Air Horns on the BMW R1100RT
to install another set of louder of horns on my RT has been
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
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
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
6. 1/8" x 1 1/4" x 2' aluminum flat stock from hardware
store used for the following brackets.
that need to be made:
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
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.
the brackets that were described previously.
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
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.
fairing is installed, you should have space all around: gas
tank, forks, and fairing.
vinyl covered mounting clamps location shown.
installation and location.
mounted and hose routing shown.
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,
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.
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.
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.
shown mounted in place of old BMW horn.
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.
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!
ride and have a blast!