That's a good point about the angles wrt *gravity*. My original variable
pivot idea would help here but, as I said before, the end is eventually
inevitable. The only way out, as you say, is to change that force direction
on the front wheel (pedalling or rear brake) to put the pivot in tension.
Cheers Daryl - it's the nature of the beast :-)
From: "25hz" <25hz@xxxxxxxxxx> Reply-To: python@xxxxxxxxxxxxx To: <python@xxxxxxxxxxxxx> Subject: [python] Re: frontend Date: Wed, 16 Nov 2005 21:10:31 -0500
On a downhill, while coasting, in my experience, two things happen. Gravity
is operating in the same plane regardless of road angle, but what does
change is the python's angle, and more critically, the pivot angle. In
general terms, the more the pivot is angled back, the more stability the
rider's weight gives the bike. On a downhill, the angle is stood up closer
to 90 degrees and you lose the effect of self centering because gravity is
still pointing in the same plane but now the pivot angle relative to the
plane of the gravity has been decreased. The second problem is that while
pedalling, the contact patch on the front wheel is pulling the bike forward
and trying to straighten out the pivot, but while coasting, the contact
patch is now pushing back against the pivot and making it want to bend
either left or right and produces a spooky hydroplane feeling on downhills.
Add to this the fact that now your CoG is higher in relation to the pivot,
the axle and the contact patch and it adds to the instability. Lastly, on
downhills, the traill also increases if measured along a line that is
perpendicular to gravity's plane. Larger negative trail figures tend to
make the bike slow to respond to steering input as well.
While all this maybe sounds complicated, a rider can get used to the downhill sensations and improve their reflexes to cope with it. Also, just about everyone might agree that a fairly successful solution to the problem as well is to keep pedalling to keep the contact patch pulling the pivot straight, and also to lightly ride the rear brake to add extra drag to the rear end which also wants to straighten the pivot. These two techniques at least give some "weight" back to the pivot to aid in stability.
> >> With increasing NT, the coasting force that wants to turn the front part > >> backward will increase as well - most probably much more than > >> the centering force, thus easily zeroing this force out. > > > But why this? The python has proved that the self-centering effect is > > stronger than the stabilizing effect induced by positive trail. What > > is the reason for you to think that this coasting force will suddenly > > take over and increase in force much faster than the self centering > > effect? > > Self-centering force does not change its value or even decreases > when coasting downhill. Coasting force is growing with > the speed. So at a specific speed both forces get equal and > with more speed the coasting force finally takes over. > > >> I wish some ingenious physicist will join this list and help us > >> calculating > >> these effects ;-) > > > I hope to become an ingenious physicist in time ;) As I said earlier, > > I think this is a great subject for my thesis next year, so with some > > luck I'll find something useful then :) > > Looking forward to this ... > > Jürgen.
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