Star May/Jun 08 Fea.qxd:TheStar
4/17/08
2:35 PM
Page 72
A Question of Balance?
TAKING ADVANTAGE OF THE LATEST IN TIRE BALANCING TECHNOLOGY.
BY MICHAEL SALEMI
INTERNATIONAL STARS SECTION
Y
ou know what I’m talking about! You
are accelerating onto the highway,
approaching your car’s “sweet spot”
of speed. You settle in for the drive, and
then you notice it—that slight shake and
shimmy. You speed up, it goes away. You
slow down, it goes away. You just can’t
cruise at your speed without those annoying vibrations! You try and blame the pavement or actually believe for a moment that
you are imagining things—but here’s the
reality: you are unbalanced.
You are a victim of tires and wheels exerting forces on your car that have not been properly identified, measured, and corrected. “Wait?
I just had my tires balanced!” you think. What’s
going on here? While you may have had your
tires and wheels “balanced,” they have not
been “road force balanced”—where all the
forces that the wheel and tire combination
exert on your car’s suspension (and thus what
you feel in your steering wheel) have been
taken into account and corrected. That takes
special equipment and a dedicated operator—
not someone paid minimum wage and charged
with balancing X number of tires per hour on
yesterday’s equipment.
If you are a serious driver (aren’t we all?),
with a serious car, tires, and wheels, I’m
going to explain it to you. I’ll show you
what’s going on, and how and where to get
your tires and wheels properly balanced with
state of the art technology encompassing
“road forces.” If you have ever experienced
shimmy and shake that just won’t go away,
and you have never had your tires “road
force balanced.” Once you do, you’ll never go
back to conventional balancing again.
The importance of road force balancing
cannot be overemphasized. Only your tires
and wheels are between your wonderful
72 THE STAR | MAY/JUN 2008
Mercedes and that cruel pavement of America’s crumbling infrastructure. If they are
not properly balanced, your entire driving
experience could be shaky.
When I got the idea for this article I
called my friend Satish Tummala, a fellow
MBCA International Stars member, and asked
him about road force balancing. Satish
offered the space for us to work and to take
pictures, plus the use of his equipment as
well. He also put me in touch with Bill Keyes,
the local independent representative for
Hunter Engineering of St. Louis, Missouri.
Seeing an opportunity like a revivalist
minister in a den of non-believers, Bill was
gracious enough to bring over Hunter’s
latest and greatest machine and do the
balancing on my new tires and wheels, help
out with the photo shoot, and explain to
me how everything worked. Frankly, he made
it look easy! I can only hope that your tire
experience will compare.
Bill set up a brand-spanking new GSP9700
Road Force Measurement System in moments.
Then he calibrated the machine with a few
simple procedures. The entire setup, connection, and calibration procedure took less then
30 minutes. If you own your own shop, or
work at a facility that might use tire balancing equipment, you would be pleased with
this “out of box experience.” You’d be productive within an hour for sure.
Before we get into the actual procedure,
I’ll explain the three important forces that
work on tire and wheel combinations, how
they impact your road feel, and how they
get corrected.
BALANCE: The first thing to consider is
balance, or more properly, “static balance.”
Remember the old static balancers which
consisted of a mounting cone and a circular
The new GSP9700 is
shown in the foreground,
the older GSP9700
owned by Motorwerks is
shown just behind it. The
stand of multi-colored
items are wheel flanges
for different sized
wheels. ◆ The OEM
280SL steel wheel is on
the left, weighing about
31 pounds. The pressed
aluminum rim is on the
right. It weighs just over
10 pounds. You wouldn’t
know one from the other
except by lifting or using
a magnet. ◆ Using quick
release mounting hardware the tire and wheel
assembly mounts on the
GSP9700 in seconds.
bubble level? That was “technology” from
a bygone era. It does nothing for our
modern cars and tire and wheel combinations. After all, you can statically balance
a square wheel, a broken rim, and a defective tire with a brick inside but of course
you couldn’t ride on them… but they would
be statically balanced!
RUNOUT: The second thing to consider is
“runout.” This is the side-to-side (which
causes shimmy) or up-and-down (which
causes shake) motion of your tire, your rim,
or a combination. Alarmingly, this is hardly
ever checked. As you’ll see later, it’s a good
thing we decided to check the rim runout
on my wheels.
RADIAL FORCE VARIATION: The final
thing to consider—and it’s the big one—is
“radial force variation.” Radial force variation is the amount of change in sidewall
stiffness and footprint when a load is placed
against the tire. This variation is most often
caused during the tire’s manufacture. There
are differences from batch to batch, from
tire to tire, and even within a single tire
that can cause changes in mass and density
around the circumference of the tire. You
might think a tire is fairly uniform, but they
generally are not. These changes in mass
and density translate into forces when the
tire is put into motion—forces that you feel.
There are specifications and acceptable
limits from tire manufacturers on this variation. Until recently with the creation of
the latest machines such as this Hunter unit,
force variation was measured only by OEMs
in a factory setting using extremely expensive equipment. It was not done at any place
that sold or balanced tires. Why is this so
important, and why is the measurement and,
more importantly, the correction of radial
Star May/Jun 08 Fea.qxd:TheStar
4/17/08
2:35 PM
Page 73
Radial Force Variations
must be measured and corrected with the tire under
actual conditions; so pressure must be accurately
set. Here, we see the
machine setup with my
rear wheel at 34 pounds
cold. ◆ After a first spin,
which only takes moments,
the machine tells us how
and where to place the
weights. We can see here
that only a small amount of
correction is necessary.
This tire/wheel combination shows a correct road
force (see the lower right
of the screen) of only nine
pounds. ◆ The laser line
clearly shows the desired
weight placement location.
force variation the state of the art? Because
a tire and rim could be perfectly round when
measured with a dial indicator (called free
or geometric runout); it could be statically
balanced, but the wheel will still vibrate
when driven due to excessive force variation! That is precisely why your set of
“freshly balanced” wheels and tires can still
give you the shakes or a shimmy.
I’ll let you in on a little secret. Many tires
today have a “paint dot” or mark on the tire
that indicates either the high side or low side
of force variation. Sometimes, tire shops will
crudely try to solve vibration problems by
blindly “lining up the dot” with the valve
stem. The operator assumes the “dot” is the
high point of radial force of the tire and the
wheel valve stem hole is the low point of
runout of the rim. Not only is this untrue,
but it takes no force measurement into
account. Also, many tires are not marked and
many wheel manufacturers do not use the
valve stem as the low point. Other tire OEMs
use the marks to indicate low points instead
of high points. Hunter’s machine eliminates
any guesswork. The locations of runout and
tire force are quickly located and measured.
The unit also automatically determines if
these inherent “defects” can be used to solve
an existing non-balance related vibration,
before any extra work is performed. Yes, sometimes the “defects” in the tire and rim are
actually “put to work” to make corrections,
using force matching as described later.
THE FIRST THING WE DID was mount
each of my “new” old wheels on the machine
to check them out without tires mounted.
This is typically not a normal procedure, but
if you have questionable rims, or very expensive rims like forged RENNtechs that sell for
$1,500 each, this might be a good procedure to persuade your tire specialist to do.
My rims are older 14" pressed-aluminum
rims. Popular with vintage racers, and well
known amongst 190SL owners, I found these
after two years of searching. My goal was
to reduce the unsprung weight on my 1969
280SL to improve its handling. Each wheel
saves over 20 pounds of unsprung weight.
Three of the four rims were perfect in
that they exerted no measurable impact on
road forces, and they were true with no
runout. The Hunter machine confirmed this
by measurement. What this means is that
all corrections and all radial force variations
using tires on these rims were from the tires
alone—the wheels had no effect. However,
the fourth rim showed .050" of runout and
hop, and also added .25 ounces to the overall balancing. While I was a bit alarmed—
you could actually see these variations as
the wheel was spinning, Bill only raised an
eyebrow and then indicated that he would
not want to see any more runout in a rim
than this. He assured me we’d be able to
correct the issue. He set that rim aside. As
he did I noticed his fingers were crossed.
Using another mounting machine with rubber arms to protect my wheel’s fresh powdercoat, Bill mounted one of my reproduction
Phoenix tires on a rim, filled it with nitrogen
to nearly 28 pounds, and then mounted the
tire/wheel assembly on the GSP9700. He then
connected another nitrogen line and more precisely the machine filled the tire to exactly 28
pounds. The machine has a “virtual” analog
pressure-gauge on the color LCD screen. He
then removed the nitrogen line, closed the
safety cover, and pressed start.
The GSP9700 went to work, first rolling
the tire to ensure the bead was well seated,
then spinning it to check balance, measured the runout, and finally—at a bit higher
speed—it measured the road force variations. Information on the screen tells the
operator what is happening at all times. It
is quite remarkable how quickly all this happens, literally in a matter of moments. Three
of my four tires and rims were done this way
quickly and efficiently, without any issues.
Using Hunter’s SmartWeight Technology,
the shake and shimmy corrections are computed independently and the operator is given
a choice of where to place the weights to
correct all radial force variations. On the one
wheel that had runout, we were informed to
place a single six-ounce weight on the inside
center of the rim. This would be completely
invisible to the eye with or without a wheel
cover. However, Bill’s long experience in the
industry told him that this was a lot of correction weight, more than he cared to use.
So, he pressed a button and the machine
re-computed another correct balance with
two significantly smaller weights, one of
these was placed on the inside edge of the
rim and the other on the outside edge. The
outside weight would be hidden by the wheel
cover and we got by with less than half of
the original weight required.
The GSP9700 shines a red laser light in
the location where the weight is needed,
and tells the operator exactly how much
weight to install. After the quick weight
installation, the wheel/tire is spun up again,
and one clearly sees that everything is in
balance. The SmartWeight system minimizes
weight usage by optimizing all forces at
work and correcting not by weight, but by
vibration reduction. This allows fewer
weights to be used. That wheel with the
runout? Believe it or not, this was not the
MAY/JUN 2008 | THE STAR 73
Star May/Jun 08 Fea.qxd:TheStar
4/17/08
2:35 PM
Page 74
After balancing
another wheel, after
weight placement, and
a second spin, we see
that the GSP9700 tells
us that all is OK. We
see that this tire and
wheel combination,
again on the lower
right of the screen,
shows a corrected road
force variation of 17
pounds. ◆ If you
have a tire and wheel
showing excessive
road force, the
machine tells you. ◆
Bill Keyes is explaining
the new screen setup
to Satish Tummala who
owns a slightly older
model GSP9700.
“worst” wheel in terms of road forces. This
is important to note because what you see
does not always translate into a problem.
THE GSP9700 ALSO CALCULATES the
total amount of “corrected road force” that
the balanced tire will exert on your car’s suspension system. A near perfect tire and rim
combo might show a corrected road force
of four pounds, a more conventional combination 12 pounds, and a combination
approaching the limits of acceptability—
but still correctable—might show 26
pounds. If you or your car are particularly
sensitive to wheel balance it is important
to write these numbers in white crayon on
the inward-facing side of the tire or rim.
You place the wheel with the smallest
corrections on the driver’s front, next best
on passenger front, third best on the
driver’s rear, and the tire with the highest
corrected road force on the passenger rear.
This will ensure that the driver feels as
little impact as possible from the tires. My
tires and wheels showed just such a range,
from four pounds to 24 pounds, and we
placed them accordingly on my SL.
MENTIONED PREVIOUSLY, one feature of
the GSP9700 is a technique known as ForceMatching. This is a procedure where a tire
and wheel combination—first rejected
during road force balancing as out of spec—
is brought back into spec. The machine
calculates the point on the rim of low force
variation, and the point on the tire of high
force variation. It gives the operator the
ability to mark the tire and rim, he or she depressurizes the combo, lines up these two
spots, and lets the machine recalculate. If at
all usable, the tire and rim can then be road
74 THE STAR | MAY/JUN 2008
force balanced. This is using the inherent
“flaws” in the tire and the rim to self-correct
the combination. If it sounds like matching
the paint dot to the valve stem, it is akin to
this, but instead of simply guessing it is accurately computed using road forces.
When initially mounting these same
Phoenix tires on my steel rims two years ago,
we had to do this on one combination, and
we found one tire that simply was so out of
specification it would not balance using force
matching. I sent it back to the manufacturer
as a defect to exchange. Bill also mentioned
to me that he “likes steel wheels,” because
they do have more variance than light alloy,
and sometimes this variance can be put to
use. Note that on three of my four current
alloy wheels—the ones that had no measurable forces or runout—using ForceMatching
would not have been possible. They were too
“perfect.” Thankfully, we didn’t need to
employ this technique.
THE NEW 3M WHEEL WEIGHT SYSTEM
that Hunter recommends was delivered with
this equipment. Instead of a myriad number of weights of different sizes and types for
steel and aluminum rims, usually in toxic
lead, 3M developed a conformable, flexible
composite material that is lead free. Delivered on a roll with a storage and cutting
system, the operator simply rolls out what
they need on the ruler, which is computed
in ounces and grams instead of inches. Then
they cut exactly what they need. No more
running out of one type of weight or
another, or mixing weights on a wheel. Plans
call for integrating this system directly with
the GSP9700 such that the system will automatically cut and dispense the proper
weight(s) upon request. Compare the
photos of the 3M system versus the other
weights that were on my wheels.
JUST WHEN YOU THOUGHT that this system as described offers you everything,
there’s one more patented feature: StraightTrak LFM (Lateral Force Measurement). This
is another feature of the GSP9700 that
allows the operator to correct for pulling
when related to tires. While you might have
believed that pulling was caused only by
improper wheel alignment, some pulling can
be tire related from the shaping of the
profile of the tires. This can be caused by
manufacture or uneven wear.
One solution is shaving the tire to a uniform profile. Another way is to use StraightTrak LFM. My resident mathematician is on
break at the moment but understand this: the
GSP9700 also measures lateral (side-to-side)
forces. These measurements and inherent computations are used to calculate where to place
each tire and wheel to minimize the pulling
as related to tires. Thankfully, I had no pulling
at all so we did not need to employ this
feature in our procedures here.
If we hadn’t been taking pictures, and
stopping to learn and understand the
machine and its procedures, the mounting
and balancing of my tires would have taken
less than five minutes per wheel. When we
were all done, my car cruised home all the
way up to, well, past the speed limit and all
my vibrations were a distant memory.
You can find a local shop or learn more
about Hunter systems by visiting their website at www.gsp9700.com. The GSP9700 is
in use at many Mercedes-Benz dealers
throughout the country as well. Find your
local GSP9700 at a shop near you, and you
too, can finally get rid of the shakes. •