rear subframe mounting tabs - where are they supposed to be?
 

My '02 R1100S Boxer Cup Prep was repaired several years ago when the rear sub-frame mounts (upper tabs) on the main frame cracked off. The welder did a decent repair job (held up for 4+ years), but I recently noticed that one side has started to crack off again.

I've got a guy who's ready to do an A1 job on the re-weld, but this time I want to make sure I have the tabs in the optimal position. The welder will grind both off, and relocate them per my specification.

So my question is, can anyone take some measurements from their factory-original frame and locate the correct spot for the center of the upper rear sub-frame tabs? Straight line (vertical) distance relative to lower tabs would be helpful, as would a straight line measurement from some other landmark on the main frame (lengthwise). I'm not too worried about width between the two tabs, but while you're at it . . .

The reason for all this is that after the first repair, I felt like I had to wrestle the rear sub-frame back onto the mounts (sure, the sub-frame may be bent), and the seat angle never felt right (I know, even the factory original bikes have this problem), so I always wondered if they were welded back on in the wrong position.

I'd even be willing to move the sub-frame tabs from the factory spec position if someone has some good reasoning on this. Like maybe move them up and back (toward the rear) a couple millimeters to flatten the seat out?

In any case, thanks in advance if anyone can reply with some quick measurements/suggestions.

Tony

Bump.

Tony - Good luck with your frame.

Can someone with a tape measure and a R1100S help Tony? Anyone?

I wouldn't do it the way you're suggesting...or maybe I missunderstand.

1st...straighten the Rear Subframe as best you can.
It may help to have the frame bolted to the two unbroken tabs to discern alignment.
It may help to mount all the plastic to see if the lines flowing between front and rear parts line up.

2nd...mount the broken subframe tab to the Rear Subframe.

3rd...Once the rear subframe is straightend, position the assembly up against the Main Frame and mark the position of the broken subframe tab where it rests against the Main Frame.

4th...weld on the broken subframe tab.

My reasoning for taking these steps is that you may eliminate any stresses involved if everything is lined up in a non-stressed position before welding. Also you still have the two un-broken tabs to help.

As far as taking measurements from another bike...no two bikes are exactly the same...best to make yours work.

What R111S says would be the way I would approach it. It is 1:35am here at the moment, If no-one has posted any measurements by morning I will measure mine. Should be pretty much a straight forward job. Wasn't there some bolt on tabs that some were using on the boxer cup race bikes? All the best with it anyway.

Some info from a search.
http://forums.pelicanparts.com/bmw-r1100s-r1200s-tech-forum/349649-looking-upclose-n-personal-pic.html?highlight=bolt+frame+tabs
http://forums.pelicanparts.com/bmw-r1100s-r1200s-tech-forum/419916-cracked-rear-frame-tab-fixes-other-than-insurance-claims-p.html?highlight=bolt+frame+tabs
See bigscience's post this page re bolt on tabs.
http://forums.pelicanparts.com/showthread.php?t=251007&highlight=bolt+frame+tabs& page=2

Main Line, who made the bolt-ons, no longer exists. They were only using them on race-bikes and would not do them for a street bike for liability reasons.

I performed this repair once. I agree, if you have mounting points that are still intact, locate the new mounting points using a straight rear subframe. Mock fit the body panels to ensure that the body lines are correct. Once 2 new mounting tabs have been fabricated, then use the subframe again to redo the other two (intact) tabs.

Good luck!

SchnellerSmileWavy

Thanks everyone for your input.

I actually just finished up with the welding guys (they are a mobile service), and they did a solid job. And since these guys are professional welders, they were able to explain a couple of different things the previous guy had done wrong, like leaving some air space underneath the weld (as evidenced by dark brown oxidized aluminum on the broken surfaces, instead of clean shiny aluminum).

I did take your advice, R11S (and others), and just had them re-mount the cracked tab where the subframe wanted to be, based on the position of the other three tabs. In the end it looks like a solid weld and the subframe bolted on with no fuss. As far as the seat angle, I'll figure out some other way to get that right (since even the factory didn't), like getting a custom seat maybe.

Thanks again.

Tony

Glad it worked out well for you...have a good ride!