Hi all,

I am working on assembling the tailpiece of my K100. I cant seem to get the front of the red tail piece to sit anywhere near flush tot he plastic frame screw inserts when the rear mounting wing nuts are tightened. This can be seen in the picture below. Upon close inspection, it looks like the frame underneath the seat bows down slightly. Could it be bent? I figure this is unlikely because there is no sign of significant damage and the frame has been powder coated by the previous owner. I doubt they would go through the trouble on a bent frame. what are your thoughts?

Thanks

Do you have the rubber mounts under the fix points? The frame looks suspect but... someone else who knows more about frames needs to comment on that.

First, is there any history of this bike being "looped" to build a cafe "racer"? 

Second, the fastenings for this part of the bike are quite extensive.  Have you followed any installation instructions?  The parts fiche at MaxBMW is a good reference for how all the bushings, spacers and washers stack up. 

Third, if the bike has ABS, there is a monster bundle of wires that you need to deal with.  Double check how they're routed.

Thanks for the reply all. First of all, I know this bike had a rough life before starting restoration. I know this because the untouched parts show signs of UV exposure and lots of corrosion. I dont think it was ever "looped" but I wouldn't know for sure because I bought it after it had been painted and powder coated. Also, it is a non ABS bike.

Here is how I understand the mounts to work from the MAX BMW Fiche. The cone shaped bushing in the back, flat rubber washers in the middle and nothing but the plastic frame inserts up front.


Also. Here is a better shot of the bend with a straight edge. I held it flush to the frame in the middle and you can see the droop in the other pictures:

A 1985 K100 frame.

Take a more distant side profile of the frame, with a straight edge secured to it if possible.

Laitch wrote:Take a more distant side profile of the frame, with a straight edge secured to it if possible.

Hmmm compared to yours, mine is not looking healthy. Here is a better shot:

That is definately bent. I took a good look at mine today, but Laitch beat me to it.

Dai wrote:That is definately bent. I took a good look at mine today, but Laitch beat me to it.

Yep you're right. Now I have to take a close look at the rest of the frame to see if anything else is bent. Hopefully its just this non critical area. Would this be the kind of thing that a bike frame shop could straighten out? Hoping it can be done without cracking the powder coat

Tjsunny2 wrote:

Dai wrote:That is definately bent. I took a good look at mine today, but Laitch beat me to it.

Would this be the kind of thing that a bike frame shop could straighten out? Hoping it can be done without cracking the powder coat

I think it could be straightened but the gussets in the angles would likely need to be cut loose from that portion of the frame to allow that, then welded again. The finish in that area would be removed for that process.

From here, there's no sign of buckling in the gusset so I'd guess that it has been looped, but the original frame was welded back on. However, it was butt-welded instead of being slugged and welded; slug'n'weld would have resulted in a straight frame. You'd probably get away with cutting it behind the gussets and then slugging it to ensure a straight join, but you are going to lose the powdercoating. If you go that route, get a a good known measurement between the two seatbase securing points and use that to drill two bits of flat steel to ensure everything comes out the right length.

[edit] Ah, it has been looped and rewelded. Look at the state of the powdercoat beind the gussets; not smooth at all, like it should be.

Dai wrote: Ah, it has been looped and rewelded. Look at the state of the powdercoat beind the gussets; not smooth at all, like it should be.

If the frame was dropped from the rafters or hit from the rear after being powdercoated, touchup might have been the order of the day but certainly it should have been braced by a jig before welding. Looped and rewelded—a case of artiste's remorse.

I think you may be able to raise the back.

A short piece of 4x4 across the frame behind the tank used as a fulcrum for a 2x4 under the rear of the loop.  The other end of the 2x4 extends out above the steering head.  A strap pulls the front end down to a point under the steering head and(hopefully) bends the loop up enough to regain the proper alignment.

No chance, point75. That's either 2mm or (most likely) 3mm wall tubing. I know from years of building bikes that tubing that thick is a bitch to bend even with a Staffa TE5372 (a proper hydraulic bender, not a Harbor Freight wannabe). Still got the Staffa, even through I haven't bent a main frame tube for over a decade now! 

Dai, I realize that the tubing is pretty strong, but there is a chance the welding heat has drawn the temper enough to be able to bend them a little.  It's worth a try.

Update: My first attempt to bend it back failed. I think the jack has enough power to d the job but the bike lifts off the ground in the read due to the moment applied. I'm open to any other ideas. I'd really rather not cut and weld but I have the capability if necessary.

You're up against another problem there too. When you bend a tube, the metal on the inside of the bend has to thicken to allow the tube to curve. That's not so much of a problem with a piece of bare tube, but the 'thick-zone' for you is right where the gussets are strengthening the frame. You also need to push the loop passed the point of it being straight and allow it to spring back. Rinse and repeat until it is straight.

I never did get very good that that bit.

Don't be tempted to use the crossloop at the back for more leverage. It's only 5-10mm into the main frame tubes.

Good photos, the one in #7 seems to show that the loop isn't bent, neither is the main frame. They've just been welded together at the wrong angle.
I would say there's no point trying to correct things by bending back - 'cause there's no 'bend' to straighten. If you try to bend it you might be able to get the mounting lugs near to where they should be, but you'll be introducing bends into frame parts that should be straight. It could end up being a mess and you'll subject the remainder of the frame to loads of stress in the process.
Clean the paint off, hacksaw 9/10 of the 'wrong' join down from the top (two blades in the hacksaw, looking at what you've got) pull the loop up straight, and tig weld it. 

It's only paint!