[python] Re: some thoughts

Some gyro observations below

From: "25hz" <25hz@xxxxxxxxxx>
Reply-To: python@xxxxxxxxxxxxx
To: <python@xxxxxxxxxxxxx>
Subject: [python] Re: some thoughts
Date: Wed, 12 Oct 2005 21:38:42 -0400

You are in fact correct, I only changed the rear wheel.  So, as I didn't
think about the effect of the front wheel as far as gyroscopic effect is
concerned, there was an easy way to find out.

I took the BHP and clamped it on my steel table with the front wheel hanging
free and clear over the edge.
I next took the following:
- my electric drill and my high speed grinder
- a hockey puck with a 1/4" hole drilled through the center with a long 1/4"
bolt through it

I put the hockey puck/bolt in the electric drill. I steadied the python's
front end with one hand, turned the drill on (variable speed) and held the
puck against the front wheel. I went from 0 to 48 kph in 10 kph increments
except for the last, 8 kph, and while the wheel was turning, I tried to see
if I could feel any gyroscopic torque on the front end by slowly rotating it
from side to side. The result? Zero.

I tried this with a spinning 20" wheel in my hands with outstretched arms and when I pivot my arms to each side (like a python pivot) I don't feel it at the pivot per se but the wheel wants to lean left or right depending on direction (i.e. pivoting right produces left lean). This would manifest itself as a very slight twisting of a rigid front boom frame (which wont be felt if the bike is clamped to the table). I'm not sure what this would do for stability.

When I tried pivoting the wheel on an axis through the center of the wheel (like a normal fork) it "fought" me. This scenario as we all know does produce stability.

I did experience much gyroscopic behaviour back in my youth when I rode a Suzuki 1100. Leaning into turns definitely got "damped" at speed. Normally not that noticable but at 100mph - 150mph things got REAL slow. At the time newer bikes came out with smaller front wheels which had much quicker reaction. Partly the smaller size made them more "squirrelly" but partly they had less gyroscopic impact for a given speed. What I learned soon on but never completely understood is why when in a lean and you then hit the brakes the bike would FREEZE in its lean. I learned this in the first week coming off the freeway in excess of 100mph and leaned into the offramp I hit the brakes (luckily dual disks bleed off speed fast). The offramp got progressivly tighter but I was FROZEN in my line/lean and the bike would *not* respond. It was like the bikes wheels were cast in concrete. I eventually saw the writting on the wall, got off the brakes, the bike immediately straightened up and I left the offramp in a safe vertical and straight orientation probably at only 30mph. It was a simple matter of reverting to my motocross days, coming in for a landing, turning around on the grass and I was on my way again albeit scratching my head as to what had happened. I guess it was something like "P" factor on large airplanes when accelerating big props. Unfortunately I think this subtle phenomenon is what likely does in a lot of the novice riders simply being reported as "loss of control". A long "shaggy dog" story way of saying be careful what you wish for, stability is not always a good thing ;-)

Anyway, getting back on track, I have yet to try a python so I'm not sure if pivoting right leaning left and vise versa is a good thing or not but if it is bigger wheels may be better.

BTW I have now taken the plunge and obtained 1/2" rod end bearings for my pivot and am exploring how I can revector my existing design. I think it will be worth it though. The ideas of simplicity & tunability convinced me. :-)

Cheers
Daryl

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