BMW R80ST Modifications

Written by Rich Morin.

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Precis:  modifications I've made to my BMW R80ST

See Also:  A Mirror Bagatelle

I bought my BMW R80ST (a street/trail “airhead”) back in 1984. It’s a great, nimble bike, ideal for the “twisties” I like to ride around the west side of the San Francisco Bay area. Over the years, I’ve made a number of additions and modifications, for safety and convenience. This note presents the changes, including my rationale and approach. (I needed to document things anyway; why not make it public?)

Physical Modifications

Most of the physical modifications are too minor to warrant much discussion. For example:


I’ve been experimenting with various ways of mounting mirrors. I think I may be converging on something I’ll like, but the details are unlikely to be interesting to most readers. That said, I’ve written them up in A Mirror Bagatelle.


I’m vaguely considering adding pneumatic suspension, to even out some bumps on the twisties I ride. Specifically, an economical air shock and something like this Motorcycle Air Ride Suspension Modification Kit.

Tail Box, etc.

I added a tail box to the rack, then mounted eight J-bolts on the lid for use with bungee cords, etc. Held down by a rope to some of these mount points, there is a canvas satchel which has compartments for various commonly needed objects (e.g., scarf, water bottle). So, the satchel provides low security, “soft” storage and the tail box provides (slightly) higher security, “hard” storage.

The threaded portions of the J-bolts extend well into the interior of the lid. So, I took advantage of this by using them as mount points for an elastic storage web. This does a nice job of retaining light (e.g., medical) supplies, as discussed in the Crash Scene Field Reference. The lower section of the box is thus freed for heavy and/or lumpy items.

Web Details

The web is strung by a single piece of 3/32” shock cord, passing through metal loops at each of the eight (two back; six side) mount points, as well as a bolt at the center of the forward (hinge) wall. This lets me ease the tension on overtight links by borrowing slack from their neighbors.

For the mathematically inclined, I’ll note that the cord’s path forms a 9-node Eulerian circuit. That is, all of the nodes are fully connected to each other, with no duplicate links. Being lazy, I let a Python script work out the path. Conveniently, I was able to use this site as my starting point.

Wiring Overview

Most of the significant changes to the bike have taken place in this area. In general, they have been motivated by adding accessories or increasing the bike’s visibility. However, I also replaced the fuse block, to ease maintenance, increase robustness, and accommodate new wiring.

Fuse Block

I’ve been adding assorted lighting and such to the R80ST. In the process, I became concerned with the original fuse block and wiring. There were several issues I wanted to resolve. For example:

So, I bought and installed a 10-circuit Waterproof Blade Fuse Box. It’s quite a bit larger than the original block (5 x 3.6 x 2 inches), so I:

In the process, I replaced each of the original pair of fuses with a pair of new fuses. I now have four, LED-indicated fuses covering the original two circuits. Two more fuses supply the USB car charger and the power circuits for the add-on horn and signal lamps. At this point, only seven of the ten fuse locations are in use, and some of these could be shared, in a pinch. So, there is room for growth.

Fuse Layout

The fuse block is organized as two columns, numbered 1-5 and 6-10. Fuses 1-3 and 6-8 are driven, respectively, by the bike’s grey and green wires. The other fuses are wired directly to the battery’s positive terminal. Here is a rundown on the fuse block’s wiring:

FuseAmpsWiringAssociated circuit(s)
17.5#18 to green/blackBMW tail lights
27.5#18 to green/blackBMW marker (aka parking) light, etc.
37.5#18power for add-on (LED) signals
47.5#18Jelly Comb USB car charger
530.0#10 yellowpower for Denali SoundBomb air horn
67.5#18 to grey/blackBMW rear brake
77.5#18 to grey/blackBMW front brake, horn, turn signals

All of the “#18” wiring is done with black zip cord. The use of numbered wire labels and the smooth/ridged distinction on the insulation helps in keeping track of which wire is which.

If all six USB ports on the Jelly Comb were running at maximum current, it could be putting out 65 Watts and drawing 13 Amps. However, I don’t expect this situation to arise in practice, so a 7.5 Amp fuse should be more than adequate.

Wiring Bay

The R80ST has a wiring bay, located under the gas tank. This area contains most of the bike’s electrical infrastructure. Conveniently, it has an unused, rectangular, 2” by 3” hole. Hearing no objection, I used this as a place to install:

Specific Devices

Brake and Tail Lights

I replaced the stock brake/tail light bulb with a LED version. I then added a (4 x 4 inch) panel containing a 3x3 grid of LEDs. The center column serves as a tail light; the outer two serve as brake lights.


The halogen bulb that BMW used is actually fairly bright, as such things go. And, although there are LED headlight bulbs that fit the fixture, the really powerful ones have a huge cooling apparatus on the rear. Aside from the fact that this doesn’t fit, it implies that there is a lot of heat that has to be disposed of. Trying this inside the sealed BMW headlight shell wouldn’t work well.

Since I try not to ride my motorcycle at night or in bad weather, my motivation for getting a brighter headlight comes down to visibility. So, I decided to go in a completely different direction. This headlight modulator causes the high beam to flash a few times each second, greatly increasing my visibility.

Note: The other day, I was cruising down a rural, two-lane highway. As usual, I had my high beam and running lights on. Imagine my surprise when some idiot decided to make a U-turn from a cross street, in the middle of the highway I was on! Although I was able to maneuver around the idiot, the experience told me that I still wasn’t visible enough.


The stock horn on the bike is functional, but kind of meh. Given the experience described above, I decided to get something LOUD. So, I installed a DENALI SoundBomb Compact Dual-Tone Motorcycle Air Horn. Every review I’ve found indicates that this is loud and robust, if installed as directed.

The horn has a built-in air compressor which draws far more current than the stock horn requires. The vendor supplies a 30 A relay, which solves part of the problem; I’m solving the rest by giving the horn its own 30 A fuse. Another issue with air horns is that they aren’t instaneous: the relay and compressor need to kick in and the horn itself needs to come up to pressure.

If I’m trying to get some idiot in my path to stop moving, milliseconds could matter. So, I wired the air horn’s relay in parallel with the stock horn. That way, the stock horn will still do its best while the air horn is coming up to speed. This may also give me a way to give folks a friendly toodle without blowing them off the road.

Turn signals

I’ve done a lot of work in this area. First, I replaced the stock turn signal and indicator bulbs with LEDs. In order to make the indicator LEDs fit, I had to trim off some plastic. I also had to replace the blinker unit, to accommodate low-current bulbs.

I then added four 4.15” diameter (truck-style) turn signals. Each of these has 21 substantial LEDs, facing both front and rear. They have two light-level circuits: running and signal. I use the latter for both purposes, providing either continuous or intermittent light, as explained below. A simple switch and a pair of diodes also adds emergency flasher capability.

The added signals are connected to a (right/left) pair of SPDT relays, using the normally closed (NC) contacts to invert the sense of the flash. This causes the added signals’ LEDs to be on full, by default, giving me very visible running lights. They also blink off (rather than on) when the flasher relay closes, introducing both contrast and a slight appearance of motion. In short, these lights are extremely visible.

Relay Wiring

I wired up the signal relays using #18 zip cord, relying on the ridged/smooth distinction to keep track of the wires. There are two pieces of zip cord, handling the input power and the two (L/R) sets of turn signals, respectively. The relay harness wires are color coded and numbered. Here are the connections, descriptions, etc:

power / ridged - fuse #3black30armature
ground baryellow86relay coil
no connectionred87NO contact
signal / ridged - add-onblue87aNC contact
signal / smooth - stockwhite85relay coil

Electrical Accessories

There are all sorts of handy electrical accessories for motorcycles. Conveniently, there are also two popular, low voltage power interfaces that make sense to have on a bike: SAE and USB.

Note: Some years ago, I purchased and installed an ISO 4165:2001 (aka BMW Accessor) socket. I never ended up using this for anything and it has now been removed.

SAE Connector

The SAE connector is a small, hermaphroditic, and well designed lump of metal and neoprene rubber. It can provide (or accept) several Amps of 12 Volt DC power. A wide variety of accessories support this standard (e.g., air compressors, battery chargers, emergency lamps).

The bike has a fused (15 A) SAE connector which is wired directly to the battery. I also made up a couple of “power distribution hubs” by wiring together a few pigtailed connectors.

USB Type-A Socket

A USB Type-A socket can provide a small (but negotiable) amount of current at 5 VDC, driving an immense number of devices (e.g., cell phones, tire pressure gauges). (USB was dragged into popularity by Apple; Microsoft had been tacitly ignoring it. However, it has grown far past its origins.)

The bike has a Jelly Comb 6-port car charger, mounted on the left windshield support. Originally supplied with a cigarette-lighter adaptor, it is now wired directly into fuse #4 in the fuse block.

Minor Materials

The notes above call out only the major components used in these changes. Here is a list of some minor but important (i.e., supporting) materials.