Another broken rear frame tab thread

[Lars]

Coming 14 years after BMW ceased production of the R1100S I realize that this thread may not be that interesting, but maybe it will help someone trying to keep their old bike roadworthy...

Oh-so-long ago I started this thread. I was concerned that the weld-back-on rear tabs on my bike were in the wrong place.

Recently I had the opportunity to measure the rear tabs on a couple of BCRs that a friend owns, while I was in the middle of resurrecting my old steed. It had been parked forlornly in a corner of my hangar, having only accrued about 500 miles in the last three years. A couple of careful measurements of said BCRs, compared to mine, made it obvious that the dumbass who welded the tabs back on my bike either wasn't careful or didn't care (probably both) about getting them in the right place. He managed to add a bunch of material, effectively lengthening the tabs by about 6mm. No wonder the tupperware never fit right. There was a gap of about 15mm between the bumpers on the front of the saddle and the back of the tank plastic; I could sense (and see) the front of the saddle collapsing to close the gap when I rode.

I decided to finally take action to correct the problem that I'd been mulling in the 11 years since I started the above-linked thread. This photo, gleaned from yet another thread about broken frame tabs, inspired me:

[Lars]

I have a bunch of 4130 chrome moly plate and bar stock lying around from various aviation projects, and a MIG welder. I figured it would be too difficult and expensive to recreate the parts shown in the photo, but I'm comfortable welding and fabricating.

I committed myself to the project by cutting the messed-up ears off my frame and filing the remains off. Cut out some rectangles out of 1/8" steel plate and commenced to cold-forming them (AKA beating the crap out of them with a hand sledge) until they conformed to the shape of the frame. Turned some spacers out of 3/4" round stock on my lathe, to mimic the cylindrical part of the now-removed tabs. Cut some tabs and bits out of more 1/8" flat stock to create new ears.

I was originally planning to through-bolt the tabs I was fabricating to the frame, but I realized that the airbox was in the way, and there was no way to solve that other than unbolting the entire frame. Which I didn't feel like doing. So drilled 3 holes per side and tapped them for M6 fasteners.

Used a bottle jack between rear tire and bottom of seat pan to get the rear subframe located correctly, then tack-welded the now-bolted plates and tabs together. Finish-welded them off the bike, painted with some zinc primer and voila.

They are now secured with 3 M6 screws per side, with a little red Loctite on the threads for good measure. For the first time in the 12 years I've owned my R1100S the tupperware and saddle fit correctly. It may be the placebo effect but the saddle feels more comfortable as well.

Some photos (welds are ugly but they won't break):

[N41EF]

Wow, first though... Can ya make a bunch of these? My seat leaves a gap as well, so I'm guessing mine are off as well.

[feisty]

Great work. Good to see work being done to reinvigorate your love of the bike.
Re you point on the age of the bike and how it effects the activity levels on the forum I get you. Having just bought a 95 r1100gs I went out seeking an active forum to discuss the issues that come with owning a 24 year old gs. Unfortunately the gs world has moved on to wizz wheels, tft screens and water cooling. The same can be said about aftermarket accessories. All those catalogues of parts for my gs/s/rt that I have kept over the years are now no longer available.
Yet there are still a lot of new used 1100s / 1100gs etc owners about seeking current information. The information that you posted is gold in my eyes. Good on you!

[BadToTheBown]

Originally Posted by Lars

...this thread may not be that interesting, but maybe it will help someone...

I thought you did a pretty good job of fixing it. I got lucky finding someone who could weld it without shrinking it so it continued to line up, your welds look better than mine, nice idea/work...

[tm1100s]

Haven't had the opportunity to do one of these, yet. But how thick is the aluminum in the area of the tab and where you placed the bolts? Is that section of frame hollow inside and thick enough to drill and tap or is it a solid piece of aluminum?

[Chris Canning]

Originally Posted by Lars

Coming 14 years after BMW ceased production of the R1100S I realize that this thread may not be that interesting, but maybe it will help someone trying to keep their old bike roadworthy...

Oh-so-long ago I started this thread. I was concerned that the weld-back-on rear tabs on my bike were in the wrong place.

Recently I had the opportunity to measure the rear tabs on a couple of BCRs that a friend owns, while I was in the middle of resurrecting my old steed. It had been parked forlornly in a corner of my hangar, having only accrued about 500 miles in the last three years. A couple of careful measurements of said BCRs, compared to mine, made it obvious that the dumbass who welded the tabs back on my bike either wasn't careful or didn't care (probably both) about getting them in the right place. He managed to add a bunch of material, effectively lengthening the tabs by about 6mm. No wonder the tupperware never fit right. There was a gap of about 15mm between the bumpers on the front of the saddle and the back of the tank plastic; I could sense (and see) the front of the saddle collapsing to close the gap when I rode.

I decided to finally take action to correct the problem that I'd been mulling in the 11 years since I started the above-linked thread. This photo, gleaned from yet another thread about broken frame tabs, inspired me:

I'll digress for a minute!! over the years I've built up quite a file of 1100s stuff but the only thing I never took a photo of was an 1100s with near identical mounting brackets as the photo made by a an Independent BM guy here in the UK that good they looked factory a much better idea than what BM came up with here's the bike the frame and motor are somewhere in Ireland!!!

[BadToTheBown]

Originally Posted by tm1100s

...how thick is the aluminum in the area of the tab...

They are hollow and about a 1/8" thick were you see the bolts....

[brentw1]

just a idea it looks like it would make sense to make it go over the top of the frame
and put at least one screw in the top,

[tm1100s]

Originally Posted by BadToTheBown

They are hollow and about a 1/8" thick were you see the bolts....

Thanks for that. I've been thinking about this for a while just in case I need to deal with the problem. While the bolt-on fix looks good, I'm thinking the two bolts tapped into only 1/8" worth of aluminum may not be safe considering that when you're bouncing around the forces appear to be pulling the fittings away from the frame, especially with the lower mounts acting as a pivot. The top strap and bolt seems to be a good idea. My original idea was to produce a rectangular hole into the frame just large enough to hook an L-shaped piece into that hole so it can't pull out, then weld around the outside. The part sticking out would serve as the new tab. Simple, but requires welding, which I think is necessary for a safe fix. Tried to make a drawing but it appears I'll need to relearn Inventor to do it.

[Lars]

Thanks for comments.

Making more would be problematic, at least the way I did it. The plates that bolt to the frame required a lot of massaging with a BFH in order to conform to the somewhat complex shape of the frame. To be mass produced it would probably be better to revert to machining from aluminum billet after modeling the things in a CAD program (which I did a lot of in a former life).

As for the thickness of the frame rails, I measured it where I drilled the holes. about 5mm at the upper bolt, 4mm next one down, 3.5mm bottom. I too was nervous about relying on threads into the aluminum, rather than through-bolting. I'm still not crazy about the idea but I figure if the bolts start loosening I'll give in and take the bike apart, drill out the threads and install longer bolts. Meanwhile I'll monitor them.

[tm1100s]

Thes are crude drawings and I think you can see the general idea but this is what I had in mind (tab size, frame cutout placement/size and especially my graphic design needs work):

[BadToTheBown]

Originally Posted by Lars

...thickness of the frame rails...about 5mm at the upper bolt, 4mm next one down, 3.5mm bottom...

Little thicker in upper portion than I thought, which is better than my observed guess, looking at the bolt, they look pretty close to being about 5mm bolts, which offers at least 1 diameter thread engagement in the top bolt, to get at the backside is a problem, you'll need small hands with some stuff removed to get a nut on the backside if you really felt you needed to, so I'd probably called it good enough here as well...

[CARacic]

Seems like a good application for a riv-nut if your threaded holes don't work out.
Good Luck

[BadToTheBown]

Careful with the rivnuts, they are VERY easy to overtorque and spin in their hole...as me how I know...

[Lars]

Originally Posted by CARacic

Seems like a good application for a riv-nut if your threaded holes don't work out.
Good Luck

Or possibly a K1000-5 plate nut and some long (and expensive) CherryMax rivets, though fishing them into place could be interesting.

[ejfbmw]

Hi Lars,

Finally, I've been looking all over for this thread and would love to see more pics of the brackets in the first post.
My frame tabs are broke off as well and as I have a couple home made brackets in play already, the machined ones here look to be the best. Does anyone have drawings or a cad model?

Also, I've seen here several comments regarding wall thickness for the tapped holes required to bolt on these type brackets and what my experience is using four 5mm bolts on each side is that the bolts/tapped holes hold fine; it's the bracket that tears and breaks the next time you over stress the seat, or flip the bike

Please let me know about drawings.

Thanks!

[Lars]

Originally Posted by ejfbmw

Hi Lars,

Finally, I've been looking all over for this thread and would love to see more pics of the brackets in the first post.
My frame tabs are broke off as well and as I have a couple home made brackets in play already, the machined ones here look to be the best. Does anyone have drawings or a cad model?

Also, I've seen here several comments regarding wall thickness for the tapped holes required to bolt on these type brackets and what my experience is using four 5mm bolts on each side is that the bolts/tapped holes hold fine; it's the bracket that tears and breaks the next time you over stress the seat, or flip the bike

Please let me know about drawings.

Thanks!

No drawings. The steel parts I made were completely ad-libbed in my shop. Once I had the right side base worked out with holes I made a second piece, held it to the first with vice-grips and match-drilled. I can say that a few thousand miles later they haven't budged.

And yeah, a set of those nicely machined ones from some BMW shop back east would've been nice, but I was way too late for that. Necessity was once again the mother of invention, in my case.

[Chris Canning]

Bump

[Lars]

Originally Posted by Chris Canning

Bump

...and grind.