Speaking of railbike, I have two Peugeot UO-8s. One seems to have the fork bent forward, and measures out to a 70 mm rake with 30 mm trail! The other is a little more conventional, but I haven't ridden it yet. I think it's going to become DIY Rando Cheapo. The long rake one DOES ride as if on rails - with bedsprings!

I need to test it going downhill, to see if it shimmies much.

No worries!

My '74 Raleigh Comp has both a long wheelbase and a huge amount of BB drop (8.5cm by my measure). I would say it does everything compromisingly well, with the exception of very high speeds (>46mph). Once I spin out, I pretty much have to press one knee to the top tube to damp out the vibration. My 2001 Spectrum Ti, OTOH, does everything UNcompromisingly well, including high speeds. It has a much shorter wheelbase and a higher BB.

If you're thinking abotu stability issues at that speed, you pretty much have to be talking about descending. When you descend, the nose-down condition of the bike makes shimmy more likely, though I doubt thats all there is to it. The difference between your Comp and the Spectrum is significant, but consider, the Spectrum is a modern rigid desigh with oversized tubes, while the Competition is standard diameter and very likely to be more flexible.

the reason I was looking at the scientific literature was to see if there was an explanation of shimmy. There doesn't seem to be any rhyme or reason to it, and nobody has ever looked at the phenomena from a modelling point of view. There are a lot of opinions though. Interesting that you should mention descending as being a factor, never really thought about that.

I've seen people talk about flexibility entering the picture, but I really don't think that's it. I'm pretty sure that the front wheel tracks the shimmy to some degree, and certainly most of the motion is in between the forks and the frame. I think every bike I've ever owned would shimmy, my current bike shimmies violently on steep downhills if I let it.

^^^^
FWIW
My first bamboo bike was quite flexy and it had very little in the way of shimmy.
My salsa is much more rigid but shows more shimmy when loaded than my bamboo bomber (it tracks rather well when unloaded).
It may have something to do with balancing the load between the front and rear wheel is my best guess.

Probably some relationship between wheel flop, descent angle/weight distribution and tire compliance. My commuter shimmies badly if I lean back no hands. I should check the alignment on that frame though.

The standard "common sense" model of bike handling has been conclusively debunked so most of the comments in this thread can be disregarded.

If you have a look at the equations in this paper from Cal polytechnic two things become apparent:

Increasing the height of the BB (and by extension the centre of mass of the bike / rider) decreases the trail for a given desired steering "feel" while increasing the trail tolerable for a given amount of wheel flop.

The other effect which is usually not considered is that a higher CG will therefore more further WRT the steering centre with a change in effective gravitational vector (braking, riding hills) .

That's an interesting assertion, but I'm not sure that I follow your conclusions. And your last sentence doen't make sense.

Thanks for the link.

You're right, it's not the distance to the centre of mass that changes, it's the distance between the steering centre and the line of force through the centre of mass due to the effective gravitational vector.

I poked around on that guy's site, and the reason he made those equations is that he is interested in designing recumbents. He says that diamond frame builders have good rules for designing their frames, but bent designers don't. One thing I've learned is that equations don't always help you with people's perceptions, although they can complete the puzzle.

It's not like there are two views, one right and another wrong. There are a number of "traditional" views, there's the Cal Poly one, there's a very extensively researched one by Andy Ruina, Jim Papadopoulos and their students, and in their body of work, an extensive literature review evaluating their new work against a very broad collection of historical papers on bike handling. This history dates back to the 1890's if not earlier.

So which traditional view are you talking about?

This link https://en.wikipedia.org/wiki/Bicycle...cycle_dynamics is a very good summary of single track dynamics. Reference 1 in that article is the Ruina/Papadopoulos paper. It had a significant hand in debunking a lot of old explanations. Recommended reading, though the math is not the easiest.

Equations are good tools for engineers, if they're simple enough. Combined with an understanding of the human factors, they can improve the engineer's ability to design a well-liked bike that also handles well.

I wonder if he goes on to describe his various handling criteria in more descriptive terms, and if he has tried to use his methodology on an upright bike design. Seems to me he has the ability to fine-tune a design at teh outset, and this could possibly benefit the design of conventional bikes.