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Frames and Framebuilding -- 1970 - 1979 (Part 3; Stability)

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Frames and Framebuilding -- 1970 - 1979 (Part 3; Stability)

Old 02-04-20, 10:09 AM
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SpeedofLite 
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Frames and Framebuilding -- 1970 - 1979 (Part 3; Stability)

Four articles, mostly by Fred DeLong, concerning bicycle stability.









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Old 05-09-22, 04:40 PM
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steelbikeguy
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I appreciate the link to these articles! Bike stability is a topic that many of us can relate to, especially if you've ever had a frame shimmy on a fast downhill!
I've read the articles, and came away thinking that very little is actually known about bike stability.
Let's review these articles and what they say.

The May 72 article by Mr Soron discusses trail (a.k.a. caster) and the effect on stability. The conclusion is that more trail provides more stability. The author doesn't define stability, but seems to equate it to the ease of maintaining control in emergencies.

The May 72 article by Mr. DeLong discusses stability in high-rise bikes (like Schwinn Stingray). There is some discussion of using hands-off riding as a proxy for stability. There is some discussion of braking too, which seems to be more of a discussion of safety than stability.

The June 72 article by Mr. DeLong looks at some testing by Dr. Jones. This hits on a few topics that relate more to what might make a bike unrideable. There is some talk about using an extra counter-rotating front wheel to cancel the gyroscopic effects of the original front wheel. The bike could be ridden still. Same for tests with a wide range of trail... the bike can still be ridden. No particular mention of stability per se.

The Sept 72 article by Mr. DeLong actually does get into some aspects of frame design, such as trail and frame drop as a function of headtube angle and fork rake. There is a mention that the American Bicycle Standard has a spec of 3/16" of frame drop. There is no actual discussion of stability; either to define it or discuss how trail or frame drop affects it. There is a graph, figure 3, that graphs trail versus some undefined quality that varies from poor to excellent. Is this handling? Is it stability? Perhaps Mr. DeLong knew, but I don't.

These articles are better than most of their era in regards to discussions of handling and stability. Still, I feel they fall far short of what they could be.
First, they should define stability! What is stability, and how to we measure and quantify it.
Once they've defined it and specified how to measure it, they should vary the parameters of interest (trail, frame angles, etc.) and show how changing a parameter changes the stability.

Perhaps I'm more critical because of my background in electronics. In control theory, there are specific criteria for what constitutes a stable control loop. There are detailed ways to measure a control loop's stability, and there are less formal methods that provide quick and useful indications of stability. The world of bicycles appears to completely lack this, which does surprise me. I'm fairly sure that some mechanical engineers are trained in control theory, especially back in the 70's when control loops were mechanical and not electronic (i.e. the governor on a diesel engine, or even the one on a steam engine). I hope that one or two of these mechanical engineers have been consulted by the bike industry??

Well, to be honest, the bike industry does seem to do its best to avoid any actual involvement by engineeers. Perhaps someone can prove me wrong and show me some sort of actual analysis of a bike's stability, the feedback that affects stability, and the gain and phase margins that define the acceptable threshold of stability?

Steve in Peoria
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Old 05-09-22, 08:33 PM
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panzerwagon 
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Originally Posted by steelbikeguy View Post
These articles are better than most of their era in regards to discussions of handling and stability. Still, I feel they fall far short of what they could be.
First, they should define stability! What is stability, and how to we measure and quantify it.
Once they've defined it and specified how to measure it, they should vary the parameters of interest (trail, frame angles, etc.) and show how changing a parameter changes the stability.

Perhaps I'm more critical because of my background in electronics. In control theory, there are specific criteria for what constitutes a stable control loop. There are detailed ways to measure a control loop's stability, and there are less formal methods that provide quick and useful indications of stability. The world of bicycles appears to completely lack this, which does surprise me. I'm fairly sure that some mechanical engineers are trained in control theory, especially back in the 70's when control loops were mechanical and not electronic (i.e. the governor on a diesel engine, or even the one on a steam engine). I hope that one or two of these mechanical engineers have been consulted by the bike industry??

Well, to be honest, the bike industry does seem to do its best to avoid any actual involvement by engineeers. Perhaps someone can prove me wrong and show me some sort of actual analysis of a bike's stability, the feedback that affects stability, and the gain and phase margins that define the acceptable threshold of stability?

Steve in Peoria
Although the concepts are similar, the stability of an electronic feedback loop is much easier to quantify and analytically examine due to the inherent closed system it encompasses. For example, an electrical current only passes through a conductor, so it's easy to trace and map out and quantify all sources of stimuli into the feedback loop.

On a bicycle, in addition to the dynamics of the frame, we have a rider of unknown mass, dimensions, weight distribution, strength in each active body part, velocity, tyre size, road surface friction, gradient, not to mention wind velocity and direction. I'd imagine that in order to compute metrics like gain/phase margins, each component would need to get modeled analytically and approximated, as many of them are nonlinear. Even if the equations were tractable, the variables are so many that the loop order would be too high to meaningfully glean these metrics.

More importantly, would it actually yield anything practically useful? I suspect most frames are created with ample margin of stability for most real-world cases, through an empirical process. Having two different (positive) phase margin numbers for a couple frames would not necessarily point to superiority of one over the other, and differences would likely be dominated by other qualities.

Last edited by panzerwagon; 05-09-22 at 08:39 PM. Reason: typos
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Old 05-10-22, 09:41 AM
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Originally Posted by panzerwagon View Post
Although the concepts are similar, the stability of an electronic feedback loop is much easier to quantify and analytically examine due to the inherent closed system it encompasses. For example, an electrical current only passes through a conductor, so it's easy to trace and map out and quantify all sources of stimuli into the feedback loop.
true.. if the loop is completely electronic with well quantified components, then it's just a matter of knowing control theory and doing the math. In other cases, such as thermal control, there is the need to characterize the device being heated, the heater itself, etc. The goal is usually to come up with some simple linear model of the device and heater, so as to make suitable for analysis. There will still be a need to run some tests and see how it turned out, and possibly tune the control loop parameters afterwards.

Originally Posted by panzerwagon View Post
On a bicycle, in addition to the dynamics of the frame, we have a rider of unknown mass, dimensions, weight distribution, strength in each active body part, velocity, tyre size, road surface friction, gradient, not to mention wind velocity and direction. I'd imagine that in order to compute metrics like gain/phase margins, each component would need to get modeled analytically and approximated, as many of them are nonlinear. Even if the equations were tractable, the variables are so many that the loop order would be too high to meaningfully glean these metrics.
The frame, wheels, bar & stem, etc. do act as one or more springs. The rider would be considered the primary mass of interest, and might even add a bit of damping. I think it's too early to say how difficult it would be to model, since we haven't even defined what "stability" means yet.

As for variables such as velocity, tire size, road surface roughness, etc., it's good to be thinking of those things. I imagine that it would be best to start with some standard values for a nominal case. If there is a desire to characterize over a range of one of the variables, that could be done too. There would certainly be value in knowing how each of the variables affects stability, since there may be a desire to change one variable without changing overall stability, so you'd want to change another variable to compensate. An example would be changing tire width in order to compensate for a change of frame stiffness.

Originally Posted by panzerwagon View Post
More importantly, would it actually yield anything practically useful? I suspect most frames are created with ample margin of stability for most real-world cases, through an empirical process. Having two different (positive) phase margin numbers for a couple frames would not necessarily point to superiority of one over the other, and differences would likely be dominated by other qualities.
We've had people discussing "stability" for decades without being able to define or measure what they mean by that term. Bike manufacturers are designing bikes mainly by sticking to what has worked in the past. Does this work? Maybe?? I know that when I started riding recumbents, it took about 1000 miles to feel reasonably proficient handling them, which suggests that the standard for stability or handling is much different from upright bikes.

As I mentioned earlier, there are metrics beyond phase margin for evaluating stability in electronics. The response to a transient can indicate whether it is overdamped or under-damped, suggesting how close it lies to a neutral stability point or how close it is to breaking out into oscillations. I've used this informally to evaluate a bike's tendency to shimmy.

But... the first hurdle is to figure out what the desired characteristics are, and then how to quantify them and measure them. Without this first step, the rest has no meaning.

Steve in Peoria
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Old 05-10-22, 09:56 AM
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Late 70's and I think presented in 1980, the Italian Cycling Federation FCI concluded an evaluation of race bike design and handling, made the results available to the Italian mfgs. Gios and Pinarello come to mind as brands that bought in. The report has long been taken offline but it resulted plainly to the shorter top tube, slightly steeper and resulting longer stem extensions to make the weight distribution more equal on the wheels. I am not sure that was a good thing as the pro peloton just looks more "nervous" to me compared to earlier times, the bars are much wider too, so plenty of elements to track.

A bit later the Eddy Merckx brand announced a 500,000 euro academic grant for a study of bicycle design and stability, with the results to be made public. Never saw a result of that unfortunately. Merckx did have opinions of bike design from his experience and the "century" geometry as the brand called it was reasonably conservative.
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