Wheel
Vibration
Diagnosing
Wheel Vibration
How to tell if
your wheels or tires are bent
by Sean Philips
One of the most annoying
things that can happen to any car is when it picks up some kind of vibration.
While a vibration is not
usually a safety issue unless it becomes very bad, a shaking car can be no
fun at all to drive, and can sometimes be screamingly frustrating to diagnose
which of the many complex components that govern the car's contact with the
road is causing the steering wheel shimmy.
For a
car to run smoothly at speed requires the contact with the road and the
transmission of the contact forces to be achieved within very tight tolerances.
The majority of vibration problems are caused by the wheels or
tires being out of tolerance in some way, usually because of an impact.
When
I go to diagnose a vibration, I always check the wheels first, then the tires,
followed by alignment and suspension.
Alignment
and suspension issues will require other articles, so we'll address how to
diagnose wheels and tires first.
I
generally start with a couple of questions for the driver:
DO YOU
FEEL THE VIBRATION IN THE STEERING WHEEL OR IN THE SEAT?
The answer can give us an idea of whether the vibration is
coming from the front end, which will generally transmit vibration directly to
the steering wheel, or from the back end, which will transmit vibration more
through the frame of the car and into the seat.
This is not always 100%
indicative, as there are a number of variables involved in car vibrations.
Certain alignment issues in
the back end can cause the steering wheel to vibrate as it shakes the car from
side to side, for instance.
DO YOU
FEEL THE VIBRATION AT A CERTAIN RATE OF SPEED?
Many people come to me already saying, “I get this weird shake between X and Y miles per hour.”
I am immediately fairly
certain that either a wheel is bent or a tire is out of round.
A vibration that has a “sweet
spot” at a certain speed range is a classic symptom of harmonic modulation
caused by a small bend.
A wheel and tire assembly that is out of round will have a
specific harmonic frequency as it spins, depending on how many bends, the
severity of the bends, tire wear and other factors.
As
the speed changes, the harmonic changes, or modulates, as well.
At
certain speed ranges this modulation can reach a frequency that will overwhelm
the vibration-dampening capacities of the suspension.
That's
the point at which you begin to feel a vibration in the car that was previously
being damped out.
DO YOU
FEEL A VIBRATION IN THE BRAKE PEDAL UNDER HARD BRAKING?
If under moderate to stiff braking pressure you can feel the
brake pedal shake under your foot, this is a good indication that what you have
is a warped brake rotor or other brake-related issues.
The brake rotor must be
either replaced or re-lathed to make it perfectly flat.
Once
we understand the history of the vibration, the next step is to inspect the
wheels and tires.
The best way to do this is to
remove all four wheels and spin the wheel and tire assembly on a balancer.
Once the wheel is on the
balancer, it should be spun by hand. With the wheel centered and spinning, we
look carefully at the outer edges of the wheel on both the inboard and outboard
faces.
Factory tolerances for a wheel are about .030” (30 thousandths
of an inch) both lateral (side to side) and radial (up and down).
Most
deflections or bends outside that range will be visible to the naked eye while
the wheel is spinning centered.
If
the wheel is straight, the line formed by the outer edges of the rim should be
relatively stable, and it should not wobble from side to side.
If
the wheel is straight, determine if the tire is out of round.
Put your eyes level with the
tread surface as it spins and look straight across the surface.
Does the tread bounce up and
down without an equivalent motion in the wheel? The tire is probably out of round.
A steel belt may be bent or
delaminated inside the tire, or the tire may be wearing
irregularly.
Look at the tire straight on;
do the tread blocks wiggle from side to side?
This indicates that the tires are getting lateral wear, usually
as a result of an alignment issue.
Of course, it may be difficult to convince your local tire
shop to let you go out back and watch your wheels spin on their balancer.
Different shops will have different policies on this, as
insurance regulations are often involved.
If yours won't, I can only suggest trying smaller shops who
can make exceptions if you explain what you're trying to do.
Alternately, you can jack up the car or put it on jack
stands, put the transmission in neutral and spin the wheels on the car, or have
a friend spin them while you look under the car at the inboard side.
It's not as accurate, because the suspension will move a
little, but it's a quick and (very) dirty way to get a rough idea.
Sean
Phillips is the Operations Manager at Rim and Wheel Works, a family-operated wheel and
tire repair shop located in Waltham, Massachusetts.
EXPERIENCE
Sean has
been straightening, refinishing, welding and replacing wheels for more than a
decade, making him one of the most experienced wheel repair specialists in the
world. He has repaired thousands of wheels, sold thousands of tires, and
trained multiple wheel straighteners and tire technicians. As a skier and
Colorado native, he has a particular interest and expertise in snow tires and
winter driving.
“At
Rim and Wheel Works, our focus is on all kinds of drivers; the daily driver,
the enthusiast, and those that want their ‘bling.’ I myself am less of a
journalist and more of a hands-on repair guy, and that is the kind of focus and
perspective I intend to bring to About.com Tires and Wheels. Tires and wheels
tend to be things you don’t really think about until you have to. I think about
them every day. Our customers appreciate our honest, no-nonsense advice and
that we go out of our way to save them money, and I hope my readers will
appreciate that as well.”
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