[python] Re: trail vs wheelsize

I think there are a whole lot of variables involved and more importantly, it
can be tough to separate out what change is doing what.  I don't think wheel
size has all that much to do with anything providing you can reach the
pedals.  After that, the rest is pivot angle, CoG height, wheelbase and
possibly BB height.

Sluggish or heavy steering is more likely a result of pivot angle than wheel
size unless a big old heavy and thick steel rim is used on the front.
Downhill speed is a function of rider experience (and possibly
craziness/bravery) as well as pivot angle, I don't think it has anything to
do with wheel size.  A gyroscopic calculator link was posted and going
through all the different wheel sizes and weights, the changes in gyroscopic
torque were extremely small and changed little with wheel size.

We started with a 26/28 python, then went to 26/26 and then I ended up with
a 26/20.  Marcel went from a 26/26 to a 26/16.  On the PC2, I made it for a
26/16 combination from the start (which I still need to take finished photos
of).  Changing wheel sizes does little because the frame can simply be
designed hanging in the air to have a certain pivot, WB, seat height, ground
clearance, etc and the wheels can be any size simply by adjusting where and
how they mount.

The three biggest factors we found in python stability were:
1)  Seat height.  The higher the seat, the more time you have to react and
regain your balance.
2) Pivot angle.  Setting this at the right angle determines how much
steering weight/self centering effect it has and whether the python likes
the flats better or rolling roads/downhills better.
3) Wheelbase.  This will determine how fast the python reacts to your
turning inputs.  Long wheelbases are stable and smooth, but difficult to
learn on because they take a long time to turn and react slowly.  Shorter
wheelbases will give you quicker responses to turning, but it also requires
constant steering corrections to maintain a straight line and is a little
twitchier at higher speeds.

The logic behind different wheel sizes was the realization that there was no
reason to have a big wheel behind the seat doing nothing more than making
the bent longer and heavier.  We wanted at least a 24" wheel up front so we
could get some decent high speed gearing with normally available chainring
sets.  Once we decided to go with smaller rear wheels, the thing that
determined the size was what was the smallest wheel we had kicking around,
and could we get a good high pressure, smooth rolling tire for it.  Once
riding, you had no way of telling the size of the tire behind you, only when
it came time to try to pack it into a small space or a vehicle.



| >> when I make a python with the data of for example P3, same trail,
| >> same pivot angle and same wheelbase but with 18 inch wheels can I
| >> expect a bike with the same riding behaviour or wil it be completely
| >> different. (so has the wheelsize also something to do with the
| >> steeringbehaviour)
| >
| > The steering behaviour is surely influenced by the wheelsize.
| > Bigger wheels make the front part heavier and this counteracts
| > the centering effect.
|
| I find both Dirk's and Lennart's pythons more heavy-steered and
| sluggish than monty, but that could have other reasons than wheel size.
|
| > Also small wheel python riders report
| > downhill coasting speeds of 60 kph and above while bigwheelers
| > have their limit around 50 kph.
|
| I have no computer on monty, but going downhill at around 40 kph I
| start imagining about ambulances, nurses and cruches. The bike feels
| not safe.
|
| > But as there are too few experienced riders around who can try
| > different pythons there is no scientific basis for a good theory
| > yet.
| >
| >> when I shorten the wheelbase and keep all other specs the same
| >> does that influence the self centering effect or only the
| >> turningcircle.
| >
| > If you shorten the wheelbase more weight shifts to the front part
|
| Then why do people say that with longer wheelbase you get less wheel
| slip? What piece of the puzzle have I lost?
|
| > resulting in less centering effect - I think.
| >
| >> I'm planning the 18inch bike based on the bike on the foto, only
| >> adding a second front fork and an optima bike seat. because my
| >> weldingskills are a little bit less than zero I'll have to find
| >> someone to weld it for me. hopefully this month
| >
| > Keep us in the loop Dennis, the Netherlands should be a perfect
| > hunting ground for pythons ...
|
| Ach, hilltops are so beautiful ...

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