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.

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

... 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)

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

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

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.

6

Fig. 13

Fig. 14

Balance weight

Smallbalance weightRim

unbalance

Heavyrim spot(unbalance

Tyre unbalance

Heavytyre spot(unbalance)

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

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).

8

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

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!

9

A

B

C

D

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

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".

11

Fig. 23

Fig. 24

Fig. 25

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