The apparent stability of a mechanical system: a bike on railroad tracks

[Spinnor]

I watched a video of a rail bike and if I had to I would have guessed that what I saw in the video was not a stable mechanical system, stable to the rear wheel coming off the railroad track. Please watch from 1:54 to 2:30 to understand the setup of this rail bike below,



I would have guessed that the back wheel would be constantly be coming off the rail track. In the comment section it was asked,

"I am surprised that the back wheel stays on the rail even more so since the rail is slightly crowned. Does it ever come off while you ride?"

The owner of the rail bike replied,

"The long story is that it never came off...ever. until my last trip where the wheel was off center, the rails were very worn, and it was raining. That's caused us a lot of grief."

It seems there is some inherent stability in this particular setup to the rear wheel coming off the track, without giving it away does anyone see it?

Thanks.

 

[Baluncore]

It is explained at 28:30 in the original video. Read the transcript.
"One of the most common questions I get asked about the rail bike, is how does the rear tire stay on the rail, and that's because it's not fixed, it actually sways back and forth. I usually respond in the comments, and tell people it just follows the front fork, but they keep asking over and over and over again, and I think they know that I'm lying.
Let me tell you the truth the way that the rear wheel stays on the railway track is through a technology known as Insanotron 5 million, it's a patent pending, unreleased system, that uses a circumferential electromagnetic pulse, to keep the tire attached to the railway. It's powered by the crankshaft of the bike, so as you pedal, it sends power to the device in the center of the wheel, transfers it out to the perimeter, to the circumferential magnet, and that's what attracts the metal to the tire, now you're probably wondering, Steve, with such a high rotational mass, how do the poles not reverse and resist each other? That's easy, what we do is we fill the tire with an inert gas, such as helium or hydrogen, it's a very safe system, it's never let me down, and now you know the truth. "

 

[Spinnor]

I eventually watched most of the video, must have missed that part, thank you.

 

[Baluncore]

If you analyse it for long enough, you will eventually reach an explanation.
https://en.wikipedia.org/wiki/Pursuit_curve

 

[256bits]

I eventually watched most of the video, must have missed that part, thank you.

The late Arthur C. Clarke famously said, “Any sufficiently advanced technology is indistinguishable from magic.” magnets."

 

[256bits]

Another rail rider with a different setup:

 

[Lnewqban]

It seems there is some inherent stability in this particular setup to the rear wheel coming off the track, without giving it away does anyone see it?

Perhaps the self-centering effect of the crowned rail on the contact patch of the rear tire has something to do with proved effect of crowned pulleys on flat belts:

https://woodgears.ca/bandsaw/crowned_pulleys.html

 

[Baluncore]

Perhaps the self-centering effect of the crowned rail on the contact patch of the rear tire has something to do with proved effect of crowned pulleys on flat belts:

Then it should track equally well if you ride backwards.

The bicycle wheel is more crowned than the top of the rail, so the rail track must be under tension, with one side of the track stretching more or less than the other.

 

[Spinnor]

Perhaps the self-centering effect of the crowned rail on the contact patch of the rear tire has something to do with proved effect of crowned pulleys on flat belts:

https://woodgears.ca/bandsaw/crowned_pulleys.html

Suppose the rails were wide and with no crown and consider the following,

When you are peddling as long as the rear wheel does not follow the path of the front wheel there will a component of the force from the rail on the wheel pushing it towards the front wheel path.

Now add the crowned rail with the wheel off center, the weight force will now have a component tending to move the rear wheel away from the front wheel path. Add a wet rail and stability becomes more of a problem. With the pulley problem I don't know if the same "steering" effect comes into play?

I will have to think more about when you are coasting how the forces change on the rear wheel.

Thanks.

 

[Ivan Nikiforov]

It seems to me that the mechanism can be explained as follows. The bike receives movement energy only when the right or left pedal is pressed. In the interval between pressing the pedals, the bike moves by inertia. Thus, in this mechanism, during the time between pedal depressions, the system moves by inertia and since the center of mass is in the front part, a stabilizing moment acts on the bicycle frame, which aligns the rear wheel on the rail. That is, the cart pulls the frame by inertia and the wheel aligns.

 

[Baluncore]

So long as the front of the bicycle is positioned above the centre of the rail, the rear wheel will follow in a pursuit curve, close to the centre of the rail.