[python] Re: More virtual pivot thoughts (tire scrub)

Hillel,

Hopefully very soon there will be two prototypes (Laurent's and mine), and a lot of things will become clear.

I don't think that this will be much of a problem though. I wrote about it a little in an earlier email, which I will include here and then elaborate upon.

On Sep 8, 2004, at 10:03 AM, Michael wrote:
AxOfx = The offset of the center of the wheel along the x axis. It is zero when the steering angle is zero by definition. I think this might make the bike hard to steer, particularly at low speeds or when stopped, because the wheel's contact patch has to move as the steering angle changes. Of course, the normal Python has this same effect, only to a greater degree (since you are swinging the contact patch through an arc as you steer), and I haven't heard anyone complain about it, so this might not be an issue. Still, smaller is probably better, particularly when it comes to balancing at very low speeds or making crazy parking lot maneuvers. The more I think about it, it seems like this will just mean that your CoG will move and the tyres will stay still -- I assume that this what happens when you steer the python sharply when stopped -- it bends in the middle, so steering left will move your weight to the right, rather than forcing the front wheel's contact patch to move to the left (or I suppose since most of your weight is in the front it would be more likely that the rear wheel's contact patch would move to the left, but in any case your weight would shift before either of those things would happen).

You can read that email online at http://www.freelists.org/archives/python/09-2004/msg00003.html (you probably have a copy of it in your email as well, from 2 weeks ago)

I haven't gotten any specific feedback on the details of that email, so if you or anyone else would like to comment on my calculations or speculations, I would be most appreciative. I know it's very long, but I thought it was pretty clear and well written -- certainly by my usual standards! I would also appreciate feedback like: "You might think it's easy to read, because you wrote it, but it's actually long and painful!". ;-)

Maybe after the prototype is built, I will put together a webpage with illustrations showing the different measurements and so forth, so that it isn't necessary to visualize it all in your head.

So, I think that quote says it all. I could be wrong of course, but to me it makes a lot of sense. Oh, and another way of thinking of the effect of a wheel trying to move sideways: I used the concept of your CoG (center of gravity) moving sideways, but perhaps better is to say there will be some rotation around the rear tire. Same thing, really.

When comparing the Python or the Vython to a trike, remember that there are only two wheels, not three, which means that translation of one wheel can result in rotation of the other, unlike a trike.

Michael

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