[python] Re: Stability Theory
- From: "25hz" <25hz@xxxxxxxxxx>
- To: <python@xxxxxxxxxxxxx>
- Date: Thu, 17 Feb 2005 21:29:45 -0500
So, from looking at the pictures and comparing the stats (prior to
shortening it) the only differences are CG height and pedal to seat height.
So, with the pivot being where it is, is it not safe to assume that the CG
height is important when coasting at speed downhill? With the drag on the
front wheel while coasting, it would be similar to the effect of light
braking with the front wheel. In my head, it sort of looks like this . .
If most of the weight is close to a horizontal line produced by connecting
the front axle and the CG, the drag from coasting can't really move the CG
closer to the front (like braking) and the line connecting the CG - axle,
and the CG - contact patch is pretty much 90 degrees. Possibly the only
direction the mass can shift is left or right, which causes the instability?
Even though the "fulcrum" might be the contact patch, the axle is still the
point that the bike can rotate forward around because it's not actually
braking, rather it is coasting.
Now, if most of the weight/CG is either above or below a horizontal line
going through the axle, it is easier for the weight shift to go forward and
down, or up and forward, which means less weight is trying to shift to the
right or left, which means it's more stable at higher speed?
Perhaps more simply, the closer the CG is to the axle height, the less
stable coasting at speed it might be. If the CG is raised, it might become
more stable, but after a certain point become more unstable again because it
is un-weighting the pivot point too much. ? Conversly, the lower the pivot
point, the more stable it should become because the drag from coasting
should want to push down on the pivot, increasing the reistance to turning
left or right, and making it more stable?
Position of the BB relative to the axle and CG might also be important but
only pedalling, and not coasting (as there is no force being applied while
coasting). If you have a low CG and low pedals there might be an increased
force unweighting the pivot point, while a higher BB might produce more net
downforce on the pivot? If this is true, the P1 should be a little more
stable while pedalling hard than the P3 is?
I guess once we get enough bare road, I can test out this theory because the
two pythons we built are pretty low. Added to this though is the slightly
higher, and less stable, steering angle which should reduce stability.
Maybe the two will cancel each other out, and they will be as "unstable" or
"scary" at speeds around 50kph as the P3.
> Some input for our theory:
>
> http://www.python.tieflieger.net/pics/gallery/Python_P1_side.jpg
> http://www.python.tieflieger.net/pics/gallery/Python_P3_side.jpg
>
> CoG P1: 63%
> CoG P3: 63%
>
> Measured with the seating position to be seen in the two
> pictures. I wonder why CoG is the same, although the
> seat angle is quite different.
>
> Note that BB-FWA of P1 is 30 mm shorter than it was, when
> I rode it. Had to shorten it for Eveline.
>
> Cheers,
> Jürgen.
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