With the extended swingarm and KTM shock there's no way the OE subframe will fit, and the subframe is something I thought I could replace with a lighter and simpler design.
There were a lot of things I needed to consider for the subframe design. It needs to support the seat, rear fender, tail light, license plate, luggage racks, muffler, etc., in such a way that they all fit and work together, plus it also needs to clear the tire when the suspension bottoms out and somehow needs to connect to the main frame and be strong enough to hold the rider and luggage. With just blank nothingness at the back of the bike the task seemed a little daunting at first. Here's what I was faced with:
To get an idea of what would work I made this mock-up with some aluminum flats I happened to have. I wanted to have as much as possible just straight tube so it would be easy to build and so it would have good load bearing for the weight.
Once I had an idea of what would work I made up this subframe drawing
to work with. The GS subframe is made of 18mm thick wall tube. My idea was to make a lighter weight unit of 3/4" (19mm) thin wall 1018 steel that may not be as strong, but had a simple design that could be easily replaced when damaged or updated with a chromoly
or aluminum version without much effort. The design has only two bent tubes, the right front support needed to bend out to clear the shock reservoir, and the rear connecting tube needed to loop up and around the fender.
To prepare the tube ends for welding I used a 3/4" end mill on a vertical milling machine to plunge into the tubes at the proper angels. I didn't have a 3/4" hole saw but the end mill worked OK. The other cuts I could do in a standard milling vice but for the shallow cut on the lower front supports I needed to use this angle plate setup. It wasn't a very solid setup so I used a 4 fluted mill and went in real slow. I've since bought a 3/4" ball end mill
that will allow me to do shallow angle cuts in a vise.
I have access to a Hossfeld tube bender
. It only has press bending
dies, but I thought that it would be enough to do the front support tube. I'll make up a set of 1/2" rotary draw
dies to do the rear connecting loop and a top luggage rack. This photo is just a setup I did later to show the bender. When I did the bend I filled the tube with sand and sealed the ends with PVC electrical tape. The sand supports the tube from the inside and gives a smother bend.
With the tube ends done and the front support bent I did a trial fitting to check the tire clearance and figure out what kind of mounts were needed on the frame.
Here are the frame mounts I added. To get a shape and location that would work with my subframe I started with some templates from thick paper card stock then when I was satisfied with them I made up a set from 1/8" steel plate.
To add some strength at the mount points I put on a set of lugs made from 7/8" thin wall tube slid over the 3/4". The welded subframe would be too big to fit into the sandblaster I have access to, so I sand blasted the individual tubes in preparation for painting.
Here's a detail of a lug. I slid the outer tube over the main tube then crimped the end in an arbor press. I used a large open end wrench to form a rounded crimp edge which I thought would give a stiffer side-to-side crimp. I couldn't get enough force out of the press to completely flatten the tube and lug so I finished the crimp with a hammer and anvil. To finish the lugs I ground the crimped area to have a nice profile and have some clearance for the upper mounts, then welded a bead along the tube ends. In retrospect, I would have liked to put some relief on the ends of the lug to reduce the stress at that junction, similar to the way old lugged bicycle frames were done.
With the tubes prepared I did another trial fitting to check the tire, fender, and shock clearance. When I setup the shock I put it as far inboard as possible so that there would be enough clearance to allow a straight lower subframe support as seen here.
I used this carpenter's level
to get the top tubes parallel to the main frame just before tack welding.
Here are the sides welded up. This photo shows the simplicity of the design.
To add some strength to the joining of the support tubes I made up this set of gussets.
I used this hole punch to punch the hole and cut out the reliefs.
And here's the subframe all welded up. I added a small plate on the middle cross tube to mount the fender.
And the subframe with a UFO enduro fender. I still haven't decided on a rear fender yet...
Here's a detail of the fender mount.
And a shot with my old seat pan, which unfortunately, will no longer work. I'll need to make a new one.
To finish the subframe I still need to make up the rear fender mount loop. I also plan to make a small detachable top luggage rack and two light weight detachable side luggage racks. All of these though depend on the rotary dies which need to be made.