chassis recipe, introduction.

twin-shock chassis recipe, discussion.

This thread will not be to everyone's taste.
It's contents will provide a comprehensive strategy to improve the handling characteristics of a twin-shock air-head ÔÇô
a not entirely my own and not-so-secret "BMW Road Kill Cafe Recipe".
A plethora of information, mis-informatiom, and idle after-market advertising hype exists regarding this topic,
and together, maybe we can sort it all out.
All discussion is welcome, but please note:
My recipe, after all, is not intended to be THE definitive menu but rather just an example of one rather tasty "dish" ...
an inexpensive dish that would suit my taste, may suit yours, but will still prove too spicy for many.
So, if "spicy" ain't your style ... perhaps best to pass this one up ...
for the rest, let's get on with it then ...

The twin-shock air-head chassis was designed by the factory with then available technology
to be flexible in order to cope with a variety of road surfaces and conditions,
to be fairly competent in all possible riding situations, albeit extremely proficient in none.
As good and well received as this marketing strategy was,
it remains a compromise intended to appeal to the largest number of potential customers / riders most of the time.
The truth of this statement will be witnessed by the extensive chassis modifications that altered the Butler-Smith / Udo Gietl 1976 R90S
in order for it to be a successful competitor and Superbike championship winner.
The first point to note, then, will be that
modifications here are intended to improve chassis behavior when ridden fast on paved roads,
and that performance on dirt roads and marginal surfaces will likely suffer.
The resulting chassis will be more focused ÔÇô and less versatile.
Next, THE FIRST & ONLY PLACE TO START WITH CHASSIS IMPROVEMENT
WILL BE A STOCK CHASSIS THAT IS OPERATING AT 100% EFFECTIVENESS.
If your riding skill and style does not approach or surpass the limits of stock chassis design capability,
any improvements to the system will prove moot.
And finally, improvements will be relegated to stock components as if we were considering building a bike
to (loosely) satisfy AHRMA Vintage 750 Sportsman Class rules relative to chassis modification ÔÇô
i.e. no late model forks system transplants, custom frames, or mono-shock conversions.
Why do I emphasize "twin-shock chassis" ?
Because the factory effectively dealt with many, but not all, of the issues we will address here
with the introduction of the mono-shock platform and improvements continued
throughout the rest of the air-head model run.

Intro:
The motorcycle chassis and chassis dynamics must be considered as an INTEGRATED SYSTEM.
Having said that, I will attempt to order this recipe in hierarchical terms of (my perceived) importance.
I will deal with these three sub-systems individually ÔÇô
Part 1, forks system and front suspension,
Part 2, main frame, and
Part 3, sub-frame and rear suspension.



stay tuned ... Part 1 to follow ...

featherbed knock-off

PT9766 said:

It is my understanding that the twin shock "airhead" frame is basically a copy of the Norton "Featherbed" frame, according to Hans-Gunther von der Marwitz - as quoted in "Bahnstormer" (p.141) by L.J.K Setright.

PT9766

Click to expand...

Norton Feathebed:


The design is quite similar,
including the rather odd method of routing the main-frame's "spine" to the bottom of the neck-stem tube –
a concession to CoG in order to keep the weight of the gas tank and it's fuel lower in the frame,
and this to the detriment of neck-stem area rigidity.
Norton compensated for this flaw, however,
and this will be an important and major distinction between the two frames.
The Norton featherbed frame has an integral design feature,
what is referred to as a "head-steady" –
A rather stout bracket that connects the frame just below the neck stem
to the head casting of the engine.
This attachment, and the engine mountings on the bottom frame rails
allow the engine case to function as a structural frame element / reinforcement,
dividing the rather rectangular profile of the frame in half on a diagonal
from the neck stem to the swing-arm pivots.
This use of "triangulation" greatly increases the Norton frame's ability to resist distortion,
and is an important element in the success of the featherbed design.

BMW engineers were apparently absent that day ...
as no such attempt to triangulate the Type-247 frame was incorporated.
This will be the reason why the BMW frame flexes significantly at the neck-stem requiring reinforcement of this area.
And also why "frame-braces" have become so popular.

A relevant quote concerning the handling qualities of the Norton featherbed, considered excellent in it's day:

"What does allow the handling stability of most British bikes is the
normally minimal amount of power, relative evenness of its delivery and the
fact that the state of technology in the rest of the world during the
British heyday was worse."

This should proove to be very interesting. I am certain that we can all learn something from this thread. I look forward to reading your "Recipe". Do you plan on any simple drawings or references to such so that we can follow along? Some of us need pictures to stay focused.

Lookin' forward to what you're gonna share...so far, in my one month of BMW ownership, I haven't been overly impressed about the engineering of simply things on my R100/7...maybe this will help me be impressed a little more...

Can't wait for more!

+1

Well!!! We are waiting!

Just think--after all this effort an Oilhead will still be better!

Nudge, nudge.

The terms are well defined

lkchris said:

Just think--after all this effort an Oilhead will still be better!

Click to expand...

A statement that I have myself made numerous times,
but also one that will be irrelevant here, Ichris.
The subject is clearly stated:

"It's contents will provide a comprehensive strategy to improve the handling characteristics of a twin-shock air-head –"

... not intended to be a comparative analysis of disparate technologies,
or a philosophical and economic justification of pursuing the goal.
It is intended simply as a "HOW TO"
and a discussion of the practical merits of those procedures

Quite busy at present, re-roofing the shop ... then headed to Hunter.
Will continue with this ASAP.
In the meantime –
put the popcorn down. Anyone is welcome to share their ideas ... regarding the topic –

lkchris said:

Just think--after all this effort an Oilhead will still be better!

Click to expand...

This post made me chuckle.

Anyone who is a member and participates online in this forum is engaged in a past time which the vast majority of the world considers irrational at best. Therefore; as a member the idea of the thread as you laid it out makes perfect sense to me.


Your posts in the past have often included the why of doing some modification along with the how you did it. I donÔÇÖt plan on running out and starting a twin shock BMW project but find that information very valuable for approaching other projects ÔÇô real or imagined. I hope you will continue to focus on telling and discussing the whys of your strategy along with the hows.

lkchris said:

Just think--after all this effort an Oilhead will still be better!

Click to expand...

And so will a number of other modern motorcycles. I love my oilhead but enjoy my airhead for what it represents. I will check in now and again to see what transpires. Perhaps I will gain an idea or two.

I haven't even read every word yet, and I've already learned a few things.

This will be cool.

vanzen said:

Quite busy at present, re-roofing the shop ... then headed to Hunter.

Click to expand...

hope to meet you there, i'll be the young doofus making a weathered R80ST look like a circus bear bike.

vanzen said:

The subject is clearly stated: ...

Click to expand...

And you think you can tightly control the subject and the responses?

How about you start by listing your engineering background that qualifies you to do this ... you know, as part of the ego trip you're so kindly "volunteering" to afflict on the rest of us?

Oh, and please spare us the "reality TV" junk, like working on your roof. We don't care.

lkchris said:

And you think you can tightly control the subject and the responses?

How about you start by listing your engineering background that qualifies you to do this ... you know, as part of the ego trip you're so kindly "volunteering" to afflict on the rest of us?

Oh, and please spare us the "reality TV" junk, like working on your roof. We don't care.

Click to expand...

MOD ON

Please let vanzen say his peace, then if you find things you don't like, you're free to criticize. One man's "ego trip" is another one's opportunity. Let him have his thread, k?

MOD OFF

Wow. Like, chill.

Vanz - I'm impressed - you've started a hot thread with only barely a word out of your mouth - Just the premise of the thread was all it took - good one!

Since when do you have to have an engineers degree to have input in this forum? If you feel his thoughts are being ÔÇ£inflictedÔÇØ on you don't read them. I for one would like to hear his thoughts on the subject.

RecycledRS said:

Since when do you have to have an engineers degree to have input in this forum? If you feel his thoughts are being ÔÇ£inflictedÔÇØ on you don't read them. I for one would like to hear his thoughts on the subject.

Click to expand...

+ 1. Henry Ford did not possess an engineering degree.

One pompous troll wants to inflict his opinion on everyone who deigns to improve a bike whose time has passed some time ago. Stick to your own bike and leave the hot rodders be. Nothing would be improved if we all accepted the current state as the only possible way.