& ROUND WE GO
By Mike Perkins
BMW CCA/Houston Chapter Technical Advisor
Owner - Bavarian Machine Specialties
This article is also available
as a PDF file:
"How can my steering wheel still shake!" she exclaimed.
"I've had the tires balanced several times and even had
the alignment checked." Does this sound all too familiar?
This is one of the most common complaints I see, and unless
properly addressed, can lead to expensive front end repairs,
or worse, a loss of driving enjoyment from ones BMW. Her 1997
528i recently had new tires installed and thereafter developed
a slight shimmy, nothing big, just an annoyance in the 50-60
mph range. 2000 miles later, the car had a full blown steering
wheel shake. How can this seemingly simple job of mounting
and balancing tires end up becoming such a problem situation?
Simply put, there is a lot to having the job completed successfully.
This article will point out some of the basic concepts involved
in achieving the smoothest ride and longest term tire wear
from your BMW. When diagnosing and correcting front end related
vibrations, there are three major areas that must be investigated.
These areas are steering and suspension components, tire and
rim run-out, and tire and rim imbalance.
I - STEERING AND SUSPENSION COMPONENTS
is essential to start with an initial inspection of the front
end steering and suspension components. Minor wear in suspension
bushings, ball joints, tie rods, center links, idler arms,
or steering gear can amplify or even induce a front end shimmy.
For example, weak front shock absorbers are the classic problem
when it comes to irregular tire wear. Tires that exhibit a
"scalloped" outer edge or are extremely "cupped"
in places are the result of shocks that can not handle the
bounce of the wheels. This can also be a cause and effect
issue. By virtue of an out of balance tire, the shock wears
out prematurely. Wear or play in any component can only be
detected by a detailed inspection of each piece. A push/pull
or clamp/release method is usually employed to test joints
for wear. However, it must be recognized that if a car is
on a lift with its wheels hanging down, there will be a different
joint reaction than one on a lift that leaves the wheels in
a loaded position. Generally speaking, Houston roads are brutal
on a highly tuned and responsive suspension such as that found
on BMWs. It has become the norm to see worn out shocks, ball
joints, and broken bushings by 60,000 miles. The nimble suspension
we all appreciate degrades quickly without upkeep and will
definitely wear your tires.
pressure has a major influence on tire wear and can influence
tire bounce especially if the tire is over-inflated. A simple
rule of thumb: For every 10∞F of temperature change,
your tire pressure changes 1 psi. The pressure increases with
higher temperatures and conversely with lower. It is best
to only check tire pressure when the car has not been driven
or has not been sitting in the sun. Each BMW has a handy little
label on the driver's "B" pillar (inside the door
jam) giving you a recommended tire inflation pressure to follow.
2 psi can make a big difference.
component that can greatly affect tire shimmy or shake is
a worn wheel bearing. Usually a BMW wheel bearing will let
you know when a problem exists by a growling noise above 25
mph which is especially noticeable when driving on smooth
pavement. This noise is not be confused with a tire roar from
uneven tread wear. Some tires, because of tread design, are
inherently louder and actually worsen with mileage due to
the "blocky" tread design. This kind of tire requires
more frequent rotation.
kind of shimmy is when the brake is applied at speeds above
50 mph. If you are thinking rotor shimmy, you would be partly
correct. Add another dimension to it and you have rotor induced
suspension shimmy. This is where the applied braking force
collapses the strut arm bushings and starts an oscillation
that produces a perverse front end shake that makes it seem
like the dash is vibrating.
last item I consider part of the steering and suspension category
is the overall alignment. Many times I have been asked to
check the front end alignment because the steering wheel has
a shimmy. From experience, I know that no amount of re-aligning
the front end of your BMW will change the degree of steering
wheel shimmy. A properly aligned car will help prevent a shimmy
problem down the road by keeping tires wearing evenly but
it will never correct the actual shimmy. This is definitely
a cause and effect issue.
II - TIRE AND RIM RUN-OUT
second area that must be investigated is tire and rim run-out.
This is the most common and most overlooked problem. Tire
and rim run-out are identified as radial and lateral run-out.
Radial run-out is an excessive amount of "hop" in
the wheel and lateral run-out is an excessive amount of "wobble"
in the wheel. No matter how well a wheel is balanced, it will
always cause a vibration if either type of run-out exists.
This run-out can be a bent rim or simply a tire that has not
retained its original round shape. New tires can even have
excessive radial or lateral run-out. It is a prerequisite
for the person installing your tires to test for any rim run-out.
This, unfortunately is where the job has problems before it
even gets started. The rim must be measured with a dial indicator
to identify any problems. Minor rim run-out can be accepted
if the rims are to be used in the rear of the car but must
be marked to avoid future problems when rotating. Rims to
be used on the front of the car must have less than .040"
run-out as measured on the inside of a dismounted rim lip.
This means the person installing your tires must dismount
all the rims and pick the best two for use on the front of
the car before even considering tire mounting. However, some
rims on late model BMWs are of different widths. If any rim
in this case exceeds the maximum run-out, the rim can possibly
be straightened and then installed in its original position.
the tires are mounted, the radial and lateral testing process
must now be applied to the tires. Tires must not exceed a
limit of .060" run-out after final mounting. If not,
a process of phase matching should be used. This process corrects
for radial or lateral run-out in tires. Dismounting the tire
from its wheel position and remounting it matching the minimum
tire run-out point to the maximum rim run-out point will produce
a rounder tire/wheel combination. In some extreme cases, a
new tire is required. To further complicate this situation,
most higher performance tires are pre-marked for mounting
position. Specifically, the manufacturer has indicated a mounting
position in reference to the valve stem. A relatively new
style of balancer has been marketed by Hunter which actually
applies a rolling force to the tire to measure uniformity
of the wheel. This is the latest in balancer technology and
certainly helps the operator obtain a true balance.
III - TIRE AND RIM IMBALANCE
third major area of concern when investigating a front end
vibration is actual tire or rim imbalance. All wheels must
be balanced using an on-the-car or off-the-car dynamic spin
balancer to achieve any decent level of overall balance. There
are many good brands of balancers being used in shops today
and most are off-the-car types. However, the key to receiving
the best job still lies with the skill of the technician.
Balancers can be set to read in grams or ounces, rounded or
precise numbers, and display at least four different positions
where the weight can be placed on the rims. Wheel weights
also come in specific profiles to exactly fit the rim lip.
You must be sure the shop has the correct style weight to
fit a BMW rim, otherwise they will fly off and you will immediately
develop an imbalance. The factory 2 piece wheel weight is
always a very good system, although very few places use them
because of the cost and additional time to install. There
is also no way of hiding a factory weight on the inside of
the wheel as you would a tape weight. For those of you who
prefer that the weight not be visible, please realize not
only does it take more weight when located in the middle of
the rim, but the wheel balance will never be as accurate.
Every wheel has 2 planes for balancing, the inside and outside
of the wheel on the outer edges of the rim. The idea of balancing
a wheel is to use the least amount of weight (usually below
thirty grams) and in the most accurate position. To achieve
this, stay with the conventional style of balancing, one weight
on each side of the wheel. When I am confronted with a rounded
or polished outer rim profile, I will use a "drop center"
method (hiding the outer weight behind the spokes) but still
maintain the inner weight in the normal position. In addition
having the weight in the correct position on the rim, it is
imperative to use the exact amount of weight needed. Many
shops that balance wheels allow the balancing machine to operate
in a "round off" mode. This means there is a margin
for error of + / - five grams. Therefore, your wheel might
be ten grams out of balance on at least one side of the rim
and yet show the operator it is perfectly balanced. This allows
for quicker balancing but over time and mileage, the tire
will show the negative results of an imprecise balancing job.
Have your wheels balanced to non-round off specs. If you really
want to see if a good job is being done, look for a digital
scale next to the balancing machine that the operator uses
to check the weights he trims by hand! As long as we are being
picky, make sure all pebbles are removed from the tread prior
to balancing. Another thing to mention about balancing machines
is the way the wheel is attached to the balancing arbor. Most
machines and shops use a cone which lends itself to inaccurate
readings. A finger type hub that engages the tapered lug holes
is a much more accurate method of attachment. These hubs are
expensive but an excellent investment for a dedicated shop.
tire balance situations come to mind that should be avoided.
Anytime you use a liquid fix-a-flat product consider the tire
to be worthless. It either hardens on one side of the tire,
or never hardens and continually changes the tire balance.
In either case, it is next to impossible to balance. The second
situation is a warning about gas stations that do not drain
their air compressor (air compressor tanks fill with water
from the hot air condensing and the tank needs to be maintained).
Each time you fill your tires with air from one of these compressors,
you will also add a small amount of water into your tires.
You can guess the result, so make sure you have a reliable
a final note about balancing tires, I strongly advocate rotating
and balancing wheels every 6000-8000 miles. It is within the
normal scope of everyday driving that tires and rims will
change shape and balance, not to mention hitting pot holes,
railroad tracks, or broken concrete. For those of you who
have either different size wheels (front to rear) or directional
tires, you can still at least rebalance on a regular basis
without rotating. It is also a viable option to dismount your
wheels and swap the tires from side to side. It is understandable
how someone might think "How hard can it be, it's only
tires?" This might be true when it comes to a truck or
a car with a less sophisticated suspension. However, what
makes the cars you and I drive so special is the precise nature
in which a BMW commands the road. To expect that level of
precision requires a mind set above the average approach to
a seemingly simple job.