SHOCK RESEARCH UPDATE
Last month I announced that I am undertaking a project to explore
sensitivity of suspension dampers to acceleration, jerk, and
perhaps other factors. I am still looking for shocks to test,
particularly groups of two or more shocks that produce similar
results in typical crank dyno tests but act different on-track.
I do have some preliminary feedback. One shock company tested
one of their dampers at the usual 2” stroke and 100 cpm (5/3
Hz), then at 1” stroke and 200 cpm (10/3 Hz). These two tests
produce equal ranges of velocity, but at any velocity the second
test produces 2 times the acceleration and 4 times the jerk
(change of acceleration). In this case, the forces were identical
in both tests, within the window of accuracy attainable. This
doesn’t mean the test was a failure. It means that the shock
tested is insensitive to acceleration and jerk, at least at
the values tested.
I think it is quite possible that many shocks are acceleration-insensitive.
My object is to devise ways of systematically testing to find
out, and also to find out whether acceleration sensitivity can
be a performance advantage if used correctly.
The shock dyno company I’m working with at this point, Performance
Data Systems, also tested two shocks that were provided to them
by a different shock company, which were identical except for
gas pressure. PDS has a unique dyno design that allows unusually
precise motion control, and will follow almost any desired motion
pattern, since it uses a linear motor rather than a crank. One
test this dyno can do is a step test: the shock is rapidly accelerated
to one velocity, held at that velocity for a given distance,
then accelerated abruptly to a higher constant velocity, held
at that velocity for a specified distance, and so on.
The acceleration zones between one step and the next can be
programmed to have defined limiting values for acceleration
and jerk. The machine can also be programmed to reach a particular
peak acceleration with either maximum jerk at the ends of the
acceleration zone, or minimum jerk for a desired mid-zone acceleration,
within a specified acceleration time between velocity steps.
If jerk is set at maximum, then jerk is zero in the middle of
the acceleration zone. If jerk is set at minimum, then peak
jerk is much less, but there is still a non-zero jerk value
in the middle of the acceleration zone. This means that this
test can produce points where both velocity and acceleration
are the same, but jerk is either zero or some known value. This
allows isolation of jerk effects from acceleration effects,
which is not possible in sinusoidal testing. Alternatively,
a shock can be tested at different known accelerations, with
identical velocities, and zero jerk. This allows isolation of
acceleration sensitivity from both velocity and jerk effects.
In the test of the similar shocks with differing gas pressures,
PDS reports that varying accelerations did not produce different
forces at mid-acceleration, but varying jerk values did produce
differing forces. And the difference was greater in one shock
than in the other. In other words, the shocks appeared to be
jerk-sensitive without being acceleration-sensitive, and the
jerk sensitivity appeared to vary with gas pressure.
Stock car teams are reporting that shocks with a given piston
and shim package definitely feel softer to the driver when gas
pressure is reduced.
Some caveats here: I was not present at the tests I am describing.
I am relying on the accounts of others. Also, we are not looking
at results of an exhaustive, systematic testing program. What
we do have is preliminary, anecdotal evidence that suggests
there are effects worth measuring and exploring through unconventional
MORE ON REAR WHEEL PLACEMENT AND TRACTION
Simon McBeath, whose comments regarding rear wheel placement
and its effects on traction prompted my remarks in the October
2002 newsletter, writes:
I've just been catching up on some overdue reading and
noticed in your October newsletter that you picked up my suggestion
for a discussion on the above. Many thanks a) for doing that
and b) for reading the feature (on the DJ Firehawk hillclimber)
where the suggestion was placed!
I read what you had to say in your newletter with great
interest. But is there also another mechanism at work with swung
back rear suspension? The Firehawk's designer mentioned to me
something I was very unclear about, and hence did not go into
in the article, but it involved the suggestion of gyroscopic
effects aiding traction, and it was in reference to buggy racing.
Have you heard of this effect being exploited this way? I couldn't
figure how that would work, to be honest.
I tried an experiment in the workshop with a hand held
grinder, angled back, as it were, as if the grinder's disc was
a wheel swung back on its suspension, and as you move such a
tool around up and down you can feel gyroscopic forces, but
when the tool is held still (but powered up) there are no sensations
or reactive forces.
But when you first power the tool up there is, obviously,
a reaction force. I wondered if this instantaneous response
could be usefully exploited for improved traction - to add to
the weight transfer under acceleration and make the tyre dig
in harder, initially at any rate. I have a feeling as I type
this that what you might gain on one side of the car you'd lose
on the other, but I can't figure it out in the middle of a Sunday
afternoon! Any thoughts would help still my curiosity and soothe
my confused brain!
What you’re feeling when you turn the grinder on is mainly
the grinding wheel acting as a flywheel, not a gyro. The body
of the grinder is more analogous to an axle housing than a semi-trailing
arm in a buggy rear suspension, because drive torque reacts
through the grinder body. The arm on the buggy only reacts thrust
under power. Drive torque reacts through the powertrain mounts,
and does not act through the suspension.
Wheels on a car do produce gyroscopic forces, but only when
their toe or steer angle changes, or their camber angle changes.
Rotational acceleration or velocity about the wheel’s main axis
(axle axis) does not produce gyroscopic forces. When we steer
the wheel to the left, it tries to lean to the right. When we
lean the wheel to the left, it tries to steer to the left. These
effects are called gyroscopic precession.
The precession force depends on the wheel’s angular velocity
in the plane perpendicular to the force. That is, when the wheel
steers left, the magnitude of the rightward camber-wise or roll
torque about the wheel-longitudinal axis depends on the wheel’s
velocity (not acceleration, not position) about the vertical
axis. The wheel’s rotational speed on its axle also matters.
More rpm, more precession force; wheel not rotating, no precession.
Lastly, the wheel’s moment of inertia about the axle axis matters.
More flywheel effect, more precession force.
In a motorcycle or bicycle, precession forces are an important
factor in vehicle behavior. We use them to hold the vehicle
upright, and to steer it. But in a tricycle or a car, we just
live with these forces; we don’t harness them. If anything,
they’re a problem, because they are part of the reason for shimmy
in steering systems.
With the grinder, you are holding the device by the body, which
is not quite in the same plane as the disc. Consequently, the
grinder may try to move in a complex manner when you power it
up. It may try to tilt the disc as well as rotate the body about
the spindle axis. If the disc tilts, then there will be some
In any case, gyroscopic precession does not increase traction.
As for transient (short-lived) forces that try to lift the
car momentarily increasing traction, that’s possible. However,
the brief traction improvement is followed by a corresponding
unloading of the wheel a fraction of a second later. What counts
for this is the vertical acceleration (not position, not velocity)
of the sprung mass (F = ma). The sprung mass is lifted a bit,
but only to a point. So its velocity upward increases to some
value, and then decreases to zero again. That means its acceleration
is first upward, and then downward. When the sprung mass acceleration
is upward, there is a wheel load increase. When the sprung mass
acceleration is downward, there is a wheel load decrease.
It’s probably better for traction not to have such an effect.
In certain instances, the driver may be able to time the momentary
traction increase to occur when it’s most needed, but in general
the car is limited by its instants of poorest traction, rather
than its instants of greatest traction. Therefore, we would
like the wheel loads to vary as little as possible.
There is also another effect when the car is being carried
in a lifted position: the center of gravity is higher, and that
increases rearward load transfer. So anti-squat does improve
traction, but not as much as many people imagine.
Now, if you move the rear wheels back on a buggy, what happens
to the anti-squat, and other properties? The answer depends
on what type of rear suspension the car has, and exactly what
you change to move the wheels back. Traditionally, buggies have
semi-trailing-arm rear suspension, derived from the design on
late VW beetles. However, this is not always the case any more.
Assuming we have semi-trailing arms, there are a number of
ways the wheelbase could be lengthened, and the various methods
have different effects on the rear suspension geometry. Probably
the simplest method, on an existing car, would be to merely
fit longer arms, without modifying the frame. If we do this,
we get the following effects:
- The static rear percentage decreases. As previously noted,
this hurts traction.
- The static anti-squat diminishes, assuming the trailing
arm slopes up toward the front.
- Changes in anti-squat with suspension motion are reduced,
because of the longer side-view swing arm.
- Changes in camber over bumps are reduced, due to the longer
front-view (rear-view, end-view) swing arm. Also, there is
less bump steer.
- The rear roll center is lower.
- In all likelihood, the suspension will be softer with a
given spring and shock package, due to a decreased spring-to-wheel
The last five of these effects could all improve traction,
especially while cornering, and on bumpy surfaces. This might
account for perceived or reported improvements. Note, however,
that all of these effects could also be achieved by moving the
pickup points forward, and leaving the wheel location unchanged.
That would probably involve redesigning the frame, of course.
And a better approach yet is to forget about using semi-trailing
arms altogether, and build a proper five-link, or short-and-long-arm,
If we do that, we can have any rear geometry we want, with
any wheel location, and we can have much less variation in anti-squat
than with any semi-trailing arm system. And any arguments for
moving the wheels back that might apply with semi-trailing arms