R100RS – Headlight upgrade

I noticed the other day that the headlight lens on the R100RS was smoky on the inside. It looks like maybe a bulb blew up at some point. I’m a firm believer that you can’t have too much light on a motorcycle at night, so I need to clean it.

The headlight assembly does come apart – the lens separates from the reflector – so I decided to take it apart and clean it.

There is a rubber gasket between the lens and the reflector and it more or less glued the two together. I tried applying a little heat, but that didn’t really seem to do much. I ended up running a knife around the edge to cut the gasket. That worked great and after the lens came off the gasket just peeled off the two parts.


I don’t know if you can tell in the photo, but the lens is a little grey and the reflector has some smoky spots on it too. You can also see the remains of the gasket.

Some window cleaner on the inside of the lens and it’s sparkly clean. I used a cloth and some alcohol to clean up and polish the reflector.


Some interesting reflection there…

After I got done re-assembling the headlight, I moved on to the other project to increase the light output – adding headlight relays.

Most motorcycles have pretty light gauge wire running to the headlight bulb, and the electricity runs through several connectors and the handle bar switch. All these things cause a voltage drop – sometimes a significant one. The solution is to run heavy gauge wire from the battery to the headlight directly and use the old headlight wires and a pair of relays to control the high and low beams.

I’ve done this in the past and it can make a pretty large difference. I was actually surprised to see that I had not done it to the R100RS in the past, since I thought I remembered doing it, but apparently not.

So, step one, figure out where to mount the relays.


This is the headlight bucket before I’ve done any work. You can see there are two relays in there already (I’m not sure what they are actually for…) There is not a lot of room inside for two more relays so I decided to just mount them to the fairing supports on the right and left. They will be sheltered from the weather and not very visible, so I’m good with it.

I also discovered that the headlight shell has several extra holes in it with rubber plugs in them, so I picked two that were in good spots, removed the plugs and inserted some grommets.

Now that I had the relay locations mapped out and the way I was going to get the wires in and out of the headlight shell, I could do most of the work on the bench instead of on the bike.


Wiring. The green and yellow wires on the left go to the old headlight wiring for the high and low beam circuit. The red/brown/green wires in the center go to the new ceramic headlight connector and the fat red wire heading off to the right is a two conductor wire heading back to the battery. Not shown is the fuse holder on the battery end.

I tidied it all up with some cable wrap and installed it.


The relays are cable tied to the faring supports, the green and yellow wires (in the blue heat shrink – I need to get some black heat shrink) go in the side of the headlight bucket and are connected to the old headlight wires, and the red/brown/green wires go into the bottom of the bucket and are connected to the new bulb connector.

I snaked the power feed back to the battery, added some crimped on rings and a fuse holder and tested it out.

It worked the first time! Woo!

So now I should have the maximum available light output.

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