I think that front-end shimmy is a harmonic.



I think that  shimmy occurs when some multiple of the resonant frequency

of the bicycle is induced.



I think the test of this is to vary the resonant frequency of a bike that

shimmies to see what effect that has on that shimmy-state.



So those who have bikes in shimmy-state are the fortunate few who

can engage in such tests.  So I propose that they become the subjects

of tests that the rest of us can help with.



Who's interested in participating?



My thinking is that if a load's mass or position are varied significantly,

that a given shimmy-state will be altered (eliminated or modified).



I yield the floor and encourage you, dear reader, to step up and share

your thoughts.



Matthew

On Sun, Jun 29, 2008 at 8:19 AM, Kogswell Cycles

<kogswellcycles@...> wrote:

> I think that front-end shimmy is a harmonic.

> I think that  shimmy occurs when some multiple of the resonant frequency

> of the bicycle is induced.



That is how I understand it as well.



My cargo bike generates a shimmy when loaded, and my experience has

shown another variable, which I will describe here for your

consideration.



When I have a heavy load attached very rigidly to the front rack, the

shimmy is very strong; but when the front load is loose (for example,

15 pounds of things loose in a giant cardboard box with packing

materials, or inside a large bag on top of the rack with the potential

to slide around), shimmy is reduced or eliminated. I think of this as

being like base isolators on a building - the ability for the weight

to move a bit with respect to the rigid bicycle frame/rack defeats the

resonant effect.



So I suggest everybody try loosening their loads inside bags or

what-have-you, and see if this helps. This is counter to the

convential wisdom that all loads should be tied to racks as rigidly as

possible.



I haven't had a chance to compare different rake/trail values yet with

heavy loads, but I will soon, as I now have the bike set up so that I

can easily switch between approximately 25mm and 40mm trail.



This is a subject I would like to understand better, as I perceive it

as the chief deficiency of the design of my cargo bike, and I would

like to know how best to deal with this both for my use but also as a

theoretical bicycle design problem.



Traditional sport touring bikes can also exhibit this shimmy - my low

trail Nishiki International also shimmies at medium speeds; so did my

high trail Fuji Sagres; and my other Nishiki International did as

well. I reduced shimmy on the Fuji by tightening the headset to

introduce some drag on the steering, but it did not go away

completely. My Swift Folder (low trail, shallow head angle) has never

shimmied, and rides great no-hands, but is unrideable with a handlebar

bag load. My ALAN cyclocross (mid trail) does not shimmy.  Nor did the

Karate Monkey, to my recollection.



I begin to think that shimmy is a result, or a corollary fellow

traveller, of the steep-head-angle low-trail concept that we all like

for various other reasons. Aside from the shimmy, the handling of my

cargo bike is spot-on perfect.



One other thing about shimmy - on my sports touring bikes, riding

no-hands would introduce shimmy if I just let go of the bars but

continued to be leaning slightly forward; if I consciously lean back

as far as I can, shimmy is reduced or eliminated.



James Black

Los Angeles, CA

--- In KOG@yahoogroups.com, "james black" <chocotaco@...> wrote:

>

> When I have a heavy load attached very rigidly to the front rack, the

> shimmy is very strong; but when the front load is loose (for example,

> 15 pounds of things loose in a giant cardboard box with packing

> materials, or inside a large bag on top of the rack with the potential

> to slide around), shimmy is reduced or eliminated. I think of this as

> being like base isolators on a building - the ability for the weight

> to move a bit with respect to the rigid bicycle frame/rack defeats the

> resonant effect.





James, I hear what you're saying, but as a counter point my P/R  also shimmies

with a light

load in a Candy Bar bag strapped to the handlebars.   That bag is probably the

epitome of

the "loose-fastening" idea you outlined, where the load is not rigidly attached

to the

bicycle and therefore has some independent movement.  However, the shimmy with

the

Candy Bar bag isn't as strong as with the rack-mounted front bag -- although the

Candy

Bar bag is significantly lighter in weight.



To be fair, that bag causes most of my other bikes to shimmy as well.



-Jim G

On Sun, Jun 29, 2008 at 12:49 PM, Jim Gourgoutis <jimg@...> wrote:

> James, I hear what you're saying, but as a counter point my P/R  also shimmies

with a light

> load in a Candy Bar bag strapped to the handlebars.   That bag is probably the

epitome of

> the "loose-fastening" idea you outlined, where the load is not rigidly

attached to the

> bicycle and therefore has some independent movement.



Hi Jim:



That is unfortunate to hear, but I guess I'm not too surprised. With

my Nishiki (73 deg. HA, 40mm trail), I had no shimmy at all before I

installed a Blackburn front rack, but some shimmy with the rack even

unweighted. It seems that the weight of the rack alone is enough to

contribute to the problem. A u-lock firmly attached to the rack makes

it essentially impossible to ride no-hands, but isn't so bad that I

notice while holding the handlebars.



Imagine you had a 10" cube cardboard box on top of your rack with a

tiny 20 pound weight in the center of it, and a bunch of loosely

wadded newspaper or soft foam all around it - that is the kind of

flexibility that I think could help with the shimmy effect. The

resonant frequency of something like that is way lower than that of

bicycle frames and racks.



James Black

On Sun, 29 Jun 2008, Kogswell Cycles wrote:



> I think that front-end shimmy is a harmonic.

>

> I think that  shimmy occurs when some multiple of the resonant frequency

> of the bicycle is induced.

>

> I think the test of this is to vary the resonant frequency of a bike that

> shimmies to see what effect that has on that shimmy-state.

>

> So those who have bikes in shimmy-state are the fortunate few who

> can engage in such tests.  So I propose that they become the subjects

> of tests that the rest of us can help with.

>

> Who's interested in participating?



I may be interested, but I've got my bike set up just the way I like it,

so I don't want to modify much.

A bit of data:

I lent my P/R to a list member for a test ride and he complained of

shimmy. I never witnessed it, and have not experienced any shimmy myself.

I had to raise the stem and saddle a couple cm for him, so maybe that's

what made the difference? I'm using the 30mm trail fork, Trimlines, a

Nitto M12 and big boxy

handlebar bag. No shimmy at all whether the bag is empty or overflowing.

I haven't ridden it without a front rack and bag.

I've even coasted no-hands and reached down and whacked the bar sideways

and it just wiggled for half a second and settled down again.



Ryan

On Mon, Jun 30, 2008 at 3:14 PM, Ryan Watson <rswatson@...> wrote:

> I lent my P/R to a list member for a test ride and he complained of

> shimmy. I never witnessed it, and have not experienced any shimmy myself.



Interesting. Any ideas what would account for the bike shimmying with

one rider, but unwilling to shimmy with another? Is the other rider of

similar body size to you?



I find that the bikes that exhibit the worst shimmy do so with or

without pedalling, therefore pedal stroke style would seem to be not a

determining factor.



James Black

--- In KOG@yahoogroups.com, "james black" <chocotaco@...> wrote:

>

> I find that the bikes that exhibit the worst shimmy do so with or

> without pedalling, therefore pedal stroke style would seem to be not a

> determining factor.

>



On my bike, the frequency of the shimmy seems to be somewhat in sync

with my pedal cadence.  And if I stop pedaling and touch a knee

against the top tube, the shimmy stops.



I haven't had a chance to ride the bike w/o the front bag to see if

the shimmy still happens with the bike unloaded, I will hopefully do

so some evening this week.



-Jim G

On Mon, Jun 30, 2008 at 3:46 PM, Jim Gourgoutis <jimg@...> wrote:

> On my bike, the frequency of the shimmy seems to be somewhat in sync

> with my pedal cadence.  And if I stop pedaling and touch a knee

> against the top tube, the shimmy stops.



You reminded me that I did notice on my cargo bike that pedalling

really slowly, less than 60 rpm, seemed to lessen the shimmy when I

had a huge front load.



James Black

--- In KOG@yahoogroups.com, "james black" <chocotaco@...> wrote:

>

> On Mon, Jun 30, 2008 at 3:46 PM, Jim Gourgoutis <jimg@...> wrote:

> > On my bike, the frequency of the shimmy seems to be somewhat in sync

> > with my pedal cadence.  And if I stop pedaling and touch a knee

> > against the top tube, the shimmy stops.

>

> You reminded me that I did notice on my cargo bike that pedalling

> really slowly, less than 60 rpm, seemed to lessen the shimmy when I

> had a huge front load.

>

> James Black

>



James & Ryan, just to clarify -- do your bikes shimmy even with hands

on the handlebars?  To be clear, mine doesn't: the only shimmy I'm

experiencing is when attempting to ride no-handed.



-Jim G

On Mon, Jun 30, 2008 at 4:49 PM, Jim Gourgoutis <jimg@...> wrote:

> James & Ryan, just to clarify -- do your bikes shimmy even with hands

> on the handlebars?  To be clear, mine doesn't: the only shimmy I'm

> experiencing is when attempting to ride no-handed.



My road bikes typically don't shimmy with hands on the handlebars, the

shimmy usually gets going shortly after I start with the no-hands.



My cargo bike does shimmy with hands on the handlebars when I have 35

pounds of newspapers rigidly strapped to the front rack. It's pretty

gnarly. I now have a giant bag on the front rack - when I save up

another 35 pounds of newspapers, I'll try it again with the bag, and i

expect it will shimmy less since the connection will be less rigid,

but probably still shimmy with no-hands.



James Black

On Jun 30, 2008, at 5:49 PM, "Jim Gourgoutis" <jimg@...> wrote:



> --- In KOG@yahoogroups.com, "james black" <chocotaco@...> wrote:

>>

>> On Mon, Jun 30, 2008 at 3:46 PM, Jim Gourgoutis <jimg@...> wrote:

>>> On my bike, the frequency of the shimmy seems to be somewhat in sync

>>> with my pedal cadence.  And if I stop pedaling and touch a knee

>>> against the top tube, the shimmy stops.

>>

>> You reminded me that I did notice on my cargo bike that pedalling

>> really slowly, less than 60 rpm, seemed to lessen the shimmy when I

>> had a huge front load.

>>

>> James Black

>>

>

> James & Ryan, just to clarify -- do your bikes shimmy even with hands

> on the handlebars?  To be clear, mine doesn't: the only shimmy I'm

> experiencing is when attempting to ride no-handed.

>

> -Jim G



No shimmy for me either way.

Jamie, are you still here? If so, when did my bike shimmy for you?



Ryan

Hi Ryan and all, The mystery rider finally comes out of the woodwork. Sorry I've

been

absent for so long but I have been lurking around the dark edges  of the

discussions here.



The ride I took with Ryan on his P/R was great. I'm mostly a commuter rider and

was

looking for the next bike to take me to another level. Ryan's offer was too much

to pass up

and we did a 25 miler with little problem or soreness. The only issue I

encountered was

the shimmy.



I weigh about 185 and run a seat height of 77-78cm. This was up a good 8-10 cm

from

Ryan's setup, but we decided it was too much work to also change the bar height.

I was

riding the Trimlines with a Jintesha front bag with a fairly minor load to

it(5-6 lbs?). I was

checking out the bike to see how it fit and handled to help me with my decision,

so I was

doing things with it to see what behaviors ensued. One of those was to give a

shake to the

bars to see what that felt like. The shimmy that it induced was of a bit greater

amplitude

and duration than I expected or was really comfortable with. It was especially

something

that I noticed on downhill runs where I just didn't feel the solidity that I've

known on other

bikes. I feel that the added leverage and higher center of gravity of the

increased seat

height were contributing factors in the shimmy, as well as my inexperience with

the bike. I

always have found it takes many more miles to find the little adjustments of

bike and body

that give the harmony we all seek. I have little doubt that with a bit more work

and

experience things may have worked out that way.



As for my conclusions about the bike, I didn't end up buying one for a variety

of reasons,

most of which didn't involve the bike or the ride. The main reason I didn't buy

a Kogswell

is that the old Frejus that I bought in high school and traded on another bike

in the late

80's came up on Ebay. I bought it back and put together a dandy single speeder.

I also

found a set of NOS Reynolds 531 tubes and a mentor that I'm in the process of

building

my first frame with. (I'm a skilled metalworker and this just got under my skin

a bit.) I'm

working to design a frame that echos the P/R, but seems to more closely match my

odd

body proportions. I'll let you know what happens with it. I still think the P/R

is an amazing

machine and if finances would allow I'd have one now.



When I embarked on this, I bought a 30mm trail 650B P/R fork that I'm now

planning on

selling.  Drop me a line if you might be interested and we can work out a price

or trade.



Cheers,

Jamie Hascall

Santa Fe

>

> No shimmy for me either way.

> Jamie, are you still here? If so, when did my bike shimmy for you?

>

> Ryan

>

On Mon, 30 Jun 2008, james black wrote:



> On Mon, Jun 30, 2008 at 3:14 PM, Ryan Watson <rswatson@...> wrote:

>> I lent my P/R to a list member for a test ride and he complained of

>> shimmy. I never witnessed it, and have not experienced any shimmy myself.

>

> Interesting. Any ideas what would account for the bike shimmying with

> one rider, but unwilling to shimmy with another? Is the other rider of

> similar body size to you?



No idea. He's pretty much my size. About the same height overall, but

longer legs, I guess, since I had to raise the seat.

Maybe something in riding style or how/where he holds the bars?



Ryan

On Mon, 30 Jun 2008, Ryan Watson wrote:

>

> On Mon, 30 Jun 2008, james black wrote:

>

>> On Mon, Jun 30, 2008 at 3:14 PM, Ryan Watson <rswatson@...> wrote:

>>> I lent my P/R to a list member for a test ride and he complained of

>>> shimmy. I never witnessed it, and have not experienced any shimmy myself.

>>

>> Interesting. Any ideas what would account for the bike shimmying with

>> one rider, but unwilling to shimmy with another? Is the other rider of

>> similar body size to you?

>

> No idea. He's pretty much my size. About the same height overall, but

> longer legs, I guess, since I had to raise the seat.

> Maybe something in riding style or how/where he holds the bars?



I remember now that we couldn't get the bars as high as he would

like. Maybe not being quite comfortable had an effect on riding style and

led to shimmy?



Ryan

--- In KOG@yahoogroups.com, Ryan Watson <rswatson@...> wrote:

>

> I had to raise the stem and saddle a couple cm for him, so maybe that's

> what made the difference? I'm using the 30mm trail fork, Trimlines, a

> Nitto M12 and big boxy

> handlebar bag. No shimmy at all whether the bag is empty or overflowing.

> I haven't ridden it without a front rack and bag.

> I've even coasted no-hands and reached down and whacked the bar

sideways

> and it just wiggled for half a second and settled down again.





How much do you weigh, and what pressure do you run your tires at?

Also, please refresh my memory: how wide are those Trimline tires?



Thx,

-Jim G

On Mon, 30 Jun 2008, Jim Gourgoutis wrote:



> --- In KOG@yahoogroups.com, Ryan Watson <rswatson@...> wrote:

>>

>> I had to raise the stem and saddle a couple cm for him, so maybe that's

>> what made the difference? I'm using the 30mm trail fork, Trimlines, a

>> Nitto M12 and big boxy

>> handlebar bag. No shimmy at all whether the bag is empty or overflowing.

>> I haven't ridden it without a front rack and bag.

>> I've even coasted no-hands and reached down and whacked the bar

> sideways

>> and it just wiggled for half a second and settled down again.

>

>

> How much do you weigh, and what pressure do you run your tires at?

> Also, please refresh my memory: how wide are those Trimline tires?



About 180 lbs when I got the bike last fall, about 170 now. I lost 10

pounds on the Kogswell diet plan! ;-)

Trimlines are about 36mm actual width I think (labelled as 38).

I've almost worn out the rear one, luckily I have one to replace it.

About 50 psi rear, 42-44 front.



Ryan

----- Original Message -----

From: Jim Gourgoutis

To: KOG@yahoogroups.com

Sent: Monday, June 30, 2008 3:48 PM

Subject: [KOG] Re: shimmy: a call for testing

--- In KOG@yahoogroups.com, Ryan Watson <rswatson@...> wrote:
>
> I had to raise the stem and saddle a couple cm for him, so maybe that's
> what made the difference? I'm using the 30mm trail fork, Trimlines, a
> Nitto M12 and big boxy
> handlebar bag. No shimmy at all whether the bag is empty or overflowing.
> I haven't ridden it without a front rack and bag.
> I've even coasted no-hands and reached down and whacked the bar
sideways
> and it just wiggled for half a second and settled down again.

How much do you weigh, and what pressure do you run your tires at?
Also, please refresh my memory: how wide are those Trimline tires?

Thx,
-Jim G

I agree - both my low trail bikes (P/R Mk I and G with low trail

fork) have been prone to shimmy if the headset gets loose.

Otherwise, they are very stable with a front load and at low speed.

I also think they are easier to ride no handed providing there is a

front load. Without a front load, you have to lean foreward (just

like with a higher trail bike you have to sit upright with your

weight back when riding no hands).

I like the load up front and am willing to put up with things like

checking headsets and getting the load distribution right.



Bill



--- In KOG@yahoogroups.com, "Mark Vande Kamp" <mevk@...> wrote:

>

> You all are coming up with the same factors that I have

collected/observed over time. The list of things that I have directly

observed as affecting shimmy include:

>

> 1. Front end geometry.

> 2. Front load (amount and location) [others have talked about how

securely fastened it is, and that makes sense to me, too.]

> 3. Riding no-hands/one hand

> 4. Headset (tightness, roller-bearing)

> 5. Grip on bars (tense/tight = more shimmy)

> 6. Leaning forward/back (probably weight dist. on bike)

> 7. Tires

> 8. Speed

> 9. Pedaling (y/n and cadence)

> 10. Knee against top-tube

> 11. Standing/unweighting saddle

> 12. Rider shivering

>

> The basic conclusion is that a lot of factors come together and

altering any of them _may_ change shimmy. The best strategy seems to

be reducing its incidence to combinations of factors that are rare in

your own use. I think that low-trail bikes are more prone to shimmy

(my Goodrich shimmies under some conditions) but I'm more than

willing to live with it in order to enjoy the other handling benefits.

>

> Mark

>

>   ----- Original Message -----

>   From: Jim Gourgoutis

>   To: KOG@yahoogroups.com

>   Sent: Monday, June 30, 2008 3:48 PM

>   Subject: [KOG] Re: shimmy: a call for testing

>

>

>   --- In KOG@yahoogroups.com, Ryan Watson <rswatson@> wrote:

>   >

>   > I had to raise the stem and saddle a couple cm for him, so

maybe that's

>   > what made the difference? I'm using the 30mm trail fork,

Trimlines, a

>   > Nitto M12 and big boxy

>   > handlebar bag. No shimmy at all whether the bag is empty or

overflowing.

>   > I haven't ridden it without a front rack and bag.

>   > I've even coasted no-hands and reached down and whacked the bar

>   sideways

>   > and it just wiggled for half a second and settled down again.

>

>   How much do you weigh, and what pressure do you run your tires at?

>   Also, please refresh my memory: how wide are those Trimline tires?

>

>   Thx,

>   -Jim G

>

To add my two cents the only shimmy I've ever noticed on my G2 was at

very low speed - say 5 MPH. And this loaded a bit oddly with a front bag

(5 to 10 lbs) on a portuer rack, one pannier (with the laptop that won't

fit in the big ortlieb handlebar bag - so 10 lbs there) and a rack trunk

(about 5 lbs). That configuration makes the bike a bit squirrelly (I

have a 56 cm frame with a 40mm trail fork), but at not a hint of shimmy

at speed (I've gotten it up to around 30 in that config).



With just the front loaded it's shimmy free entirely (I've taken up to

around 45 in that config).



The low speed shimmy is also odd in that it has a pretty low frequency -

around a hz or so.



Greg



On Sun, 2008-06-29 at 10:19 -0500, Kogswell Cycles wrote:

> I think that front-end shimmy is a harmonic.

>

> I think that shimmy occurs when some multiple of the resonant

> frequency

> of the bicycle is induced.

>

> I think the test of this is to vary the resonant frequency of a bike

> that

> shimmies to see what effect that has on that shimmy-state.

>

> So those who have bikes in shimmy-state are the fortunate few who

> can engage in such tests. So I propose that they become the subjects

> of tests that the rest of us can help with.

>

> Who's interested in participating?

>

> My thinking is that if a load's mass or position are varied

> significantly,

> that a given shimmy-state will be altered (eliminated or modified).

>

> I yield the floor and encourage you, dear reader, to step up and share

> your thoughts.

>

> Matthew

>

>

>

>