Bracing the front of the frame

[Chuey]

Well, as you know, I'm such a good rider that..........

OK, I do think I want to brace the front end. The bike already has a very noticeable upgrade in that the frame braces connect to the top engine to transmission bolts. Who can say if that benefits speed or handling while the bike is being ridden by little old me? It is, without a doubt, the most fun bike with a motor/engine I've ever ridden. It is likely that what caused that condition is the riding position to a much larger degree than it is the stiffening up of the frame.

In the end, the result that matters is that I have fun riding the bike....as long as I haven't caused myself a safety issue. For some reason, my Cafe is my favorite bike to ride. It is stable but probably not over a hundred mph. There is no speedo but that is my estimate. It would be cool if it felt good the faster I go but it starts a kind of a weave over about a hundred. It's clear that that is an aerodynamic issue and not mechanical. The real fun to be had on that bike is well below that speed but up to the point that the air isn't happy, it's super stable and if I let it down just one or two mph once that happens, it's back to being super stable.

Chuey

[ME 109]

OK, I do think I want to brace the front end.
Chuey

How you gonna do it Chuey?

Without any experience at all, I like the idea of doubling the original gussets, and 'plating' like in Vanzens photo.

[R85/8]

I have never been sure whether the real problem is the frame needs bracing or the original 36mm forks are too flexy.

Probably both

I used to look at the leading axle forks of the original 750 Ducatis with some envy.

[vanzen]

I have never been sure whether the real problem is the frame needs bracing or the original 36mm forks are too flexy.

Probably both

Definitely both – the latter being more problematic IMNSHO.
As proof, look at BMW's handling of these issues throughout the course of later production:
Forks tube diameter increased concurrent with the switch to larger brake rotors
(even as the increase might have been justifiable with the change from ATE to Brembo calipers).
Any semblance of the antique cradle frame was abandoned
with the introduction of the (more powerful than the air-head) oil-head.
As design limits were encountered and then surpassed, changes were made.

[R85/8]

When I bought my R75/5 I was a bit disappointed in the frame because much was made of its similarity to a Featherbed at the time. (My previous bike was an Atlas). The frame design seemed to have missed the point of the brace to the cylinder head on the Norton. The other thing that was obvious riding it after an Atlas, was that the mass of the engine was too far back. The Atlas had surer handling with an even flimsier fork.

The longer travel of the BMW did swallow up all the dirt roads much better than the Norton, and was a much better bike otherwise.

I went as far as an extensive correspondence with Richard Difazio to fit one of his hubcentre steering frames, but then I crashed my R75/5. The current setup on the oilheads looks like a good compromise.

[Motorhead]

as a Point of reference one should know

Honda did lots of resurch on a solid frame Idea braceing and building to a point that the powerful motor an solid frame didn't work

Honda found that some flex was GOOD

I found my set up of motor attachment a good idea, as little frame mods are not needed, but as Chuey found some can add an effect that maybe explained as airflow can actually be be Harmonic

with my Sturgis trip I could do 80 MPH with a smooth a ride as any could want, faster Is possable.... but some feed back developed 37/11..... But that could have been the start of loseing compression...... motor trouble, but then what field does a motor that runs across the field of DIRECTION have

as my motor is sound again I found a higher MPH is possable than before, the Feed back happends now later 32/11

[vanzen]

as a Point of reference one should know

Honda did lots of resurch on a solid frame Idea braceing and building to a point that the powerful motor an solid frame didn't work

Honda found that some flex was GOOD

Believe me, too little chassis compliance will never be a problem with the air-head frame !
Doubly true given the stock air-head's potential for attainable acceleration and top speed.
And triple those odds given the choice of tires that will even fit the stock chassis !

" What is certain, though, is that any deliberately introduced compliance
must be below a level that would introduce instability problems ...
Prior to about the 1980s many chassis fell below that level
and benefited greatly from increased chassis stiffness"
~ Tony Foale


Chassis stiffness has increased dramatically in the past 30 - 40 years.
"Too much chassis stiffness" has been misrepresented and then primarily as an issue in racing –
machines which easily produce twice the HP and stick to the ground with twice the tire (in race compounds)
... than any air-head (or Sportster !) might boast.

The problem occurs when these MCs are healed over in a curve to that point where
the suspension can no longer dampen a majority of the force of a "bump".
The force of a "bump" will be approximately vertical, and while leaned, say at a 45º angle,
much of that force will remain "un-damped" and thrown into the chassis – which tends to upset traction.
The result will be "rear wheel chatter" – while healed over coming out of a curve, and applying full throttle.

Designers and engineers seem to concur that increasing chassis compliance would not be the best solution.
The real issue, they will say, is not an excessively "stiff" chassis, but rather a lack of "lateral suspension".
Some racers must agree, and still choose the default – a chassis that is "rock-solid".
Watch them MCs "wiggle" as they exit a turn !

[vanzen]

When I bought my R75/5 I was a bit disappointed in the frame because much was made of its similarity to a Featherbed at the time. (My previous bike was an Atlas). The frame design seemed to have missed the point of the brace to the cylinder head on the Norton...

I think the BMW engineers must have been on holiday
when the structural function of the head-steady to incorporate the engine as a stressed member,
and the theory of triangulation as a method to eliminate frame distortion were discussed ...
even as they were obviously paying attention during the rest of the 'featherbed' lectures !

[Major Softie]

as a Point of reference one should know

Honda did lots of resurch on a solid frame Idea braceing and building to a point that the powerful motor an solid frame didn't work

Honda found that some flex was GOOD

Yes and no. Factory racing engineers have found that a perfectly rigid frame does not handle as well as one with designed in flex, but that is designed in flex - in the chosen designed in directions. Some directions perfectly rigid is still most desirable.

So, "some flex" CAN be good, not IS good.

[vanzen]

The interest in providing a degree of Lateral Suspension
will be why swing-arms are getting taller and skinnier.
The design allows the swing-arm to flex (a well defined and controlled flex)
when going over slight bumps at max lean.

Lateral suspension will also be the reason why contemporary race frames have sh!t-canned
the X-bracing that was popular in the 80s between longitudinal aluminum frame-spars.

Lateral suspension will also be one reason that I have chosen to use wire wheels vs cast
with the CRo's (longitudinally & torsionally very stiff) X-frame

In any event, the generous amount of lateral compliance inherent in the Type-247 frame and swing-arm
much more closely approaches the operational limits of stability
vs that limit of traction loss due to excessive lateral rigidity !