My neighbor told me about an accident he was in a few years ago with a Kawasaki Nomad, fitted with a conventional fork-mounted windscreen. It was on the freeway near Sturgis and he was doing about 70 mph on a windy day.
Suddenly the front end began to wag back and forth. As he slowed down the wobble got worse and the bike went off the road to the left. He woke up in hospital with broken leg and collar bones (ouch).
What gives? I've had one speed wobble my Ultra and when I slowed down it went away. After that I had a Tru-Track installed.
> My neighbor told me about an accident he was in a few years ago > with a Kawasaki Nomad, fitted with a conventional fork-mounted > windscreen. It was on the freeway near Sturgis and he was > doing about 70 mph on a windy day.
> Suddenly the front end began to wag back and forth. As he slowed down > the wobble got worse and the bike went off the road to the left. He woke > up in hospital with broken leg and collar bones (ouch).
> What gives? I've had one speed wobble my Ultra and when I slowed down > it went away. After that I had a Tru-Track installed.
From a control system point of view, the description of your injured neighbor doesn't make sense. That sort of oscillation would normally involve some rotational inertia in the part of the steering that pivots, and a motorcycle doesn't have enough moment of inertia up front. A person should be able to stabilize it, especially as the speed decays. The wobble getting worse just doesn't make sense.
Your description of "when I slowed down it went away" makes more sense.
Even on a windy day, I don't immediately see how it could happen. Even with 20 knots across the bike ... not enough surface area on the windscreen to make a difference.
On Sun, 20 Jul 2008 04:30:54 GMT, Sean_Q_ <nos...@no.sapm> wrote: >My neighbor told me about an accident he was in a few years ago >with a Kawasaki Nomad, fitted with a conventional fork-mounted >windscreen. It was on the freeway near Sturgis and he was >doing about 70 mph on a windy day.
>Suddenly the front end began to wag back and forth. As he slowed down >the wobble got worse and the bike went off the road to the left. He woke >up in hospital with broken leg and collar bones (ouch).
>What gives? I've had one speed wobble my Ultra and when I slowed down >it went away. After that I had a Tru-Track installed.
My Nighthawk developed a head shake after I had new tires installed. My Concours developed a headshake, but it went away after I recently had new tires installed.
I have a feeling the wobbles sometimes start after the bike throws a weight, or the bike shop does a crappy job of balancing the new tire.
> "Sean_Q_" <nos...@no.sapm> wrote in message > news:2czgk.120378$gc5.94200@pd7urf2no... >> My neighbor told me about an accident he was in a few years ago >> with a Kawasaki Nomad, fitted with a conventional fork-mounted >> windscreen. It was on the freeway near Sturgis and he was >> doing about 70 mph on a windy day.
>> Suddenly the front end began to wag back and forth. As he slowed down >> the wobble got worse and the bike went off the road to the left. He woke >> up in hospital with broken leg and collar bones (ouch).
>> What gives? I've had one speed wobble my Ultra and when I slowed down >> it went away. After that I had a Tru-Track installed.
> From a control system point of view, the description of your injured > neighbor doesn't make sense. That sort of oscillation would normally > involve some rotational inertia in the part of the steering that pivots, > and a motorcycle doesn't have enough moment of inertia up front.
And obviously NO inertia in a huge front wheel either. It makes one wonder how ANYONE can have a head shake or a tank slapper on a bike doesn't it?
> A person should be able to stabilize it, especially as the speed decays. > The wobble
As the frequency decays, the magnitude increases. Have you ever seen a two bladed helicopter resonate itself into pieces while it's sat on the floor?
> getting worse just doesn't make sense.
Oh but it does.
> Your description of "when I slowed down it went away" makes more sense.
Maybe to someone who's never experienced a tank slapper or even a mild oscilation.
> Even on a windy day, I don't immediately see how it could happen. Even > with 20 knots across the bike ... not enough surface area on the > windscreen to make a difference.
>>My neighbor told me about an accident he was in a few years ago >>with a Kawasaki Nomad, fitted with a conventional fork-mounted >>windscreen. It was on the freeway near Sturgis and he was >>doing about 70 mph on a windy day.
>>Suddenly the front end began to wag back and forth. As he slowed down >>the wobble got worse and the bike went off the road to the left. He woke >>up in hospital with broken leg and collar bones (ouch).
>>What gives? I've had one speed wobble my Ultra and when I slowed down >>it went away. After that I had a Tru-Track installed.
> My Nighthawk developed a head shake after I had new tires installed. > My Concours developed a headshake, but it went away after I recently > had new tires installed.
> I have a feeling the wobbles sometimes start after the bike throws a > weight, or the bike shop does a crappy job of balancing the new tire.
> Robert
In many cases that's a bad tire. And what makes it more confusing is that they sometimes balance on machines, but replacement makes the problem disappear.
On Jul 19, 9:56 pm, "David T. Ashley" <d...@e3ft.com> wrote:
> From a control system point of view, the description of your injured > neighbor doesn't make sense. That sort of oscillation would normally > involve some rotational inertia in the part of the steering that pivots, and > a motorcycle doesn't have enough moment of inertia up front. A person > should be able to stabilize it, especially as the speed decays. The wobble > getting worse just doesn't make sense.
There are two distinct modes of oscillation that are referred to as "speed wobble".
The true speed wobble has a frequency of five cycles per second (or more) and only the forks oscillate around the steering stem when the front tire chatters over rough pavement.
The chassis remains in a vertical attitude in a speed wobble.
The rider has no time to react to the speed wobble, it begins and ends in a very short time and may be so violent it tears the handlebears out of the rider's hands.
In the early 1970's, you could still buy motorcycles with Earles type forks, and that design had a lot of mass forward of the steering axis. When they speed wobbled, it was violent.
Also, early disk brake motorcycles had the brake calipers mounted forward of the steering axis. We used to swap the fork legs from one side to the other to mount the calipers behind the for leg and reduce mass forward of the steering axis.
Then the manufacturers caught on to what we were doing and delivered motorcycles with calipers behind the fork legs.
The *other* mode of oscillation is the *speed weave*. The heavy chassis is fully involved in the weave, yawing and rolling from side to side and the forks may be oscillating less than the rest of the chassis.
The speed weave occurs at a frequency of 1 to 3 cycles per second, and the rider can regain control by leaning forward, rolling off the throttle, or tapping the rear brake.
The speed weave is the mode you're seeing in those videos that depicts a rider being thrown off to one side. If the rider doesn't let go of the bars, the motorcycle may do an endo.
On Jul 19, 11:48 pm, Robert Bolton <robertboltond...@gci.net> wrote:
> My Nighthawk developed a head shake after I had new tires installed. > My Concours developed a headshake, but it went away after I recently > had new tires installed.
> I have a feeling the wobbles sometimes start after the bike throws a > weight, or the bike shop does a crappy job of balancing the new tire.
No, speed wobbles are caused by the front tire losing grip as you cross rough pavement.
You can reduce speed wobbles by using the original equipment tires, inflated to the recommended pressure.
If you can't get the OEM tires for some reason, beware of installing a narrower or taller front tire, or one with a pointy profile
If your forks have a preload adjustment, only dial in the amount of preload necessary to achieve a static sag of about 1/3rd of the full fork travel.
Also, use the minimum amount of compression damping adjustment.
Avoid the temptation to adjust rebound damping by using heavier fork oil.
When your rear tire wears flat and your front tire wears to a point, the motorcycle will tend to speed wobble at a lower speed. Replace the tires as a set.
> From a control system point of view, the description of your injured > neighbor doesn't make sense. That sort of oscillation would normally > involve some rotational inertia in the part of the steering that pivots, > and > a motorcycle doesn't have enough moment of inertia up front. A person > should be able to stabilize it, especially as the speed decays. The > wobble > getting worse just doesn't make sense. >The *other* mode of oscillation is the *speed weave*. The heavy >chassis is fully involved in the weave, yawing and rolling from side >to side and the forks may be oscillating less than the rest of the >chassis.
That mode of oscillation makes sense to me. Thanks for all the info.
I appreciate all of the information. I wasn't trying to claim that bikes can't oscillate ... just that I didn't see immediately how and why it would happen.
>> From a control system point of view, the description of your injured >> neighbor doesn't make sense. ?That sort of oscillation would normally >> involve some rotational inertia in the part of the steering that pivots, >> and >> a motorcycle doesn't have enough moment of inertia up front. ?A person >> should be able to stabilize it, especially as the speed decays. ?The >> wobble >> getting worse just doesn't make sense.
Believe it.
>>The *other* mode of oscillation is the *speed weave*. The heavy >>chassis is fully involved in the weave, yawing and rolling from side >>to side and the forks may be oscillating less than the rest of the >>chassis.
>That mode of oscillation makes sense to me. Thanks for all the info.
>I appreciate all of the information.
The problem is, most of Krusty's info is flat out wrong. He comes across as authoritative, but he pulls most of it out of his ass. One example is when he says: "speed wobbles are caused by the front tire losing grip as you cross rough pavement. You can reduce speed wobbles by using the original equipment tires, inflated to the recommended pressure. " The OEM tire on ST1100s were notorious for contributing to a speed wobble. A wobble that seldom occured on rough pavement, as he claims.
There are numerous factors that affect speed wobble or head shake - steering head bearings, tire side wall stiffness, etc, and it can be very difficult to track down the major culprit.
On Jul 20, 2:47 pm, Turby <turbosur...@beach.comber> wrote:
> The OEM tire on ST1100s were notorious for contributing to a speed > wobble. A wobble that seldom occured on rough pavement, as he claims.
Deja vu strikes again...
Engineers have to make a compromise between stability and agility, then the marketing department puts in their opinion that the riding public wants something really sporty looking with a tiny front tire, then along comes a police department that wants to add 100 pounds of cop equipment high above the center of gravity...
If you ride an elephant on roller skates, don't be surprised at the deadly instability...
> >> From a control system point of view, the description of your injured > >> neighbor doesn't make sense. ?That sort of oscillation would normally > >> involve some rotational inertia in the part of the steering that pivots, > >> and > >> a motorcycle doesn't have enough moment of inertia up front. ?A person > >> should be able to stabilize it, especially as the speed decays. ?The > >> wobble > >> getting worse just doesn't make sense.
> Believe it.
> >>The *other* mode of oscillation is the *speed weave*. The heavy > >>chassis is fully involved in the weave, yawing and rolling from side > >>to side and the forks may be oscillating less than the rest of the > >>chassis.
> >That mode of oscillation makes sense to me. Thanks for all the info.
> >I appreciate all of the information.
> The problem is, most of Krusty's info is flat out wrong. He comes > across as authoritative, but he pulls most of it out of his ass. > One example is when he says: "speed wobbles are caused by the front > tire losing grip as you cross rough pavement. You can reduce speed > wobbles by using the original equipment tires, inflated to the > recommended pressure. " > The OEM tire on ST1100s were notorious for contributing to a speed > wobble. A wobble that seldom occured on rough pavement, as he claims.
> There are numerous factors that affect speed wobble or head shake - > steering head bearings, tire side wall stiffness, etc, and it can be > very difficult to track down the major culprit.
> -- > Turby the Turbosurfer
True that speed wobbles are ofter hard to track down. Back in the seventies they made bikes with more power & speed than their rubbery frames could handle either in a straight line or a long sweeper at 80+ MPH. Both my75 -Z1 and77-KZ-650 would wobble in sweepers above 80MPH. My 78-GS1000 was supposed to be a good handler for that era but without OEM rubber it would do a straight line wobble at 100+ which was scarey as hell. You couldn't just chop the throttle and get on the brakes or it would get worse. You graduallly had to slow it down to below the oscillation resonant speed. Nowadays the bikes (sport or ST) handle much better but tire wear Flattening can cause an ocasional light wobble at speed in a turn after hiting a tar strip or bump in the road.. I think the better stability is due to a mumber a factors: Stiffer frames, frame mounted fairings for wind induced wobbles,fat radial tires and better streamlining plus bigger forks and tapered steering head bearings.
