Frame Brace Thoughts

[Kurt in S.A.]

I like it...it will help prevent my assumption that the large "rectangle" of the original frame will collapse. Plus there's still a wide open view of the pretty engine!! It has nice "symmetry".

Kurt in S.A.

[Chuey]

Check google images for "reg pridmore bmw". It shows that the frame he had utilized the somewhat conventional front to back CC products type braces, though a little more slender version commonly see now. I tried to see if there was any extra bracing around the swing arm but could not see evidence of such.

Chuey

[Chuey]

I agree that probably causes the wobble but what are you feeling in the seat of your pants? surely somewhere the frame is moving?

No.

That wobble is caused by the worn out tire causing a difference in tracking direction between the front tire and the rear. The worn out tire is "searching" for a tracking direction. In effect, this creates a constantly changing tire misalignment. This shows up as movement in the steering head, as the relative tracking directions between the front and rear tire keep changing. The forces causing this wobble are tiny (as in: enough to move the bars a small amount), and if that wobble you feel was your frame flexing, then you'd be able to flex that frame all over the place easily in your garage with your bare hands.

Any bike can do it, but some do it much worse than others. I would speculate that the difference is in steering geometry factors - some geometries heightening the issue, and others dampening them (along with just how worn out the tire is).

I've been away for a couple weeks and just checked back in on this thread. In So. Calif. we have grooved freeways which cause what I think you are referring to as white lining. This happens not only with old tires. New ones can have the same "queasy" feeling on the groovy pavement. Newer bikes do not do this. I refer to R1100S and such. It may be the tires or it may be frame design/geometry. On the older bikes, some tires are better than others. I'm not thinking this actually answers any questions as much as it may be info that could be considered when thinking about the subject of bike behavior.

Chuey

[Nourish]

I think this latest idea is out also, I swapped the mock-up to the other side, as the engine is offset in the frame the mock-up tube won't fit between the starter motor's cover and the /5's tank's fuel tap.
So going back again, Kurt's theory of the frames rectangle gets out of shape makes sense to me.
As I can't use the side braces how can I stop the rectangle from deflecting - How about a head steady?



This would surely create a triangle from the stearing head area to the bottom frame rails via the engine block - wouldn't this keep the frame in shape?
Fitting a brace above the gearbox/engine mount would stop the rear of the spine from swinging around.

[Chuey]

I wouldn't expect that housing to have a lot of ability to withstand the loads it is designed to hold against. If it didn't break, I'd think that that would indicate it is not doing much in the first place and the load was imagined.

Chuey

[Nourish]

Well I don't know, I was thinking of welding in the bosses to the inside web (web? - the face where the cam and crank poke through!) as well as the outer shell of the timing chest- do you not think that would be rigid enough?

Just another thought - in the past I've noticed pieces of glass glued over cracks in brick work looking for movement - could I try that? - from the front cross brace under the headstock to the timing cover on my present R100RT - the paintwork is very shabby - what could I use as glue?
But then would the vibration just break it?

[vanzen]

Udo Gietl adequately summarized the deficiencies
(70s era stock BMW frame)
and
proposed a comprehensive strategy for DIY improvement
4 decades ago...
Tony Foale then explained all of that
which Udo and Rob North knew intuitively.

2 frames of my design were fabricated.
Finances and circumstances kept my project from completion.
The other frame was purchased and raced without notoriety.
The design is not patented.

[Nourish]

Ah there you are Vanzen - and there was me thinking something had happened to you!
Have you moved the Old Crow on or do you still have it?
Where can I read Udo Gietl theory and his DIY improvements on the BM frame?
I've searched for Foale's writings on bMW too but can't find much.

[vanzen]

A few observations:

Forks and the bolt-on subframe
are major contributors
conspiring to create the gummikuh effect.

K-series (or later R) forks resolve #1
but transfer (some of) those forces
To the steering head-
which then needs structural attention.

The axis created by the swing arm pivots
(Relative to the steering head & mainframe spine)
is notoriously unstable.

A bracket connecting the rear of the engine case
to the spine,
welding the subframe to the main,
and adding a diagonal brace welded across the tool tray area
... Is as effective to stabilize the SA pivots
as any compromise "side brace" that can be implemented
without additional (& major) "packaging" changes.

Too much rake?
Rake is virtually inconsequential to handling
PROVIDED
Trail is calculated correctly
to provide the needed balance
of maneuverability and stability.

[vanzen]

The best strategy to improve
a 70s airhead frame?
Trash it and start over.

The Magni BMW frame did it best
(Given the requirement of fitting all stock components)
It's design is quite similar to the Moto Guzzi V7
which handles surprisingly well for it's vintage.