Head shake

I have read somewhere that *any* kind of looseness or sloppiness in a bike's
suspension system can manifest as a head shake/tank slapper. True?

My CB900F (#1) has always been a little bit twitchy on the steering. It's
been both stable and responsive, as long as I was touching the bars. Even a
single fingertip on a bar end would keep it stable. But if I let go of the
bars completely, it would begin an oscillation that would grow in a most
alarming fashion. If I grabbed the bars it would quickly settle down, and
it never got to be a real tank slapper. This has been a steady situation
through several different brands and types of tires.

I'm now resurrecting my bike after a long sleep, and as part of another
procedure I pulled off one of the rear shocks. Lo and behold, the rubber
bushing on the upper shock mount is decayed and broken, allowing the shock
to flop around quite a bit. Could this be a source of head-shaking
instability? Probably. Why did I never notice this before? Damned if I
know. Just lazy, I guess.

So I'm replacing the bushings. I should replace the shocks, but at $500 a
side that'll have to wait. I'm also rebuilding the forks with new seals and
bushings. The parts are a week away, and it'll be longer than that before I
can ride (separate issue), but I'm excited to find out how the bike behaves
with the new parts.

-Scott

 

That could be part of it but also change the fork oil, balance the front
tire and make sure both tires are properly inflated. It also wouldn't
hurt to make sure the steering head bearings are properly adjusted.

Tony

 

The key word is *can*.

It can be a contributory factor, yes. However, one single imperfect
suspension component, on its own, is highly unlikely to cause it.

In addition, you have the problems of tyres, luggage fitment, fitment of
badly designed fairings, you name it.

Your Honda is a big, lardy old bus designed in the 1970s, when Honda
really hadn't got their chassis technology thought out. Replace the fork
components and the shocks by all means, but there's only so much you can
do to help.

And think - the swinging arm moves, at the back, up and down, in a near
vertical plane. A worn rubber bushing isn't going to affect that at all.
So the short answer is that replacing it isn't really going to make any
appreciable difference, although it's wise to do it if you're
overhauiling the suspension completely.

 

That's usually a bunch of BULLSHIT that mechanics use to get you to pay
them money to "fix" problems that are INHERENT to the design of street
motorcycles.

Oh, well, if you put your machine onto the centerstand and you can grab
the front wheel and move it back and forth half an inch, your steering
head bearings are worn out or need to be retorqued. I wouldn't worry
too much about loose swing arm bearings that allow you to move the rear
wheel side to side a quarter of an inch.

Your premise is that *any* kind of looseness or sloppiness in a bike's
suspension system can manifest as a head shake/tank slapper.

Modern motorcycle chassis engineers know that a certain amount of
"sloppiness" is actually very desireable and they are designing more
limber chasses to deal with
head shakes/tanks slappers/speed weaves.

There are a number of factors involved in the "head shake/tank slapper"
syndrome.

What you are describing is a so-called "speed wobble". That's when the
forks oscillate back and forth from side to side at a frequency of 5 or
more times per second. The chassis doesn't roll or yaw very much, it
stays vertical.

The rider who experiences a "speed wobble" has little chance to realize
what
is happening, and has no time to decide on a corrective action or to
implement the corrective action.

The sudden onset and end of the speed wobble mystifies riders and makes
money for motorcycle mechanics when they charge the ignorant customer
to change his steering head bearings (or tighten them), or he sells
them a set of trick new shock absorbers for $400 to $500 or tells him
that he needs to have his swing arm bushings or bearings replaced or
that his forks are worn out or whatever.

The real reason for a head shake/tank slapper/speed wobble is that the
front tire doesn't have enough traction to stop the motorcycle chassis
from falling over to the right or left sides.

This loss of traction usually occurs as the motorcycle goes over a rise
in elevation on the road, or crosses railroad tracks. In this case,
front tire traction is instantaneously reduced or even becomes
non-existant when weight transfers to the rear tire as the front tire
*dances* on the pavement.

The head shake/tank slapper/speed wobble is damped out as the front
tire regains traction.

But motorcycles will also experience a deceleration wobble when weight
shifts to the front end as the rider rolls off the throttle. The amount
of rake and trail is decreased slightly, and this is enough to start
the head shake/tank slapper/speed wobble.

So the real culprits are the age of the rubber in the front tire
(whether it's new and damps out oscillations, or if it's old and cannot
damp oscillations), the profile of the front tire (front tires wear to
a "V" shape over time, losing traction, while rear tires wear flat,
increasing their traction), and front suspension compliance (fork oil
that's too thick will reduce front tire traction).

It was designed that way to be light handling in parking lots and to
reduce steering effort while you are leaned over in a long sweeping
corner. A "neutral steering" machine will only take about 1 pound of
force on the handlebars to hold the machine in cornering attitude.

Did you ever hear the story of the patient that went to a doctor and
said, "Doc, it hurts when I hit myself in the head with a hammer." The
doctor replied, "Then don't hit yourself in the head with a hammer."

As I said, your Honda is designed to have light, responsive and precise
steering in low speed situations, like around town and in parking lots.
If your Honda was designed for high speed stability at 180 miles an
hour on a race track, the steering would be unpleasantly heavy in a
parking lot.

You may remember when Superbike racing started. Enthusiastic novice
racers would buy a Honda CB900F or a CB1100F or a GS1000 or a Kawasaki
KZ1000 that was designed for light, precise parking lot steering and
they would go out and try to ride 150 mph on a race track. And they
came back to the pits scared silly from the violent head shake/tank
slapper/speed wobbles.

The street oriented machines even got into a worse situation. They went
into high speed weaves that would cause the motorcycle to roll and yaw
violently, throwing the rider off to the side or over the handlebars.

I have experienced high speed weaves. They are caused by the rear tire
having a lot more traction than the front tire. High speed weaves take
place at a frequency between 3 and 5 cycles per second, and the rider
CAN do something about the weave.

He can roll off the throttle, regaining front tire traction by
transferring weight forward.

He can do the same thing by sliding forward or laying down on the gas
tank. He can also transfer weight forward by tapping the rear brake
pedal.

A last resort is to take your feet off the foot pegs, lowering the
whole center of mass of the motorcycle. This may not be an entirely
volunary action. If you are in a speed weave with your feet off the
foot pegs, the motorcycle may throw you off. So don't hold on, let it
go.

When your feet are off the foot pegs, you run the risk of getting your
left foot caught in the chain and having toes severed. That happened to
pro racer Doug Polen several year ago. Another racer, Chuck Graves,
makes am accessory "shark's tooth" that keeps the rider's foot out of
the drive chain/sprocket meat grinder.

Back to the early Superbike racers. Initially, the racers believed the
old bullshit about the problem being "chassis flex", that the steel
chasses behaved like they had a hinge in the middle because they were
"too limber". So the racers braced their chasses six ways from Sunday,
and the motorcycles STILL had problems with head shakes/tank
slappers/speed wobbles and violent underwear-destroying speed weaves.


The racers learned that they needed to keep more weight on the front
tire to make it grip properly. So they pulled the forks up through the
triple clamps as much as an inch. You'd be amazed at how much that
helps the front tire traction. They also cut the frame just behind the
steering head and leaned the steering head a little forward at the top,
reducing the fork rake.

A few degrees less fork rake, and half an inch more trail will help
stop head shake/tank slapper/speed wobbles.

The racers also learned to use a lighter weight fork oil to control
traction over small amplitude, high frequency bumps instead of using a
heavier weight fork oil to reduce
larger amplitude, lower frequency bumps.

I doubt that this is causing your problem. A guy with his own machine
shop used to
scout for work on Usenet by insisting that the cause of head shake/tank
slapper/speed wobbles and speed weaves was mechanical sloppiness. He
refused to believe that Honda and Kawasaki were finding that handling
over rough surfaces got WORSE, not better, when they removed all
sources of chassis "sloppiness".

Honda and Kawasaki began redesigning their high tech space age chasses
to be less stiff. They now claim that their newest chasses are
"improved" but refuse to say whether they are stiffer or more limber.

Can you cancel the order before the parts arrive? You're "shotgunning"
a bunch or repairs that you may not need.

 

$249.98, and free shipping! But I'm having an internal debate on "fix it
cheap" versus "keep it close to stock", and so I'm going to cheat and not do
anything about it right now.

But aftermarket progressive springs seem appealing. Are they really worth
using? If nothing else I'd be happy to have progressives up front, because
I've learned to hate trying to keep the damn air forks properly inflated.

-Scott

 

The bike's a quarter of a century old. It's not one single imperfection,
it's a long series of little imperfections. Who knows what they add up to.
But I find that I'm actually enjoying the process of going through and
fixing these little things, not because I'm trying to solve a problem, but
just because broken things ought to be fixed. Speculating on cause and
effect is more for sport at this point.

So I'll fix what's wrong, and either the head shake will disappear or it
won't. It's not as if I couldn't add a steering damper.

I'm finding the process itself interesting. This bike is an old friend, but
I've been away from it for some years. Now I find myself with a more
critical eye, and a changed lifestyle that permits me to fix things the
right way, rather than sliding by with race tape and baling wire.

Been through lots of tires. No bags, fairings, or windshield. Got a
luggage rack, though. The bike's native storage capacity is approximately
one can of beer, if you leave the toolkit home.

Agreed. It's a shame the aesthetics of the motorcycle market have changed
so much. I'd love to have a bike with modern technology, but there's damn
few bikes today that are worth spitting on, as far as styling.

Do you think a bad bushing could allow the swingarm to set up some kind of
undesireable resonance?

-Scott

 

Ah, yes -- and now is the time to look after the head bearings, since the
front end's already torn down while I rebuild the forks.

Question: Assuming price is not a factor, what compelling reasons exist for
using purpose-specific fork oil rather than ATF?

-Scott

 

Some oil is thicker than ATF and some is thinner. Thinner oil gives
less damping and a softer ride and thicker oil give more dampening and a
harsher ride. I've used ATF as a starting point in all my bikes over
the past 30 years and it's worked well for me. Keep in mind that many
1970's bikes came with fish oil in the forks and shocks. I've seen
water in the fork oil (mostly from condensation cause killer tank
slapper wobbles.)

Since you have the fork tubes out, you might as well remove the triple
clamps and repack the steering head bearings. I'm pretty sure Honda was
using tapered roller steering head bearings by the time the CB900 came
out. If not, install a set.

A new set of progressive springs would be worth it as the stock ones are
somewhat weakened from age and use. Sacked out springs effectively
shortens the front end giving a harsher ride and quicker steering. The
stock springs were progressive too but not from the "Progressive"
company that makes shocks and springs.

Make sure all the little stuff is done before you spend money on the big
stuff or you may find yourself PO'ed... ;o)

The CB900F was a good handling bike for it's year and by the time it was
made, handling was an important selling feature. Once the Japanese
discovered handling, it was all over for the old Triumph. Leaky old
shakers were not worth putting up with anymore for good handling. The
900C wasn't as good handling but still shouldn't wobble.

Tony
Motorcycle mechanic since 1976

 

It could. I don't remember if Honda used needle bearings or bushings on
the swing arm on your bike but, regardless a wobbly swing arm isn't
good. With new bushings you won't be able to feel any side to side
wobble and only a slight amount when still serviceable. By the time you
get to a quarter inch of wiggle, they're long past shot.

Roller or needle bearing swing arms should require a dial indicator to
find the side to side wobble and it should be indetectable by hand.

Tony

 

Parts & More has Taiwanese made shocks for under a C note. I've never
used them but some people I know have and liked them. However don't buy
their turn signals...

Tony

 

Christ knows.

"No" would be my gut reaction.

 

Yes. Dirt cheap and a worthwhile mod.

I had them on my old airhead BMW (admittedly a bike with a soft front
end as stock) and I've pre-loaded OE fork springs as an (even cheaper)
fix on other bikes.

 

The ability to choose between viscosities.

 

AAARRGGHHH! Another one!

We're not talking about the swinging arm bearings!

 

Jesus H Christ. Here we go again. Reams of pseudo-scientific waffle that
misses the point.

You really think yanking the forks up through the yokes is a cure-all?

And, in caser you hadn't noticed, the OP is not talking about worn s/arm
bushes *at all*. He's talking about the rubber grommets in the shock
absorber eyelets.

 

No, forget the Progressive fork springs! I bought two sets. Never
again.

Those people make a few part numbers of springs and try to tell you
that you can use the same spring in several different models. Yeah,
right!

If you have a lower rate shorter original equipment spring and you
replace it with a progressively wound spring that has more, closer
wound coils and the instruction sheet tells you to throw the stock
sheet metal tubular spacers away, then the Progressive spring is going
to take up extra space inside the fork tube.

That means there's less room for fork oil. How much less fork oil
should you use?

Progressive can't tell you, because they've never installed their
springs in every model that they claim to make a spring for!

Is that bullshit, or what? The best Progressive can do is give a you a
ballpark figure of the least amount of air space that might remain
above the oil when the fork leg is collapsed.

They said about 140 to 150 millimeters from the top of the fork tube to
the fork oil is
about the least air space that you can allow for.

So that might mean you have to remove the fork caps and go through all
the hassle of reducing the oil level and trying again and again, until
you get a fork that doesn't hydraulic lock before it uses all of its
travel.

A fork that hydraulic locks can blow the oil seals out.

 

It isn't "waffle", I know what I'm talking about because I've been
through all this chassis shit.

It's not a "cure all", but it's effective if you're performance riding
on roads with good traction and ess-curve after ess-curve. You know
you've gone to far with pulling the forks up when the motorcycle
doesn't transition from side to side easily, but wants to keep going
the way it was headed when you first flicked it into the corner.

If all you're doing is riding down an English country lane that's
pretty much zig and zag around corners where farmers would not let the
road cross their fields of rape, you never get the motorbike down into
deep lean angles for long enough to feel whether the chassis is
neutral, tries to run wide, or falls into a turn.

I don't think you have any real sportbike roads in England anyway. You
have to go to Wales or Scotland.

If you ever have the occasion to ride on the famous Angeles Crest
Highway above Los Angeles, you'll find about 60 miles of off camber
corners followed by correctly cambered corners and ess curve after ess
curve, 120 mph straight and 100 mph sweepers and winds blowing up
canyons to upset your lines and frost heaves to make the front tire
dance and laughing coyotes waltzing across the road ahead while turkey
vultures circle overhead.

It's a road that throws everything at you. Pine cones, squirrels,
landslides, ice snow, deer, mountain sheep, you name it, but no matter
what it has challenged you with, it still isn't a race track, like
nearby Willow Springs that puts handling and performance into a
completely different perspective.

Willow Springs melts rear tires on one side and the traction balance
between front and rear is especially noticeable as your race bike rocks
from side to side as you try to ride it in a straight line returning to
the paddock.

I *know* what he was talking about. The rubber grommets absorb some of
the high frequency vibration that the tire feeds into the swing arm.

If the swing arm flexed a bit, it still wouldn't cause a head shake,
because head shakes are caused by loss of front tire traction going
over bumps.

 

No but the subject came up so I addressed it.

Tony

 

Ohlins shocks and forks have two compression damping circuits. One is
an adjustable needle and orifice to control damping at rather low
stroking speeds for control over larger bumps that occur at lower
frequency.

The other compression damping curcuit is a shim stack that blows off
excess pressure when the tire hits smaller bumps at higher frequencies.

The latest thing on Formula I cars is extrenally removable valving. You
get a box of about 16 preset chrome plated valves and they go into
holes in the shock absorber body. It's laying on its side, so it
doesn't leak oil. I don't know what they do about pressurization while
they are swapping valves.

When they get spit off to one side of the bike and still hang onto the
bars they are actually in a huge, out of control *speed weave*. The
chassis is rolling and yawing at the same time. When they lose control
and don't let go of the bars, the motorcycle trips over its own front
wheel, which is pointed sideways to the direction of travel.

Freddy Spencer likes to say that there is a lot of energy transferred,
as the rider is launched 10 or 15 feet into the air and flips with the
machine down the race track.

If the rider would just let go of the bars, the motorcycle would often
recover stability and go running down the track, riderless.

The older the tires are, the less damping you get from hysteresis in
the rubber.

That's when you need a velocity sensitive steering damper. That's
unlikely to be a solution for theOP and his elderly UJM.

The main thing HE needs to understand is how old, hard rubber loses its
grip and why he needs to lower the front end slightly to make his front
tire grip better.

 

Nope. Everything I'm doing is necessary for other reasons. I just got
curious about what factors might contribute to the tendency to wobble. I'm
not doing any repairs to specifically address the wobble at this time.

Thank you for the dissertation. Very informative.

-Scott