Speed wobble on a Nomad
Sean_Q_
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.
Sean_Q_
'99 FLHTCUI / '04 S40 / various projects
David T. Ashley
news:2czgk.120378$gc5.94200@pd7urf2no...
From a control system point of view, the description of your injured
neighbor doesn't make sense. That sort of oscillation would normally
involve some rotational inertia in the part of the steering that pivots, and
a motorcycle doesn't have enough moment of inertia up front. A person
should be able to stabilize it, especially as the speed decays. The wobble
getting worse just doesn't make sense.
Your description of "when I slowed down it went away" makes more sense.
Even on a windy day, I don't immediately see how it could happen. Even with
20 knots across the bike ... not enough surface area on the windscreen to
make a difference.
Robert Bolton
My Nighthawk developed a head shake after I had new tires installed.
My Concours developed a headshake, but it went away after I recently
had new tires installed.
I have a feeling the wobbles sometimes start after the bike throws a
weight, or the bike shop does a crappy job of balancing the new tire.
Robert
Beav
"David T. Ashley" <d...@e3ft.com> wrote in message
news:jo2dnWBLrZkNWx_V...@giganews.com...
> "Sean_Q_" <nos...@no.sapm> wrote in message
> news:2czgk.120378$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.
Mmmm.
--
Beav
VN 750
Zed 1000
OMF# 19
Steve L
"Robert Bolton" <robertbo...@gci.net> wrote in message
news:3on584penrjfvmct7...@4ax.com...
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.
.
> 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.
.
> 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.
David T. Ashley
news:3e850193-9e70-4ffe...@h17g2000prg.googlegroups.com...
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.
>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.
Turby
wrote:
>"." <Rhia...@gmail.com> wrote in message
>news:3e850193-9e70-4ffe...@h17g2000prg.googlegroups.com...
>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.
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
.
> 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...
Bob Nixon
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.
Bob Nixon
Turby
wrote:
>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...
You keep spitting out the crap, we'll keep calling you on it.
>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,
Only you would consider the stock width of an ST tiny.
>then along comes a police department that wants to add 100 pounds of
>cop equipment high above the center of gravity...
You're confusing the ST11 with the ST13, but that's your normal
condition.
--
Turby the Turbosurfer
oasysco
Sean, do you think his steering bearings failed? Maybe they were bad
to begin with and he just ignored them, chalking hndling problems up
to tires or the windshield?
Greg
Sean_Q_
> Sean, do you think his steering bearings failed? Maybe they were bad
> to begin with and he just ignored them, chalking hndling problems up
> to tires or the windshield?
All I know is what he told me, and I'm no expert. I've ridden a lot
of bikes, the odd time perhaps a bit faster than HM Queen Bessie would
be pleased to hear about (er h'mm) and I've experienced a speed wobble
(or was it a "weave"? I'm not even sure) only once.
"." aka RhiannonX wrote:
> 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 *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.
When it happened I was too busy trying to survive for me to identify
which of these effects it was. Here is the text of an account I posted
to RMH. There were a lot of interesting replies, for which see:
> This morning I got impatient stuck behind two slow trucks (ungood),
> and when the slightly faster one pulled ahead I went blasting
> around it (even more ungood) until I must have been doing nearly
> 80 as I watched both of them dwindle to dots in my mirrors;
> that's when I noticed that I'd reached the sweeper leading
> to the bridge and I was going a little bit (well maybe more
> than a little bit) too fast, and just as I'm countersteering
> like mad to lean into the curve I reach some "waves" in the road
> (this is in a boggy area where some stretches of asphalt have
> sunk more than others, the effect being like long shallow
> waves at sea) and suddenly my front end starts to wobble.
>
> I start breaking, but soon I'm drifting onto the shoulder,
> with scattered marble-sized (and spherical shaped) gravel,
> so I couldn't lean all that much.
>
> Oh shee-it, I'm gonna crash, but I kept breaking (gently) and I
> finally got her under control just as I reached the white line 3"
> from the grass. Whew, that was a close one, I thought and when I told
> my bike wrench later on he said the FLH's have an inherent high-speed
> instability in the back wheel (he called it "rear steering");
> the fix being some kind of bracket that bolts to the frame
> and the tranny; either that or slow the fuck down, he told me.
> Sean_Q_
>'99 FLHTCUI <-- still in 1 piece, thankfully
SQ
'99 FLHTCUI SE3 with Tru-Track (installed subsequent to the above)
Beav
>Deja vu strikes again...
Have you ever tried to balance a 4ft long brush with the head of the brush
in your hand and the shaft of the brush pointing skywards? Then have you
ever tried it with the head up in the air while you balance the whole thing
on one finger? If you find it hard, make the shaft longer.
Now, tell us again how a high CoG makes something unstable?
.
> Have you ever tried to balance a 4ft long brush with the head of the brush
> in your hand and the shaft of the brush pointing skywards? Then have you
> ever tried it with the head up in the air while you balance the whole thing
> on one finger? If you find it hard, make the shaft longer.
>
> Now, tell us again how a high CoG makes something unstable?
Once again: Handling is a compromise between stability and agility.
The designer of a motorcycle sets up the center of gravity in relation
to the motorcycle's tire contact patches to get the agility or
stability he wants.
.
> �> Oh shee-it, I'm gonna crash, but I kept breaking (gently) and I
> �> finally got her under control just as I reached the white line 3"
> �> from the grass. Whew, that was a close one, I thought and when I told
> �> my bike wrench later on he said the FLH's have an inherent high-speed
> �> instability in the back wheel (he called it "rear steering");
> �> the fix being some kind of bracket that bolts to the frame
> �> and the tranny; either that or slow the fuck down, he told me.
That was a speed weave. All motorcycles will speed weave at some
critical speed.
When the front forks speed wobble, they start oscillating back and
forth so fast you can't react to it, you don't have time. The speed
wobble begins and ends suddenly.
Since the forks don't weigh that much, they can wobble very fast.
The speed wobble occurs when the motorcycle is vertical. The speed
weave occurs when you are leaned over, even a little bit.
The speed weave starts walking the motorcycle to the outside of the
curve.
The motorcycle chassis with the rear wheel and engine weigh a whole
lot more than the forks, so the "rear wheel wobble" you describe takes
place at a slower frequency.
Marc Gerges
>
> Have you ever tried to balance a 4ft long brush with the head of the brush
> in your hand and the shaft of the brush pointing skywards? Then have you
> ever tried it with the head up in the air while you balance the whole thing
> on one finger? If you find it hard, make the shaft longer.
>
> Now, tell us again how a high CoG makes something unstable?
>
By default, a high CG is a good thing on a motorbike. However there's an
influence from any mass far away from the CG exerting a moment on it.
There's a reason why they try putting exhaust under the engine lately -
with catalitic converters and noise regulations, exhaust start packing
weight, so it's less interesting to tie them to the back of the bike.
Top cases are bad for the same reason, you may end with comparatively
little mass, but because it's miles away from the CG, it still can
generate a big moment.
Other problem may be the point of aerodynamic attack. If that's too far
away from the CG, as a rule of thumb you run into trouble.
cu
.\\arc