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K-forks & latest R forks are fairly substantial items as far as vintage pieces go...
The plates which support the headstock of the R frame are easily bent- even with the stock 70's era spaghetti forks.
In my opinion, K or late R forks demand reinforcement in that area.
BMW might have gone there-
But they had already determined that the design package was obsolete...

...and we all know that this subject concerns an exercise in "improving" a frame we all know to be obsolete.

Hi vanzen.

Chuey

I love whippin' improved bikes on my old unimproved RS.

When I'm flying my Messerschmitt, I often wonder what all the frame fuss is about.
My frame behaves beautifully in tight banks when dog fighting, and has just a touch of aileron flutter at 180 with a full payload.

Food for thought, what forces could be expected to reside on the sub-frame, near the seat latch mount cross member, when the frame is doin' its thang?

My seat latch cross member cracked clean through at the edge of the weld at the subframe tube.
I left it for more than a year to observe.
There was zero movement at the crack over that time, demonstrated by no evidence of wear at the close tolerance crack.

Hey Vanzen.

ME 109 wrote:
My seat latch cross member cracked clean through at the edge of the weld at the subframe tube.
I left it for more than a year to observe.
There was zero movement at the crack over that time, demonstrated by no evidence of wear at the close tolerance crack.

I would wonder what forces made it crack.

Chuey

I think I would rather brace the frame rather than rely on the subframe if I could so I've been scratching my head trying to come up with some alternatives and have thought of two possibilities:-

Add a tube below the upper subframe mounts and run another from one end of this tube to the opposite side rear down tube just above the swinging arm pivot. As this would run just behind the battery then it would need to be removeable.

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The other idea would be to extend the spine tube by some 3 - 4 inches and attach this to a tube welded to the down tubes that are curving away from the spine.

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What do you think?

My thoughts?
The difficulty of packaging stock components is the primary nemesis of frame reconstruction.
As I previously suggested, starting over is quite likely the best course of action.
Reconstructing the airhead frame with the intent of achieving contemporary performance standards is a bit naive. Doing so within the confines of vintage racing regulations is an entirely different game.
I currently ride the crap out of a 2007 R1200S hexhead which thoroughly resolves all of the performance, braking, and handling issues posed by an antique chassis. My riding ability does not approach the capability of the machine in spite of my best attempts. I like that scenario- the machine is bone sober stock.
Having said that, the challenge of building and improving has always been part and parcel of my nature.
Nourish, should you decide to accept the challenge, the best solutions were proposed and implemented back in the day when these machines were new and being raced.
If the goal is to modify a stock frame, then the Butler-Smith campaigned R90S Superbike winner (thanks Bamboo) would be an excellent model. You will do no better than Udo in trying to reinvent that wheel.
ME109 poses a valid question when asking, "why?"
If you need to ask, then you do not need to bother with an answer...

vanzen wrote: ME109 poses a valid question when asking, "why?"
If you need to ask, then you do not need to bother with an answer...

Ouch!
I'm clearly still the recalcitrant one

I've never asked 'why' for my benefit. I don't need an answer.

I don't run on a racetrack either.

Chuey wrote:

ME 109 wrote:
My seat latch cross member cracked clean through at the edge of the weld at the subframe tube.
I left it for more than a year to observe.
There was zero movement at the crack over that time, demonstrated by no evidence of wear at the close tolerance crack.

I would wonder what forces made it crack.

Chuey

I don't know Chuey. That's as interesting as why it didn't move after it cracked through. The crack may well have opened up thousands of times, but the always zero clearance crack (when I looked) was as clean as a whistle.

Now let's not go on about clean cracks ZM.

Agreed 100% -
If one can run about on a cracked frame
Without notice or consequence,
Then improving the design
Is certainly a superfluous notion.

The fact of which does nothing to address the question
Of how to improve the old cradle.




And so here we are...
Just as recalcitrant
As any old rubber cows can possibly be.

ME, borrow a GoPro camera, mount it to watch the crack and go ride your favorite curvey road to see what is happening. You will probably see the crack breathing as you go through left right curves.