speed shimmy follow-up

Discussion in 'Cycling Equipment' started by Dax, Jun 15, 2003.

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  1. Dax

    Dax Guest

    ...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×
     
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  2. speed shimmys are all in the mind, and you just proved it.
     
  3. Jobst Brandt

    Jobst Brandt Guest

    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
     
  4. Dax

    Dax Guest

    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×
     
  5. Jobst Brandt

    Jobst Brandt Guest

    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
     
  6. 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"
     
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