K1600GT-L Top Case Wobble Fix
One of the most annoying features of my 2012 BMW K1600GT-L was how the top case shook and wobbled going down the road. I was shocked to see how much the top case moved upon first seeing my shadow on a freeway center divider barrier. My wife hated the insecure feeling and did not like to lean back on it for fear it would break away. When we road hard and she was leaning against me she said the box would actually hit her in the back occasionally. So, after putting up with thiscondition for 14 monthsI decided fix it.
The problem is actually two separate problems. One is the fact that the top case foot print on the mounting rack is very narrow and the second is that the entire support rack is made of plastic parts. I realize BMW was trying to keep the weight down, but adding 3 pounds to provide a stable platform would be insignificant in my opinion. If you rock the top case side to side using the corner of the upholstery (don’t use the headrest) you will see that the entire top case moves about ¼ inch back and forth on each side. If you rock the case from front to back it also moves down about a ¼ inch and can actually be lifted ¼ inch from its static position. Overall that’s a deflection of ½ inch in both directions. For a motorcycle that costs close to $30K and is touted to be on the “cutting edge” of technology, that’s a pretty sad statement with regard to design. However, this problem is not unique to the K1600GT-L. Several friends with K1600GT’s and R1200RT’s who have added the universal 49 liter BMW top case suffer the same problem.
The Fix
Step 1: Stiffening the Mounting Rack
Short of BMW offering a pair of rear grab handles and center top case mount section cast from Aluminum you’re stuck with trying to stiffen the existing plastic structure. I discovered that the black plastic center section of the mounting rack was contributing to about 90% of the total rack movement (for to aft) and the remaining 10% was flexure in the silver grab handles. My solution was to create a steel “horseshoe” that bolted underneath the existing plastic hardware in such a fashion that it would stiffened the black plastic tail section of the rack without creating an ugly looking add-on. Thesolution described below took out about 90% of the flexure out of the plastic rack components and still looks cleanIMHO (see photos).
My fix consisted of removing the rack assembly (both grab handles and center mount section). With this assembly upside down I bent a piece of welding rod around the inside recess of the black plastic mount tail piece to create a pattern. I then bent a piece of ½” square steel bar(using an Acetylene torch) to match the welding rod pattern. This bar was welded to two, 1/8 thick by 1.0 inch wide steel plates. Each plate was drilled to mount to the two fastener holes that tie the grab handles to the center section. Longer 6mm fasteners were necessary and I used some bushings to keep the steel plates from gouging into the grab handle plastic. I also drilled the black tail section and tapped the corners of the horseshoe so I could fasten the end of the plastic mount section to the horseshoe. Doing this stopped the plastic mount from flexing up and down. It also provided a mounting point for two rubber bumpers (discussed in Step 2). The new steel horseshoe moved the rack flexure point forward of the rear grab handle mounting point. The steel support eliminated 90+% of the total rack flexure. The only way to achieve a more rigid structure would require penetrating the plastic body panels and extending the steel horseshoe all the way up to the bikes Aluminum sub-frame.
Step 2: Spreading & Tightening the Top Case Contact Points
Now that the rack was solid I began removing the slop in the top case to rack contact points. The stock rubber pads on the Top Case that make contact with the rack are not a snugfit and are far too close together. To improve this I added two rubber bumpers to the tail end of the center mount plastic, picking up the fasteners that tied the steel horseshoe and plastic end of the mount together. This eliminated some of the backwards rocking. I also added a bumper on each grab handle near the outside edge to where the front of the top case passes over it. This spread the top case contact points from a little over 5 inches (stock) to almost 13-1/2 inches at the front of the case. These bumpers stopped the tendency of the top case to tip forward or rock side to side. The bumper height was sized so they compress slightly, taking up the gap between the top case and the support structure. Do not use bumpers that preload the top case excessively. I also add a 3” long piece of 3/16 inch thick by 1.0 inch wide rubber body molding to the mounting rack between the two front top case hooks. This filler helped tighten up the top case between the two mounting hooks. The end result was that the bumpers filled all the space between the bottom of the top case and the rack at each of the four corners and in the center. Installing my top case requires a very slight compression of the top case at the latch end for the claws to engage.
Conclusion:
Long term performance is still pending but driving with a wobbly top case is not only annoying, I believe it creates undue stress in all the plastic rack mount components (especially when loaded with gear). Since making this modification only a very severe bump causes the top case to move and then only slightly. My wife says the difference is like night and day.My only concern now is “what is the lifespan” of the plastic rack components. Most plastics tend to become brittle with age. I wonder how many folks will suffer a top case loss while underway before BMW will develop a fix. I don’t believe my solution is the end all fix, but I do believe it will save me from a catastrophic rack failure and top case loss. It also made my wife far more confident when riding with me and that was a big positive. The design shown herein is my personal fix and I do not build accessories for sale. Anyone who wishes to duplicate this design is welcome too, but they do so at their own risk since not everyone’s fabrication skill are equal.
Steel Mounting Plate Horseshoe Pattern
Finished “Horseshoe” Support Installed on underside of Plastic Rack
Installed Rear View
Installed Side View (side case removed)
Rear Rubber Bumpers on Rack (horseshoe underneath)
5 Rubber Bumpers (total) on Top of Rack