R100GS Trailer Hitch Project
My wife loves to ride pillion with me and wanted to do a trip to Alaska in 2012. In order to carry our baggage, we decided we needed a trailer behind the motorbike.
I couldn't find anyone in the US who made a trailer hitch for my 1994 R100GS. So, I designed and built one myself.
Mechanical Design
The design is quite simple. There are two horizonal and two vertical supports. The main horizontal supports are 3/16" x 1" mild steel. They tie into the footpeg mounts. I replaced the footpeg bolts with slightly longer versions.
I welded the two horizontal members to a piece of 3 x 3 x 3/16" angle iron that serves as the platform to which the 1 7/8" hitch ball mounts. I later added some 1/4" rods criss-crossed ahead of the platform to increase lateral stiffness (not pictured).
The vertical supports tie into existing holes in the subframe at the top, and into the horizontal supports at the bottom. They are also 3/16" x 1". I ground some small notches into the right vertical support to hold zip-ties in place for securing the trailer wiring.
I used metric hardware throughout, so that the existing BMW tool kit could be used if there was any need to make adjustments during a trip.
I am still able to remove the rear wheel without removing the trailer hitch.
In my first version of this hitch, I had ensured plenty of clearance for the tire, exhaust, and touring cases. However, I underestimated the travel of the swingarm. The first time I put weight on the bike after installing the hitch, the interference was obvious . I later modified the right-side mount to clear the swingarm by bending a piece of 1/4" x 1" mild steel into a shape that would clear the swingarm.
Electrical Design
For the electrics, I used a Hopkins 48935 converter to convert the 5-wire (ground, left, right, tail, and brake) setup on the BMW to the 4-wire (left/brake, right/brake, tail, and ground) arrangement used on the trailer. I tied into the subframe wiring harness using Posi-tap connectors from Posi-Lock. These connectors are a pleasure to use. The Hopkins converter comes with a 48" extension, which I sheathed in some split tubing and routed back on the subframe, then down the right vertical support. It terminates at a Hopkins 48595 mounting bracket hidden under the ball mounting platform. I'm pretty proud of the clean overall exterior look.
On the trailer I swapped the stock incandescent lighting with LEDs. I replaced the bike's stock incandescent tail light with an LED unit model 1157-R19W6 from SuperbrightLEDs. I believe that using LEDs in both applications will keep the net electric current draw well within the motorcycle's capacity. Not shown in the picture is a mud flap I added.
Trailer
The trailer itself is an inexpensive Harbor Freight "Tag Along." I believe it has recently been discontinued but similar models may be available from other suppliers. The trailer is essentially a car-top carrier on a small trailer frame. To allow a little more rotational movement than the ball normally provides, I turned down the shank of the ball on a lathe (not shown). I'm happy with the ball setup. I removed extra leafs from the spring packs to more appropriately match the trailer's suspension to its typical load.
A number of reviews of this trailer suggest lengthening the tongue for added stability in motorcycle applications. I have found no reason to do so.
Overall impressions
So far, I'm happy with the design, looks, and performance of hitch. It adds about eight pounds to the motorcycle. My 2012 trip was cut short by a transmission problem, but we did the trip in 2014, which was over 2000 miles, and the trailer and hitch setup gave us absolutely no problems at all.
