humble attempt at reconciling physics with frame design

[SteelRules]

Just to undust and old thread and add more up to date material on the topic of stability of bikes, please find the link:
Two-mass-skate bicycle - Wikipedia, the free encyclopedia
and other links therein.

The lab of framebuilders is the road. Their search for frame design consists of recursive building and riding (trial and error).
Notice that stability of a bike is not synonymous with bike handling as humans understand it,
although both concepts should not be far off for practical purposes.


The paper by J. D. G. Kooijman, J. P. Meijaard, Jim M. Papadopoulos, Andy Ruina, and A. L. Schwab published on
Science Magazine, April 15, 2011;
aims at explaining in rigorous terms how the leaning of bikes translates into steering
and this is how the authors define stability. The necessary condition for stability is that a bike must turn into a fall.
This means that bikes falling sideways trigger steering of the handlebars so that they are re-positioned in such a way that their centre of gravity is again between the wheels.


The mechanisms that couple leaning to steering have been exclusively thought to be:
i). gyroscopic effect of the wheels
ii). trail.
The authors show that none of the above are sufficient conditions on their own for stabilising a bike. Other things
like front mass location and steer axis tilt also contribute crucially.
All of them are necessary, none of them sufficient in isolation.

What I like of this paper is that it illustrates from a rigorous point of view how intricate bicycle dynamics are.
As such, simplifying bike behaviour by quoting trail, fork rake or head tube angle in isolation is illusory.

I think this work offers several lessons that I consider useful as a rider:
1. Gyroscopic effect, trail, weight distribution and head tube angle all work in conjunction to determine
the behaviour of a bike.

2. The importance of the above measurements and proportions on bike dynamics are unknown.

3. Each frame builder will come up with a satisfactory solution when designing a bike because the
umbrella of rideable bikes is enormous, given the vast number of
frame designs that have worked, so far.

4. Weight distribution is crucial in the design of bike frames, however its importance escapes the general audience because
it cannot be easily encapsulated in a number like angles, trail and rake can.

Dazza, how do you arrive at the stability number that you quote in your frames?

5. Elaborating point 4. further, it seems that front mass located in front of the steering axis and closer to the ground, i.e.
lower than mass on top of the saddle, further aids the coupling between leaning and steering. As such, the front of the bike falls
sideways faster than the rear and this further helps steering and realigning the bike.

digression
============
Pino Moroni seems to have nailed this decades ago. He actually designed bikes with longer chainstays to transfer weight
to the front along with shorter top tube and longer and lower stem. Congratulations, Pino.

my conclusions
===========
1. It is possible to design examples in which the absence of one parameter requires to intensify the presence of others
to compensate, i.e., bikes with negative trail might require lower front mass distribution. The number of trade-offs and combinations among these elements is infinite and all might arrive at a satisfactory bike.

2. I doubt that the research of Papadopoulos, Ruina, and Schwab can come up with a formula or collections of them that would help in designing frames that could surpass what is currently being designed. If we had been waiting for a physics explanation of bike dynamics until now, we would have missed an awful lot of riding.

3. Ultimately, it is the job of a competent frame builder to determine the harmonious proportions of trail, angles or lengths and weight distribution to offer clients the bike that allows them to ride in their preferred manner.

4. Clients should learn to trust framebuilders more. The internet disseminates information but not knowledge.

My purpose is to bring this evidence to your attention and may be trigger the discussion on frame design between framebuilders and authors of the paper. My guess is that we could benefit from the exchange of ideas.

As usual, at your service.
SteelRules.

[steve garro]

This is a classic and interesting experiment on "unridable bicycles"
http://socrates.berkeley.edu/~fajans...onesBikeBW.pdf
I came upon my conclusions by two decades of racing, bike touring the world, being a paid mechanic since 89' and a life of two wheels.
No formulas involved. It took decades, no quick way around it I figure.
Still cool stuff to ruminate upon, for sure.
- Garro.

[EricKeller]

The mechanisms that couple leaning to steering have been exclusively thought to be:
i). gyroscopic effect of the wheels
ii). trail.
The authors show that none of the above are sufficient conditions on their own for stabilising a bike. Other things
like front mass location and steer axis tilt also contribute crucially.
All of them are necessary, none of them sufficient in isolation.

I have all the respect in the world for people that write papers about bike stability. I think it's very difficult to actually do much that is useful for framebuilders. It is fairly clear that the necessary and sufficient condition for a bike being ride-able by a human is that you need to be able to have the center of gravity of the bike balance over the center of the reaction forces acting on the wheels. But only in a time-averaged sense.

[Human Epic Jolt]

Main Mike can you dig out the equation for working out target time in a yacht race?

[e-RICHIE]

I have a good friend whose name is Gene.
Once in the mid 1980s, he related this to me and it's been imprinted ever since atmo...

Sometimes the only answer to the question "Why?" is "Because."

[EricKeller]

Sometimes the only answer to the question "Why?" is "Because."

also applies to academic work on bike stability. Mathmaticians, physicists and engineers often do things that nobody sees a practical use for, only to become important years later.

[SteelRules]

Mathematicians, physicists and engineers often do things that nobody sees a practical use for

and sometimes uncovering facts does not mean that solutions are available or the general audience are prepared to accept them.
Case at hand: finite element analysis concludes that frames are vertically stiff enough but not laterally.
This means that frames get out of lateral plane due to pedalling. According to this:

1. Megatubes increase size in wrong axis, which only increases harshness of ride and weight.
However, megatubes might look cool and are perceived as aerodynamic.

2. Would be difficult to justify increasing the size in the axis that opposes forward movement
on aerodynamic grounds.

3. I take that chainstays cannot change the direction of the oval due to clearance issues with the wheel.

4. Drive side chainstay should be stiffer than non-drive side chainstay.

Then, the best compromise is old fashion round shape tubes that add stiffness in all directions.
XL round tubes help.
Our ancestors kept getting it right...

[e-RICHIE]

A rider has to fit on a bicycle. And the bicycle has to work for its intended use. For these to happen successfully and simultaneously, the window of design is very small. If someone from academia figured all of this out before the industry did, by either trial and error OR through liaisons with the sport, I would be surprised to learn this atmo. I know of no one in my trade who regards the science and studies cited in the OP. That is not to say that the findings don't speak to anything we might do. They might explain why the choices we make work the way they do, but I'd wager that the making came before the math. Ride lots. Look around at what preceded you. Leave it alone if it works, or drive it forward with your own imprint if you believe a better product will result. If you don't ride, you can't get it. I think there is no substitute for racing miles, but any miles ridden are better than an equation.

[Mike Mcdermid]

A rider has to fit on a bicycle. And the bicycle has to work for its intended use. For these to happen successfully and simultaneously, the window of design is very small. If someone from academia figured all of this out before the industry did, by either trial and error OR through liaisons with the sport, I would be surprised to learn this atmo. I know of no one in my trade who regards the science and studies cited in the OP. That is not to say that the findings don't speak to anything we might do. They might explain why the choices we make work the way they do, but I'd wager that the making came before the math. Ride lots. Look around at what preceded you. Leave it alone if it works, or drive it forward with your own imprint if you believe a better product will result. If you don't ride, you can't get it. I think there is no substitute for racing miles, but any miles ridden are better than an equation.

My fourteen month old must be a fucking genius she just figured out how to balance on a bike ....see no PHD or analysis needed

[Mike Mcdermid]

Our ancestors kept getting it right...

a stuck clock is right twice a day, not sure if second moment existed before the bicycle but i hazard a guess it did


Ps is there a "reply" button as opposed to "reply with quote" i keep replying with quote it sounds like i was having a dig in the previous post

[steve garro]

Ps is there a "reply" button as opposed to "reply with quote" i keep replying with quote it sounds like i was having a dig in the previous post

right down there in the left corner, M.
- Garro.

[EricKeller]

If someone from academia figured all of this out before the industry did, by either trial and error OR through liaisons with the sport, I would be surprised to learn this atmo.

We know they didn't. The academics are not trying to change bicycle design. They are trying to further our understanding of physics or mathematical modeling of nonlinear systems. The dynamic I see is that people in the cycling community see these stories and incorrectly run with them as being somehow meaningful to cyclists. I'm reminded of the self-righing bike of a couple of years ago, which was a very interesting result from a mathematical point of view and almost entirely uninteresting from a framebuilder's point of view. Yet people kept bringing up the story like it was going to change bicycle design somehow. Successful academics are adept at self promotion, and having a meaningful result in the field of bicycle dynamics seems to be a good way to get notice in the popular media.

When I said that this work may someday be recognized as important, I didn't mean that it would be important to to bicycle design.

[Human Epic Jolt]

hmmm..

Poe vs Heraclitus.

This line of text will consume a LOT of energy. More than it's ever been worth. /wrap