Speed wobble on a Nomad

 

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.

Sean_Q_
'99 FLHTCUI / '04 S40 / various projects

 


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.

 



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?


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?


Oh but it does.


Maybe to someone who's never experienced a tank slapper or even a mild
oscilation.

Zed 1000
OMF# 19

 



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.

 



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.