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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