speed shimmy follow-up

[Dax]

...a couple weeks back i responded to a thread about speed shimmy on this NG, stating that one cause
is an out-of-dish front wheel. There was a quick response of 'hogwash' from Mark McM and i followed
that with 'i know from experience' - then Mark: 'so do i'... .....then Jobst stepped in and said it
was unlikely. I'm not gonna argue with knowledge... so i stepped back, said i would do some tests.
And i did. I took an elderly 50cm Peugeot PB9 redone as a hybrid for a testbed, mainly because i
thought its short wheelbase might magnify the problem (tho' it's probably the opposite) I know the
bike - it tracks well. I took its front wheel, removed the locknut from one side, added a spacer on
the other, and with a washer or two so that the QR would engage the fork dropouts properly, put it
on the bike. http://gallery10526.fotopic.net/show_collection.php?id=27367 Then a friend and i took
turns booting it around town. The handling stank, but there was no shimmy for either of us. Flipped
the wheel over: same thing. I was disappointed. After festering for a few days, i wondered - maybe
the speed wasn't high enuff? This is, after all, a SPEED shimmy... So I took it up to the top of
Burrough's Falls hill (a local 2km 8% grade) and cranked it up to 58 kph.... NOTHING. Back up again.
Flipped the wheel over, zoom. NOTHING again. The front end felt like it was 'hunting' a bit and i
had the bars in a death-grip, but on the whole the bike was well-behaved. I don't know why fixing
the dish problem seemed to rectify a shimmy on my old Bobet so many years ago. It had quite a long
wheel-base and a ridiculous amount of fork rake, so maybe i should repeat this with something more
like that. however, this test shows that on its own, a front dish problem will not automatically
cause speed shimmy. SOOOO.... pass the roast crow, please. I'll try to watch it with the statements
of absolute certainty based on events a quarter century gone - Ъ×

 

[Callistus Valer]

speed shimmys are all in the mind, and you just proved it.

 

[Jobst Brandt]

anonymous writes:

http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html

> Back up again. Flipped the wheel over, zoom. NOTHING again. The front end felt like it was
> 'hunting' a bit and i had the bars in a death-grip, but on the whole the bike was well-behaved.

Well there's your first problem. Shimmy with hands on the bars is such a rare occurrence that it
makes headlines here when a terrified rider experiences it. To have that occur the rider must either
have almost no grip on the bars or it must be cold enough to initiate a shiver, which as you must
have read, is the same frequency as a shimmy for most bicycles.

> I don't know why fixing the dish problem seemed to rectify a shimmy on my old Bobet so many years
> ago. It had quite a long wheel-base and a ridiculous amount of fork rake, so maybe i should repeat
> this with something more like that. however, this test shows that on its own, a front dish problem
> will not automatically cause speed shimmy.

Try coasting down your test hill no-hands at speeds greater than 20 mph (aka >32 km/h), with the
pedals top and bottom and horizontal, and report back. That is the only valid shimmy test because
with hands on the bars, the rider is the test object.

> SOOOO.... pass the roast crow, please. I'll try to watch it with the statements of absolute
> certainty based on events a quarter century gone - Ъ×

Why, are you especially hungry? So who are you anyway, or is that asking too much in polite company?

Jobst Brandt [email protected] Palo Alto CA

 

[Dax]

On Mon, 16 Jun 2003 01:19:55 GMT, [email protected] wrote:

>anonymous writes:
>
>http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html
>
>
>Well there's your first problem. Shimmy with hands on the bars is such a rare occurrence that it
>makes headlines here when a terrified rider experiences it. To have that occur the rider must
>either have almost no grip on the bars or it must be cold enough to initiate a shiver, which as you
>must have read, is the same frequency as a shimmy for most bicycles.

Thanks for the FAQ - I hadn't read it , obviously. However, there is no WAY i was gonna take my
hands off the bars on the 39"-wheelbase Peugeot at 50kph.I note that the knee-against-top-tube
method of damping the problem is mentioned. Presumably this is done AFTER the death-grip-on-the-bars
has failed to control the problem.

>Try coasting down your test hill no-hands at speeds greater than 20 mph (aka >32 km/h), with the
>pedals top and bottom and horizontal, and report back. That is the only valid shimmy test because
>with hands on the bars, the rider is the test object.

I will - and also: i will use a bike with a longer wheel-base and smooth tires. It seems clear that
shimmy is the sum of a number of factors, and perhaps this dish thing will eventually turn out to
have some bearing on the problem after all
>
>> SOOOO.... pass the roast crow, please. I'll try to watch it with the statements of absolute
>> certainty based on events a quarter century gone - Ъ×
>Why, are you especially hungry? So who are you anyway, or is that asking too much in polite
>company? Jobst Brandt [email protected] Palo Alto CA

No, not particularly. I was trying to be witty. This is polite company? I'm Frank Harding, and I
have a small bike shop - a repair shop, really (Vélo Ayer's Cliff) in a small town in SW Quebec. I
worked for many years at various shops (Pedlar's, Bikenergy, Star) in Toronto. I don't ride
diamond-frame bikes much any more, but I LIVE on my homebuilt USS/SWB recumbent -
http://album3596.fotopic.net/photo.php?id=311492 No doubt i will think of lots of bon mots the
instant i punch "send" - Ъ×

 

[Jobst Brandt]

Frank Harding writes:

http://draco.acs.uci.edu/rbfaq/FAQ/8h.5.html

>> Well there's your first problem. Shimmy with hands on the bars is such a rare occurrence that it
>> makes headlines here when a terrified rider experiences it. To have that occur the rider must
>> either have almost no grip on the bars or it must be cold enough to initiate a shiver, which as
>> you must have read, is the same frequency as a shimmy for most bicycles.

> Thanks for the FAQ - I hadn't read it , obviously. However, there is no WAY i was gonna take my
> hands off the bars on the 39"-wheelbase Peugeot at 50kph. I note that the knee-against-top-tube
> method of damping the problem is mentioned. Presumably this is done AFTER the
> death-grip-on-the-bars has failed to control the problem.

No. It's done when coasting no-hands. The death grip is one of the prime initiators, it being the
closest thing to muscular trembling, the natural frequency of shimmy on a bicycle. Lighten up and
assess what is happening. Unless your bicycle is unique, you must wonder why no one else is
complaining. My bicycle shimmies reliably between 22 and 28 mph but I don't let it, especially when
I am coasting no-hands downhill as one does when tucked in with hands on the stem.

>> Try coasting down your test hill no-hands at speeds greater than 20 mph (aka >32 km/h), with the
>> pedals top and bottom and horizontal, and report back. That is the only valid shimmy test because
>> with hands on the bars, the rider is the test object.

> I will - and also: I will use a bike with a longer wheel-base and smooth tires. It seems clear
> that shimmy is the sum of a number of factors, and perhaps this dish thing will eventually turn
> out to have some bearing on the problem after all

I don't know what a longer wheelbase will do but longer top and downtube will be more elastic
and enhance shimmy. Also be careful which shopping cart you choose at the market, they shimmy
nicely at times.

Jobst Brandt [email protected] Palo Alto CA

 

[David L. Johnso]

On Mon, 16 Jun 2003 07:48:43 +0000, dax wrote:

> Thanks for the FAQ - I hadn't read it , obviously. However, there is no WAY i was gonna take my
> hands off the bars on the 39"-wheelbase Peugeot at 50kph.I note that the knee-against-top-tube
> method of damping the problem is mentioned. Presumably this is done AFTER the
> death-grip-on-the-bars has failed to control the problem.

Actually, should you read the FAQ you may notice that the death-grip may well contribute to
the problem.

> I will - and also: i will use a bike with a longer wheel-base and smooth tires. It seems clear
> that shimmy is the sum of a number of factors,

Yes, and far too many factors to be able to rationally control. It is a nearly-chaotic situation
(that is, small changes in parameters may produce large changes in shimmy), and not a matter of a
few controllable variables.

--

David L. Johnson

__o | "Business!" cried the Ghost. "Mankind was my business. The _`\(,_ | common welfare was my
business; charity, mercy, forbearance, (_)/ (_) | and benevolence, were, all, my business. The
dealings of my trade were but a drop of water in the comprehensive ocean of my business!"
--Dickens, "A Christmas Carol"