[python] Re: Large Virtual Trail

  • From: Michael Rienstra <ageless@xxxxxxxxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Mon, 20 Sep 2004 07:27:25 -0700

Jürgen M.,

It would certainly be a cool design, but I have no idea whether or not it would be rideable! To find out, you (or someone) would have to build one, or read up a bit on bicycle steering theory and if they say it isn't rideable, you could choose to believe, or try it anyway!

OK, that probably wasn't too helpful! :-) Here's a more concrete analysis. The most obvious thing is that the virtual steering axis will be way way out in front of the front wheel -- see your gif below, I just added some lines pointing out where the virtual steering axis will be in the two positions you illustrated.

GIF image



My initial thought is that it would be nearly impossible to steer, because of the intense self-centering effect. And after further deliberation, I forgot what else I was going to say... Suffice to say, it will act like the castor on a shopping cart -- ideal for walking the bike. Almost like the opposite of a Python? Fun to think about.

So moving on, there are several things you could do to reduce this -- basically it all comes down to getting the virtual steering axis closer to the front wheel axis. One thing that would help is attaching the linkages slightly behind the front wheel axis, instead of directly on it. Another is to increase the amount of 'toe-in', or in other words, shorten the coupler link (C) and/or shorten the side links (S) and/or lengthen the ground link (G). Oh, and of course, shortening the side links is the same as attaching the linkages behind the front wheel axis, so I just made the same suggestion twice.

Oh, although I guess you could also shorten the side links by moving the ground link forward, for example, you could have very short side links (say Y = 8cm), with the coupler link 3 cm behind the front wheel axis (let's say R = 4cm), and the ground link will be oh 12cm. That would make S = 8.94cm and C = 4cm. Whoops, no room for the drivechain. Oh well! The picture below is the beginning of an analysis, before I realized that I was getting a bit carried away! The grey star at the top of the gif shows the position of the virtual steering axis when the steering is neutral. I was going to add the wheel, and the other position's info and so forth...

GIF image



Anyhow, there are some tricky constraints, but there might be a good geometry in this range. Let me know if you need any help with figuring out how to analyze the possibilities.

Michael

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