Re: [CR]Now: Shimmy Was: on topic?

Thanks for that link Chuck. If you'd like to see more discussion on shimmy search for shimmy on the Bike Science and Framebuilders lists at http://catfood.phred.org/query.asp We also discussed it heavily here too not that long ago. Alas this is one of those topics like helmets that can cause folks to get really pissed, get people banned from the list, and/or stomp off in a huff. It's one of those things that has multiple variables involved and lots of anecdotal evidence with very few actually researched facts. good luck, Brandon"monkeywrench"Ives SB, CA

On Sunday, March 23, 2003, at 03:43 PM, Chuck Schmidt wrote:
> Greg Brooks wrote:
>>
>> Hey all, If this question isn't on topic, I guess just ignore it or
>> send me your thoughts off list. O.K.- Can someone explain the
>> mechanics/physics involved in high speed,hands-off shimy experienced
>> in cycling? I assume the root cause is a slightly misaligned frame.
>> But why does the front wheel "vibrate" back & forth rapidly instead
>> of just veering off to the right or left? (on topic material- My '72
>> Paramount has a tendency to do this on steep descents when I take my
>> hands off the bars, which I don't do too often.)Thanks,Greg Brooks,
>> Ridgetop,TN.
>
>
>> From Frequently Asked Questions on rec.bicycles.* :
> http://draco.acs.uci.edu/rbfaq/FAQ/
>
> ==================================
> Subject: 8h.5 Shimmy or Speed Wobble
> From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
>
> Shimmy is not related to frame alignment or loose bearings as is often
> suggested. Shimmy arises from the dynamics of forward motion and the
> elasticity of the frame, fork, and wheels, and the saddle position.
> Both perfectly aligned bicycles and ones with wheels out of plane to
> one another shimmy nearly equally well. The same is true for bearing
> adjustment. In fact shimmy is more likely with properly adjusted
> bearings than loose ones. The bearing or alignment concept is usually
> offered as a cause of shimmy and each airing perpetuates the idea.
>
> Shimmy, the lateral oscillation at the head tube, depends primarily on
> the frame and its geometry. The inflation of the tire and the
> gyroscopic effects of the front wheel make it largely speed dependent.
> It cannot be fixed by adjustments because it is inherent to the
> geometry and elasticity of the components. The longer the frame and
> the higher the saddle, the greater the tendency to shimmy, other
> things being equal. Weight distribution also has no effect on shimmy
> although where that weight contacts the frame does.
>
> In contrast to common knowledge, a well aligned frame shimmies more
> easily than a crooked one because it rides straight and without bias.
> The bias force of a crooked frame impedes shimmy slightly. Because
> many riders never ride no-hands downhill, or at least not in the
> critical speed range, they seldom encounter shimmy. When it occurs
> with the hands on the bars it is unusual and especially disconcerting.
> There is a preferred speed at which shimmy initiates when coasting
> no-hands on a smooth road and it should occur every time when in that
> critical speed range. Although it usually does not initiate at higher
> speed, it can.
>
> Pedaling or rough road interferes with shimmy on a bicycle that isn't
> highly susceptible. When coasting, laying one leg against the top
> tube is the most common way to inhibit it. Interestingly, compliant
> tread of knobby tires give such high lateral damping that most
> bicycles equipped with knobbies do not shimmy.
>
> Shimmy is caused by the gyroscopic force of the front wheel that acts
> at 90 degrees to the axis of the steering motion. The wheel steers to
> the left about a vertical axis when it is leaned to the left about a
> horizontal axis. When the wheel leans to the one side, gyroscopic
> force steers it toward that side, however, the steering action
> immediately reverses the lean of the wheel as the tire contact point
> acts on the trail of the fork caster to reverse the steering motion.
>
> The shimmy oscillates at a rate that the rider's mass on the saddle
> cannot follow, causing the top and down tubes to act as springs that
> store the energy that initiates the return swing. The shimmy will
> stop if the rider unloads the saddle, because the mass of the rider is
> the anchor about which the oscillation operates. Without this anchor
> no energy is stored. The fork and wheels may store some energy,
> although it appears the frame acts as the principal spring.
>
> Shimmy can also be initiated with the hands firmly on the bars by
> shivering, typically in cold weather. The frequency of human
> shivering is about the same as that of a typical bicycle frame.
> ==========================
>

> Chuck Schmidt

> L.

> A.

>

> .