[python] Re: report: pythoon first ride
- From: Dirk Bonné <dirk.bonne@xxxxxxx>
- To: python@xxxxxxxxxxxxx
- Date: Tue, 13 Oct 2009 10:27:52 +0200
Hi Dirk and Ken,
Changing the pivot angle is much less work then rebuilding the bike ;-)
You would be disappointed how little the wheelbase changes when turning
my son around and have the rear wheel between his legs. I have tried
that design out when drawing up the pythoon and the change was only
10-15cms. The advantage would have been that the passenger would have
had a better view. The downside was that using the bike for cargo would
be worse. I assume he will sit max 2 years in the box, after that he
should be able to safely sit on our (modified) tandem and take an active
part in travelling.
Dirk
Ken Pendergrass wrote:
> Hi Dirk and Dirk,
> I had a very similar thought just shorten the wheel base. I think the
> rear wheel between the passenger's legs is an elegant solution. It
> seems you may have to mount a seat which could be removable to cary
> only cargo. I like the idea of the cargo area extending back behind
> the rear wheel. You could even make the rear of the cargo area
> removable so that if you don't need a truck you don't have to ride
> one. Best of all this would get your wheel base back to that of a
> normal bike, where you know your pivot angle would work. However this
> being your bike not mine you doubtless have our own ideas of how to
> fix it.
> best regards,
> Ken
> On Oct 12, 2009, at 5:11 AM, dirk@xxxxxxxxxx wrote:
>
>> Hi Dirk,
>>
>> Great design!
>> I also think along the lines, that the self centering effect is
>> related to the torque as a driving force to bring the bike back into
>> the straight position.
>> Many dimensions, like mass of rear and front part, pivot angle,
>> distances to the wheels, weight of the driver (weight of the drivers
>> legs before the front wheel axis..), etc, have a major impact on
>> this, and not all of them are used by your program. (You should be
>> easily able to verify the correctness of your calculated data by
>> measuring pivot/ground distance in straight and defined angular
>> positions.)
>>
>> Beyond that, you would have to consider, how the mass is distributed
>> - relative mass with respect to distance from pivot (front & rear
>> part). This, and the change in seat height/turning angle, make up for
>> the correcting torque. The more mass you lift and the higher, the
>> more correcting torque you get.
>> However, when building bigger, heavier bikes, increasing the pivot
>> angle, to have less torque probably gives you more wheel flop. It
>> probably turnes out that if the overall mass of the bike rises, you
>> need more force on the pedals to work against that. You could
>> probably fine tune a little, but what happens when you ride it with
>> and without passenger?
>>
>> I wonder, if the rear wheel would sit betweeen the passengers legs,
>> far closer to the pivot, how the bike would handle. In this
>> configuration, the mass of the "optional load" would sit around the
>> rear axis, somewhat balancing itself around it, and not changing the
>> effect on the pivot that much. At the same time, the wheelbase is
>> greatly reduce, generally givin the bike a more agily feeling.
>> Also, there is the inverse pendulum effect as seen on any bike. If
>> you tip over to the left, how much do you need to steer/drive towards
>> the left side, in order to overcome this and start tipping to the
>> right? The longer the wheelbase, the more steering it takes to get
>> back to the other side. The heavier the bike is, the more pushing
>> force you need to put into the pedals, to work against the mass of
>> the bike and its load.
>>
>> It would be great, to have a program, that would caculate/simulate
>> all of this...maybe we could contact some university research folk :-)
>>
>> greetings,
>> DirkS
>>
>>
>> > The bad news is that the bike steers like a truck. Before I started
>> the
>> > project I actually made a graph comparing the pythoon, the baby python
>> > and the p3 and I didn't see anything unusual. I recalculated and
>> put it
>> > on picasa:
>> >
>> > http://picasaweb.google.com/dirk.bonne/Pythoon#5391423541426608210
>> >
>> > It shows the change in seat height when turning the front part. The
>> > pythoon has a stronger self centering then baby python, but still less
>> > then the P3 of Jürgen. But in practice the self centering feels very
>> > strong, stronger then I remember from trying a 26" python (birdcage of
>> > Serge). Either my program to calculate the curves is wrong (quite
>> > possible), or it has to do difference with weight distribution
>> caused by
>> > the long wheelbase. On this bike there is more weight on the front
>> wheel
>> > then on any other python. In any case this shows that the graphs
>> are not
>> > useful at all. What is needed is a graph depicting the needed
>> torque to
>> > sustain a turn.
>> >
>> > I am considering my options. I don't like the idea of putting a wide
>> > steer on the bike to help with turning (the current steer is very
>> > narrow). I think I am going to increase the pivot angle. It is now
>> > already on 70degrees - I could try 80degrees. This is a lot of work
>> > because I have to re-braze on both the rear and the front. But on the
>> > other hand it would be interesting to see what such a high pivot angle
>> > has as an effect. I calculated the graphs:
>> >
>> > http://picasaweb.google.com/dirk.bonne/Pythoon#5391428883434182962
>> >
>> > The graphs show that the self centering (in seat height change) should
>> > be smaller even then then baby python. In term of torque, I do not
>> know.
>> >
>> > Dirk
>> >
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>>
>>
>> http://dirk.steuwer.de
>
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