[python] Re: Jetstream

  • From: Stephan Schöling <stephan@xxxxxxxxxxxxx>
  • To: python@xxxxxxxxxxxxx
  • Date: Wed, 28 Feb 2007 19:38:48 +0100

It took over 1 hour to read that ;-)

May i try to summarize with easy words, please correct me if i'm wrong.

The frame itself isn't stable, the frontpart flops sideway (by gravity).
The self-centering effect works against that, is making the bike stable.
If I put some weight to the seat (gravity too), i increase the self-centering effect. My python frame (without rider) balances at ca. 8 kg on the Seat. (Borderline between the two effects)
The riders weight increases the self-centering effect.
The pivot angle decrease: increase wheel flop, increase self centering (up to a point). (we still have to find the proportion) neg trail increase: probably not much influence on wheel flop, increase self centering.

Luggage placed near the rear wheel doesn't have much effect to the stability of the bike, because its away from the SBB.

You wrote, your formulas are wrong ?
What is wrong with them ?


dirk.bonne@xxxxxxx wrote:
Thomas P Jahn wrote:
There is a borderline where the self centering force compensates the
"weight of front part + leg-feet weight (i.e. that what causes wheel
flop)".That is where the balance is.

I am still confused about the "compensation" or balance. What exactly has to be compensated?
Uhm, the flopping of the front part. Self centering compensates the tendency for the front part to flop over.

Turning makes the front part come out of its left-right balance and gravity will pull it down. That is the wheel flop. Self centering counteracts wheel flops, giving the tendency for the bike to straighten up. You tried Torbens bike you certainly noticed that when not sitting on the bike how unhandable the bike was, but once sitting on the bike the bike straightent up.

The borderline is where the tendancy to flop is compensated by the tendency to straigthen up.

Jürgen writes on his homepage:

"*So 63° is the best pivot angle?**

*Not really, but it is close: The optimal pivot angle is exactly the one where the weight of the whole front part (including the riders legs) is in equilibrium with the rest of the bike (including the rest of the rider). This is different with every bike and rider. To be more exact: It is the angle where the wheel flop effect is nullified by the seat rising effect.
Yes that is this borderline I am talking about. I do not agree with the 63degrees though, as trail aspect is left out.

* angle decrease: increase wheel flop, increase self centering (up to a point). * neg trail increase: probably not much influence on wheel flop, increase self centering.

The heavier the rear part is in relation to the front, the closer the pivot angle may approach the desired maximum seat rising angle. "
Not sure I understand that.

So, here it sounds as if the compensation is to increase rear weight versus front weight.
Moah, I would not say it like that. There is not a real choice anyway.

And how would it in theory relate to the pivot angle?: The heavier the rear part compared to the front, the more shallow the angle may be??? But you Dirk say something about pivot angle versus front weight. And that is probably something that cannot be put in numbers. like degree angle / kg weight???
What could be done for a reasonable weight distribution calculate a graph that plots the borderline pivot angle in function of the trail.

I made some formulas in the past that could aid in this, as they can calculate the seat rise for all the input parameters. But alas the formulas are wrong :-(

To increase the confusion:
In the other thread "81 inseam" you just said that you did not experience any difference with the luggage. So, weight plays no role??
luggage = 14kg. that is not so much. I should not have said the word heavy. I was more thinking on wind effects. I was just agreeing with 25Hz-es point.

Weight to the rear of the bike does have an adverse effect, but has nothing to do with wheel flop. When you have a lot a weight on the *far* rear part your bike gets wobbly. Every bike has that effect, python much.

Then again its sounds as if you on your babypython must be close to wheel fleep. But still it never happened. That sounds really good. And your pivot angle is 60° as I can see from the survey. What about the weight (rear versus front?) Or does it matter at all in your case??? It seems not!?
On my flevo I can ride without steering damper at all. Without steering damper the flevo with its positive trail has such a big flopping tendancy that the front part litterally bangs to the ground if you take your feets from the pedals. I find I can ride the flevo even comfortably. I guess it is all what one is used to.

more confusion


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