[python] Re: max downhill speed

----- Original Message ----- From: "Dirk Bonne" <dirk_bonne@xxxxxxxxxx>
To: <python@xxxxxxxxxxxxx>
Sent: Tuesday, May 10, 2005 10:26 PM
Subject: [python] Re: max downhill speed --lost mail, second try


The flevoracer with 28" wheels has been reported to have
"strange" high speed effects. That´s why they build it now
with 26" wheels.


Aha! No my 10cents fall. Years and years ago some guy was talking about
his bad experience with coasting downhill on a flevo race (he talked
about not being able to control the bike). At the time, it nearly
brought me off the idea of building a flevo. But when I finally build
mine (20" sized), I never ever experienced anything wrong.

I could not understand why he had problem while I had none. ...never
went over +-65kmh though, so may be the problem would also have been
there on my bike, but I just never went fast enough.

Do you have pointers to info about this? I mean when they came unto the
idea of making the wheel smaller, they probably also understood what was
going on.

May be this effect has nothing to do with the trail being negative. But
just that on centered steered bikes the effect is stronger ---just like
that loading heavy bagage on a centered steered bike is immediatly felt.


Hi Dirk,

I found the postings about this flevoracer issue in the
flevofan mailinglist:

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From: "Koen Van den Bergh" <koen@...>
Date: Tue Nov 9, 2004  6:35 am
Subject: Re: [Flevofan] Flevo statistics  koen_vdbergh
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On Mon, 08 Nov 2004 15:41:15 -0800, John Sisler <jsisler@...> wrote:

I think I read (on this site) that the Flevo Racer was dropped because of
poor high speed stability.

I think you mean the 28" version of the racer. When equiped with 26" wheels, the
racer becomes more stable at high speeds according to what I heard. The problem
wasn't the frame, but the wheels which aren't strong enough for the that speed.

At Tempelman you can still buy a racer which has 26" wheels. (
http://www.ligfietsshop.nl Dutch site)

Koen

From: "Fokke Buwalda" <fokke.buwalda@...>
Date: Tue Nov 9, 2004 1:25 pm
Subject: Re: [Flevofan] 28 inch racer (was Flevo statistics) fokke.buwalda@...
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John,

fun to read the account of your first spins on the racer!

Re. your question about actually racing a racer: I remember seeing a guy
who participated in the 1 or 6 hours race at cycle vision 2000 (?) on a
Flevo racer. He finished somewhere mid-field, I think. Just to show that
it is in fact possible to race them...

About the 26/28 inch issue, I think the problem is in the stronger
gyroscopic precession of large wheels. The gyroscopic action of a wheel
generates strange and unexpected extra forces as you steer. The larger the
wheel, the stronger this effect. Because of the extremely inclinated
steering angle of the Flevo design, relatively large sideways rotations
are necessary even for small corrections. And, as you may guess: the
faster the wheel spins, the heavier it is, and the larger and quicker
these shifts in the axis of rotation are, the stronger these gyroscopic
forces. At 45 km/h, the forces generated by a 28 inch wheel are 50%
stronger in comparison with 26 inch, assuming that there is no difference
in weight. See my messages 80 and 86, posted some time ago.

to calculate for yourself: http://www.gyroscopes.org/math.asp

Today's fast motocycles have much smaller wheels than those of, say, 50
years ago. Probably for the same reason.

Fokke - has just built up a nice new 28 inch front wheel - Buwalda


On Mon, 08 Nov 2004 23:14:51 -0800, John Sisler <jsisler@...>
wrote:

Thanks for the information, but I'm still confused. 28" not strong
enough?
Why not? Is there a larger load on the wheels of a Flevo Racer compared
to
an upright bike?


I'm not an engineer, but I guess it's because of the different way of
steering. Maybe someone else can give more information.

I know an 28" wheel can be strong too (look at the cyclocross), but a 26"
with a wider rim is obviously stronger. Maybe it's the same as with the
headset. The different characteristics of the flevo can be the reason for
much higher forces on certain components.

Koen -correct me if I'm wrong- 


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