Frame wobble - custom built bike

Tim McNamara writes:

jim beam writes:

Jose Rizal wrote:
jim beam:

well, the good news is that you've got what should be a
shimmy-resistant wheel setup - the xtr hub has a much better
bracing angle than a standard road hub.

Whatever you mean by "bracing angle", it has little effect on
shimmy which is a system phenomenon.

bracing angle affects the increase [or decrease] in tension exerted
on a wheel in lateral load, ie. when experiencing shimmy. the
smaller the bracing angle, the greater the tension increase &
therefore the greater the elasticity experienced. tension increase
rises to infinity as angle drops to zero.

Please show how there is lateral load on the wheels during shimmy.
Which wheel, and where the load comes from. Thanks.

Is'nt a lateral force required whenever a turn is initiated? Not in
the steady-state, of course, but during the transitions. How else does
the force get transfered from the tire to the frame?

On Sat, 26 Feb 2005 04:48:02 GMT, Jose Rizal wrote:

jim beam:

[snip]


"bikes" without any wheels at all still shimmy.

At what speed did you experience shimmy with these bikes without wheels?

Dear Jose,

Jim is probably referring to ski "bikes" again, vaguely
bicycle-like things with skis where the wheels would be:

http://groups.google.co.uk/groups?q=...eranews&rnum=1
or http://tinyurl.com/5glw9

I don't think that anyone has ever posted speeds or details
of shimmy on such contraptions.

I'm not even sure if the riders ever put their feet up on
these things, or just drag them on short little skis.

Jobst has also used these no-wheels ski "bikes" to
illustrate points, such as how steering works.

This would be a good time for some as-yet-unheard-from
poster to educate the rest of us about these
pseudo-bikes--how fast they go, how much they weigh, what
kind of frames they have, where the feet are placed, what
sort of shimmy (if any) occurs and whether it seems related
to the cold and body shivering, how much steering is lean
versus edging, and so on.

Carl Fogel

Faulty logic! If your first green bike was the first to shimmy, would
you
blame the color? BTW, 73 degrees and 50mm offset is not unusual
(produces
about 2" of trail).

Damn! That's what it is. Should never have bought a green bike. At
least my Orange Rambouillet doesn't shimmy. Don't know about the
blue ones

But seriously, 2"/50mm of trail for a design that was supposed to be
more of a touring design than racing was plain wrong when their racing
geometry is typically 73 deg with 43mm rake.

Orin.

Tom Sherman wrote:



One of my material's engineering professors made up some fiberglass
wrapped concrete cylinders. They exceeded 25,000 psi compressive
strength (at the load frame limit) while the unwrapped specimens
fractured at less than 5,000 psi.



Adding layers of carbon is common technique used to increase the load
rating of existing concrete bridges when there isn't much room to play
with
--
---
Marten Gerritsen

INFOapestaartjeM-GINEERINGpuntNL
www.m-gineering.nl

On Sat, 26 Feb 2005 04:36:11 GMT, Jose Rizal wrote:

Tom Sherman:


How about wrapping the tubes in the head tube/down tube/top tube are
with carbon fiber composite to add some stiffness to the frame?


It's not the stiffness that will change the frequency of shimmy onset,
but the change in mass of the system. You can hang a brick on the frame
and eliminate shimmy at the usual speed you encounter it. Not
practical, but neither is carbon fibre wrapping, for costs' sake.

Not sure I follow your logic, Jose. Isn't natural frequency a result
of both stiffness and mass (with some constants thrown in)?

f = 1/[2(pi)]*sqrt(k/m)

From http://personal.cityu.edu.hk/~bsapplec/natural.htm, Eq. 12

On Fri, 25 Feb 2005 20:55:24 -0800, jim beam wrote:

Jose Rizal wrote:
Tom Sherman:



How about wrapping the tubes in the head tube/down tube/top tube are
with carbon fiber composite to add some stiffness to the frame?



It's not the stiffness that will change the frequency of shimmy onset,
but the change in mass of the system. You can hang a brick on the frame
and eliminate shimmy at the usual speed you encounter it. Not
practical, but neither is carbon fibre wrapping, for costs' sake.

angular frequency = sqrt (spring constant/mass)

you're changing the mass, which works, but stiffening the frame alters
the spring constant, so it /will/ change the frequency.

That would be if bending or twisting of the frame tubed were the point of
oscilation, instead it's the headset turning, not the tubes bending.

Ron

Jim Smith writes:

Tim McNamara writes:

jim beam writes:

Jose Rizal wrote:
jim beam:

well, the good news is that you've got what should be a
shimmy-resistant wheel setup - the xtr hub has a much better
bracing angle than a standard road hub.

Whatever you mean by "bracing angle", it has little effect on
shimmy which is a system phenomenon.

bracing angle affects the increase [or decrease] in tension
exerted on a wheel in lateral load, ie. when experiencing shimmy.
the smaller the bracing angle, the greater the tension increase &
therefore the greater the elasticity experienced. tension
increase rises to infinity as angle drops to zero.

Please show how there is lateral load on the wheels during shimmy.
Which wheel, and where the load comes from. Thanks.

Is'nt a lateral force required whenever a turn is initiated? Not in
the steady-state, of course, but during the transitions. How else
does the force get transfered from the tire to the frame?

This doesn't have anything to do with shimmy, of course. But the
answer is basically no- turns are initiated by leaning the bike to the
inside of the turn. Unless one is deliberately riding in a strange
gymnastic style.

Tim McNamara writes:

Jim Smith writes:

Is'nt a lateral force required whenever a turn is initiated? Not in
the steady-state, of course, but during the transitions. How else
does the force get transfered from the tire to the frame?

This doesn't have anything to do with shimmy, of course. But the
answer is basically no- turns are initiated by leaning the bike to the
inside of the turn. Unless one is deliberately riding in a strange
gymnastic style.

Right, but doesn't the only path for forces to be transfered from
frame to tire involve the spokes and hub? If a wheel starts out
vertical before a turn and is leaned over at 45 degrees during the
turn it seems that some lateral force must have been applied to change
its orientation. I suspect the magnitude of this putative lateral
force is small, and may not have much to do with shimmy, but it still
seems to me that any time the bike is changing direction there is at
least some lateral force on the wheels. Or do I have this all wrong?

Tim McNamara wrote:


Is'nt a lateral force required whenever a turn is initiated? Not in
the steady-state, of course, but during the transitions. How else
does the force get transfered from the tire to the frame?

This doesn't have anything to do with shimmy, of course. But the
answer is basically no- turns are initiated by leaning the bike to the
inside of the turn. Unless one is deliberately riding in a strange
gymnastic style.



If you believe this try riding without a frontskewer
--


---
Marten

m-gineering wrote:

Tim McNamara wrote:


Is'nt a lateral force required whenever a turn is initiated? Not in
the steady-state, of course, but during the transitions. How else
does the force get transfered from the tire to the frame?

This doesn't have anything to do with shimmy, of course. But the
answer is basically no- turns are initiated by leaning the bike to the
inside of the turn. Unless one is deliberately riding in a strange
gymnastic style.




If you believe this try riding without a frontskewer

By implication, this means that you have ridden without a front skewer.
Please report on the experience.

--
Tom Sherman - Earth