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Static and dynamic unbalance, offthecarbalancing, matching,
optimization,
finishbalancing ... - terms which are connectedwith tyre service
routine and yet are not always
quite easy to understand. That is why we want toexplain those
terms and present some
fundamentals of balancing technology withoutgoing too much into
technical details.
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Contents Page
1. Static / dynamic unbalance 2
2. Possible causes of unsmooth ride 5
3. How to remedy unsmooth ride 6
4. Test your understandingof optimization 9
5. Onthecar balancing 11
1. Static / dynamic unbalance
Steering wheel flatter, vibrations sensedin the passenger
compartment ... - thereis no one who would not have experi-enced
such inconveniences someday orsomeway. Main causes for those
vibrati-ons sensed in the passenger compart-ment are the structures
of tyre and rimand the combination of those negativeeffects,
sometimes under most unfavou-rable conditions and on all four
wheels. Before we are going to study thepossible causes of those
vibrations ...
To begin with we will study the term ofunbalance and come to
understandthe difference between static anddynamic unbalance.
Then we will continue with the questi-on which factors in the
car wheel mightadversely affect smooth ride.Next we will show you
how to remedyunsmooth ride:
Beginning with offthecar balancingwe will proceed with matching
andfinally come to the patented HOFMANNoptoride optimization
technique.
It is then up to you to make a little testshowing if you have
actually under-stood the optimization technique.
In the closing chapter we will explainwhy finishbalancing, that
is balancingon the vehicle, is indispensable forsmooth and
convenient ride even afterperfect offthevehicle balancing
andoptimization of the wheels.
2
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... let us first answer the question Whatdoes unbalance mean
indeed?.
Unbalance is the heavy spot in the tyreor rim structure which
has beenproduced in manufacture (Fig. 1).
Those heavy spots (due to nonuniformmass distribution) make the
wheel andsteering system vibrate during the rideunless they are
balanced.
Basically we distinguish betweentwo types of unbalance:
1. static unbalance2. dynamic unbalance
The difference is easily explained on thetyre. Static unbalance
exists when thetyre has one heavy spot (Fig. 2). Whensuch a tyre is
suspended the heavy spotwill tend towards the bottom. Thismeans
static unbalance can be determi-ned without rotation of the
wheel.
The contrary applies to dynamicunbalance. With dynamic unbalance
thetyre has two heavy spots diagonallyopposed (Fig. 3). If such
spots areequally heavy, they compensate foreach other when the
wheel does notrotate.
But what do you expect the wheel tobehave like once set into
rotation:
Fig. 4
a) Will the wheel bounce?b) Will the wheel wobble?c) Will the
wheel run smoothly?
3
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Rimunbalance
Tyreunbalance
Staticunbalance
Dynamicunbalance
a) b) c)
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re a): The wheel would bounce if it hadstatic unbalance. Dynamic
unbalancemakes the wheel wobble (hence b iscorrect).re b): You are
right. A wheel with dyna-mic unbalance tends to wobbling onceset
into rotation.re c): As the two heavy spots arediagonally opposed
the wheel willnever allow smooth ride, but tends towobbling when
set into rotation(hence b is correct).
Important:Dynamic unbalance is only recognizedwhen the wheel is
set into rotation.The wider a tyre, the more increases theeffect of
dynamic unbalance, that is themore wobbling is noticeable during
theride. Fig. 5 and 6.
In this context let us consider the que-stion of how and where
static ordynamic unbalance can be eliminated.
Important for your understanding:Stateoftheart wheel balancers
dividethe wheel into two halves, the socalledplanes (Fig. 7).If the
wheel possesses static unbalance(one heavy spot of , say, 40 g)
thatunbalance is compensated for by atta-ching each one balance
weight of 20 gleft and right at the diametrically oppo-site sides
of the wheel (Fig. 7).
If the wheel possesses dynamic unbalan-ce (two heavy spots
diagonallyopposed) the socalled plane separationfeature is applied:
Each wheel plane(wheel half) is considered separately;the dynamic
unbalance portion can onlybe compensated for diametricallyopposed,
that is at the same wheel side,because otherwise there would
anotherunbalance.In our example the unbalance of 25 gof the left
wheel plane is compensatedfor by a balance weight of 25 gattached
to the diametrically opposed(left) wheel position. The
sameprocedure is applicable to the rightwheel side (unbalance of 30
g) - Fig. 8.
4
Fig. 4
Fig. 5 Fig. 6
Fig. 7
Fig. 8
a) b) c)
Conventional tyre
Plane separation
Balance weights 20g 20g
40g
30g
30g
Dynamic unbalance 25g
25g
Wide tyre
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2. Possible causes ofunsmooth ride
In their structure tyres and rims possessa variety of properties
that might causeunsmooth ride:First possibility:The rim possesses
unbalance, that is aheavy spot, which adversely affectssmooth ride
(Fig. 9).When the wheel is perfectly balanced,this defect of the
rim is completelycompensated for.
Second possibility:The rim possesses admissibledeformat-ion
caused in manufacture -the socalled eggshape (Fig. 10).The unsmooth
ride will be noticeddespite of perfect balancing of thewheel
(balanced egg).
Third possibility:The tyre possesses unbalance, that is aheavy
spot (Fig. 11).When the wheel is perfectly balancedthis defect of
the tyre is completelycompensated for.
Fourth possibility:The internal tyre structure presentsmore and
less flexible parts whichmeans tyre deflection will vary over
onerevolution of the wheel, hence duringride (Fig. 12).The unsmooth
ride will be noticeddespite of perfect balancing of thewheel.
5
Fig. 9
Fig. 11
Fig. 12
Heavyspot
Flexible spot
Egg-shaped rimFig. 10
Heavy spot(unbalance)
unflexible spot
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3. How to remedyunsmooth ride
What are now the techniques toremedy unsmooth ride on a
vehicle?
1. Balancing
Tyre and rim unbalance is determinedduring the measuring run
andcompensated for using balance weights(Fig. 13).
Mind:Unsmooth ride - due to deformation ofrim and nonuniform
tyre deflection - isstill possible after balancing.
2. Matching
With this technique nowadays used ingarages and motorvehicle
workshopsthe tyre is readjusted relative to the rimuntil tyre
unbalance and rim unbalanceare opposed and then - at least partly
-compensate for each other (Fig. 14).The small residual unbalance
thusachieved will be compensated for withbalance weights.
Mind:If the position of rim unbalance doesnot coincide with the
position of rimdeformation (which is very likely withalloy rims),
unsmooth ride will still bepossible after matching.
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Fig. 13
Fig. 14
Balance weight
Smallbalance weightRim
unbalance
Heavyrim spot(unbalance
Tyre unbalance
Heavytyre spot(unbalance)
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3. HOFMANNoptoride technique
The most important target of thisoptimization procedure is to
determinerim deformation, or more precisely thehighest rim spot
Fig. 15.
Why this takes three measuringruns - please see overleaf.
How can rim deformation asdetermined be compensated for?
Rim deformation can only becompensated for by nonuniform
tyredeflection (unflexible tyre spot - intechnical terms radial
force variations)Fig. 16.
As exact determination of thoseunflexible tyre spots is only
possiblewith expensive and highly sophisticatedindustrial
machines.HOFMANN follows the lawof probability:In most cases the
unflexible tyre spot isalso its heavy spot - Fig. 17.
With the optoride technique theunflexible heavy spot of the tyre
isopposed to the highest spot on the rimFig. 18.
Residual tyre and rim unbalance is thencompensated for with
balance weights.
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Fig. 15
Fig. 16
Fig. 17
Fig. 18
Rimdeformation egg -shape
Flexible spot
Heavy spot
Highestrim spot
Unflexible spot
Heavy unflexible spot of tyre
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What will happen in the individualmeasuring runs?
Important: The rim must have the sameposition before every
measuring run(e g valve in top vertical position).In a first
measuring run the rim alone,that is without tyre, is measured
andthe unbalance determined iscompensated for electrically (will
beadded again after the last measuringrun). What is left after the
first measu-ring run is the effect of deformation ofthe rim, which
is still unknown (Fig. 19).
In order to determine amount andposition of rim deformation
anothertwo measuring runs are necessary:For a second measuring run
the tyre isfitted on the rim and the wholetyre/rim assembly is
measured on thebalancing machine.The measured result is composed
oftyre unbalance and rim deformation(Fig. 20).
In order to exactly determine rimdeformation, that is the
highest spot onthe rim, the tyre has to be readjustedby 180 degrees
relative to the rimbefore the third measuring run -Fig. 2.
This means the highest spot on the rimis opposed to tyre
unbalance, whichmostly is the unflexible spot on the tyre(Fig.
22).
In the final measuring run the rimunbalance previously
compensated forelectrically is added again and read outtogether
with the residual unbalance ofthe tyre (Fig. 22).
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Fig. 19
Fig. 20
Fig. 21
Fig. 22
Rim unbalance
Correctrim centreActualcentrebore
Rimdeformation
Tyreunbalance
Tyre unbalance
Highestrim spot
Optimization
First measuring run
Second measuring run
Third measuring run
(Electricallycompens-ated)unbalanceof rim
Heavy unflexible spot of rim
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Let us sum up:
The patented HOFMANN optoridesystem does not only determine
theunbalances of tyre and rim, but alsorim deformation due to
manufacture.Rim deformation is compensated for byopposing the
highest spot on the rim totyre unbalance (where mostly the
mostunflexible tyre spot is located).With the optoride
techniqueHOFMANN offers the optimum smoothride as achievable with
garageequipment and is, therefore, able tosatisfy even most
sensitive customerswith respect to driving comfort.Do you think you
now have correctlyunderstood the fundamentals ofoptimization?
4. Test your understandingof optimization
Which of the four examples given(A - D) shows the wheel in
optimizedcondition?
correct rim centre
actual centrebore
heavy spot (unbalance)
unflexible spot
elastic spot
rim deformation(highest spot on rim)
For the solution see overleaf!
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A
B
C
D
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A: Either you did not look at the figureproperly, or you have
not understoodthe optimization procedure correctly.In figure A the
highest spot on the rim(deformation) is not opposed to theheavy
unflexible spot on the tyre -hence B is correct.
B: Congratulations! You have thecorrect understanding of
whatoptimization is like.Only in this example is rim
deformation(highest spot on rim) opposed to theheavy unflexible
spot of the tyre.The position of rim unbalance does notimport at
all for optimization because itcan easily be balanced.
C: Maybe you simply made a slip.When you look at the figure more
clo-sely you will see that rim deformation(highest spot on rim) and
the heavyunflexible spot of the tyre coincide -whereas they have to
be opposed withan optimized wheel. Hence B is correct.
D: By opposing tyre and rim unbalances(matching) residual
unbalance can bereduced.But rim deformation, which is not
com-pensated for, will continue to adverselyaffect smooth
ride.Wheel optimization is shown in Fig. B.
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A
B
C
D
Highestrim spot
Heavy unflexible spot of tyre
Heavy unflexible spot of tyre
Highestrim spot
Highestrim spot
Heavy unflexible spot of tyre
Tyre unbalance
Rimunbalance
Highestrim spot
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5. Onthecar balancing
In this last chapter we would like tostudy the question why
thefinishbalancer (for onthecarbalancing) is an indispensable tool
toachieve optimum ride of every vehicledespite of previous
offthecarbalancing and optimization of thewheels.
The fundamental difference:Both offthevehicle balancing and
theHOFMANN optoride technique are"only" able to optimize ride as
relatedto the wheel - Fig. 23.
The finishbalancer, however, handleswheels and vehicle as one
unit - Fig. 24.
Even a perfectly optimized wheelmostly presents new unbalance
whenfitted on the vehicle.It is caused by added
manufacturingtolerances of the vehicle componentssuch as:
manufacturing tolerances
of wheel hub residual unbalances of hub unbalances in brake
drum
or brake disc
The HOFMANN finishbalancerdetermines the effects of
suchmanufacturing tolerances of vehiclecomponents, which might
betransmitted to the wheel and producenew unbalance.
With the selective measurementtechnique at different
speedsHOFMANN ensures that even withdriven wheels every wheel is
consideredsingly and unaffected by influences ofthe opposite wheel
- Fig. 25.
If you are interested in details on thismeasurement technique,
please see ourseparate brochure "Precision balancingon the vehicle
at different speeds".
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Fig. 23
Fig. 24
Fig. 25
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9472 102 03.03 Printed in GermanyTechnical modifications
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Snap-on Equipment GmbHGeschftsbereich Hofmann
Werkstatt-TechnikPOB 1202 64311 Pfungstadt/Germany 061 57 12-450
061 57 [email protected]
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