Fixed gear/track frame geometry

ML said:

Quoted message said:

ML said:

Benjamin Lewis wrote:

>[email hidden] wrote:
>
>
>
>>Benjamin Lewis wrote:
>>
>>
>>>[email hidden] wrote:
>>>
>>>
>>>
>>>>If you have zero trail you can't steer. You'll fall
>>>>over.
>>>
>>>Huh? I think you need to reexamine your model of how
>>>bicycle steering works. Of course you can steer on a
>>>bicycle with zero trail. It's been done. People have
>>>even successfully ridden bicycles with negative trail.
>>
>>We aren't talking about negtive trail, we're talking
>>about zero trail. If you have zero trail and you lean
>>your bike over while riding it, how do you make the
>>bike steer back under you?
>
>
>By turning the handlebars.
>
>I'm not sure what you're confused about here. If the
>wheel is turned the front of the bike will move in the
>direction of the turn as the bike moves forward.
>

I believe you are wrong on this but can't really prove
it. Unless you have a bike with true zero trail, you
can't prove your point either so these are just mind
exercises.

If you consider your contact patch as a point on the
pavement, turning your handlebars on a bike with zero
trail will not do anything for you. It will continue to
try to go forward (I believe) and ultimately you will end
up on the ground. Why do I say this? For a couple
reasons. Consider what happens to your bike when you lean
it to the left while walking it. The front wheel swings
to the right and the bike turns into the direction it was
leaned. This is how a person is able to ride a bike no
handed. The bike naturally does the steering for you -
you affect your direction of travel by strictly leaning
the bike through the saddle. Now consider what would
happen if you leaned a bike with zero trail to the left -
nothing. The bike would just lean, the front wheel would
continue straight down the road. Why? Because with zero
trail there is no torque component in either the CW or
CCW direction. The component of the force vector which is
perpendicular to the fork (the frame of reference at this
point) is applied along the fork centerline. In order to
rotate the fork you must have a component of force offset
by some radius from the center or rotation. With zero
trail, you don't have this. So, if you were on this bike
you would fall down. You can't ride this bike no handed.
Now consider what happens when you are on the bike. You
feel the bike start to fall over as you lean so you turn
the bars to steer into the turn. This is where the
problem comes in. Since you have zero trail all you are
doing is rotating the contact point on the pavement. A
true point source does not have a directional component.
In this case though the reality is that the contact patch
looks more like a square or a rectangle than a point so
it will have a directional component. Will this
directional component be enough to actually direct the
bike into the corner? Yes - the front end will act like a
unicycle. I'm not sure how rideable this bike would be.
My instincts tell me only at very slow speeds.

BTW, I briefly looked through my copy of Bicycle Science
last night & could find no reference to any experiments
done with zero trail bikes. All references were to
various combinations of positive or negative trail bikes.
If you can direct me to references (or to the pages in
Bicycle Science) documenting zero trail bikes I'd love to
read them. I can't think of anything in normal usage that
would be equivalent. Shopping carts, cars, motorcycles,
scooters - they all have trail components.

Jobst, any thoughts on this subject?

MOO, Matt

Dear Matt,

I think that David Jones concluded that we can ride
damned near anything in terms of front-end geometry,
although most bizarre versions are good for little except
for parking lot demonstrations.

Below are some articles mentioned in a post elsewhere by
Jonathan Thornburg:

< [email hidden]>

thp.univie.ac.athome.htmlOpen ↗

Carl Fogel

David E. H. Jones "The Stability of the Bicycle" Physics
Today , April 1970, 34-40

Framk Rowland Whitt and David Gordon Wilson "Bicycling
Science", 2nd edition MIT Press, 1982, ISBN 0-262-73060-
X (paperback), -23111-5 (hardcover) ... this book has
a chapter devoted to stability,

J. Lowell and H. D. McKell "The Stability of Bicycles"
American Journal of Physics 50(12), Dec 1987, 1106-
1112

G Franke, W Suhr, and F Reisz, "An advanced model of
bicycle dynamics", European Journal of Physics 11
(1990) 116-121

John Maddox's report of the Franke/Suhr/Reisz paper in
Nature 346, 2 Aug 1990, p 407.

Carl:

The URL for jthorn is not valid. I did a websearch and
found no other information.

Thanks, Matt

Dear Matt,

Try him at this page:

aei.mpg.dehome.htmlOpen ↗

Carl Fogel