mounting clamps location shown.

Bracket installation and location.

Horns mounted and hose routing shown.

Compressor mounting instructions:
You will need to file Bracket D on the left side (looking at the bike from the front) to clear the bent stop on the old horn's mounting assembly. Once fitted, remove and install the compressor on the middle hole of the bracket (facing forward and terminals downward) with the included compressor mounting bolt, flat washer, lock washer and nut. Mount the relay on the remaining 3/16" hole with a #8 screw, lock washer, and nut.

Compressor wiring instructions:
The wiring is done as follows: Remove battery and install a Ford 14 gauge fusible link (Pep Boys) to the positive terminal of the battery. It has a loop on it that will fit the battery bolt. The other end will need to be cut off and install a covered bullet connector (female). Make a 3 1/2' long 14-gauge wire up with a bullet connector (male) and an insulated female spade connector up on the other. Make another 3' 14 gauge wire to go from the negative terminal of the battery (loop connector) to a female spade connector at the other end.

Run both pair of wires from the battery through a 3/8" plastic wire loom (Pep Boys) along the outer left side of the tank. Not too hard to do by shoving the plastic wire loom in from the front over the air intake horn.
Hook up the battery connectors (the other ends are insulated connectors I hope). You may want to use some dielectric grease to insulate and protect the connections from moisture.

Run the positive wire up to the relay's center-most terminal near the screw-attaching lug. The outer two terminals on the sides (from the attaching screw lug) go to the existing horn spade connectors (i.e. the relay coil). The far terminal form the attaching lug needs a 5" 14 gauge wire with an insulated spade connector at each end. One goes from the remaining relay terminal to the positive terminal on the bottom of the compressor. The negative lead form the battery goes to the negative terminal on the compressor's bottom.

Hooking up the horns:
Hook up the tubing from the horns using the enclosed Y-connector and vinyl and run it behind the forks and secure it with a nylon tie-wrap. I also used some tie-wrap to cinch the hose to the compressor and horn ports just to make certain they don't slip off.

Cut paint cover cap and compressor.  Relay on 1" standoff on back of the compressor.  It was sealed with silicon and mounted tabs up to connect to stock BMW horn wiring (outer two terminals on side away from mounting tab).  The tabs nearest and farthest from the mounting tabs are the positive wire in from the battery and the other tab goes out to the + terminal on the compressor.  The negative terminal on the compressor goes back to the battery.

Cap installed via Velcro to top of compressor.  This will enable you to get to the mounting nut above the compressor that attaches to the bolt on the old BMW horn mounting strap.

Compressor shown mounted in place of old BMW horn.

Other notes:
I also painted on some liquid electrical tape (similar to silicon) on the compressor (except for the compressor's air intake hole!). I then painted it and all the brackets flat black. The horns themselves are the black units. I made a rain cover to keep water out of the compressor's intake hole from an old paint can top (flat black). I cut the center section, and also some clearance needed around the mounting tab and air output nozzle areas, out with a Dremel tool and used double-stick Velcro (you need it to access the mounting bolt on the bike) to hold it on the top of the compressor.

Here are the dual trumpets installed under the fairing on the right side of the bike.  There is very little room, about the width of your small finger, around them and the fairing.  It takes time to do a good job, but the "BLAST" is worth it!

Now go ride and have a blast!