Static and dynamic unbalance, off–the–car explain …

1 Static and dynamic unbalance, off the carbalancing, matching, optimization,finishbalancing .. - terms which are connectedwith tyre service routine and yet are not alwaysquite easy to understand. That is why we want toexplain those terms and present somefundamentals of balancing technology withoutgoing too much into technical details. Contents Page1. Static / dynamic unbalance22. Possible causes of unsmooth ride 53. How to remedy unsmooth ride64. Test your understandingof optimization95. On the car balancing111. Static / dynamic unbalanceSteering 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.

2 Before we are going to study thepossible causes of those vibrations ..To begin with we will study the term of unbalance and come to understandthe difference between Static anddynamic we will continue with the questi-on which factors in the car wheel mightadversely affect smooth we will show you how to remedyunsmooth ride:Beginning with off the car balancingwe will proceed with matching andfinally come to the patented HOFMANN opto ride optimization is then up to you to make a little testshowing if you have actually under-stood the optimization the closing chapter we will explainwhy finishbalancing, that is balancingon the vehicle, is indispensable forsmooth and convenient ride even afterperfect off the vehicle balancing andoptimization of the let us first answer the question Whatdoes unbalance mean indeed?

3 Unbalance is the heavy spot in the tyreor rim structure which has beenproduced in manufacture (Fig. 1).Those heavy spots (due to non uniformmass distribution) make the wheel andsteering system vibrate during the rideunless they are we distinguish betweentwo types of unbalance:1. Static unbalance2. dynamic unbalanceThe difference is easily explained on thetyre. Static unbalanceexists 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 contrary applies to dynamicunbalance. With dynamic unbalancethetyre has two heavy spots diagonallyopposed (Fig. 3). If such spots areequally heavy, they compensate foreach other when the wheel does what do you expect the wheel tobehave like once set into rotation:Fig.

4 4a) Will the wheel bounce?b) Will the wheel wobble?c) Will the wheel run smoothly?3 Fig. 1 Fig. 2 Fig. 3 Fig. 4 RimunbalanceTyreunbalanceStaticunbalance Dynamicunbalancea)b)c)re a): The wheel would bounce if it hadstatic unbalance. dynamic unbalancemakes the wheel wobble (hence b iscorrect).re b): You are wheel with dyna-mic unbalance tends to wobbling onceset into 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 onlyrecognizedwhen the wheel is set into wider a tyre, the more increases theeffect of dynamic unbalance, that is themore wobbling is noticeable during theride. Fig. 5 and this context let us consider the que-stion of how and where Static ordynamic unbalance can be your understanding:State of the art wheel balancers dividethe wheel into two halves, the so calledplanes (Fig.

5 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 so called 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 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. 4 Fig. 5 Fig. 6 Fig. 7 Fig. 8a)b)c)Conventional tyrePlane separationBalance weights 20g20g40g30g30gDynamic unbalance 25g25gWide tyre2.

6 Possible causes ofunsmooth rideIn their structure tyres and rims possessa variety of properties that might causeunsmooth ride:First possibility:The rimpossesses unbalance, that is aheavy spot, which adversely affectssmooth ride (Fig. 9).When the wheel is perfectly balanced,this defect of the rim is completelycompensated possibility:The rimpossesses admissibledeformat-ion caused in manufacture -the so called egg shape (Fig. 10).The unsmooth ride will be noticeddespite of perfect balancing of thewheel (balanced egg ).Third possibility:The tyrepossesses unbalance, that is aheavy spot (Fig. 11).When the wheel is perfectly balancedthis defect of the tyre is completelycompensated possibility:The internal tyrestructure presentsmore and less flexible parts whichmeans tyre deflection will vary over onerevolution of the wheel, hence duringride (Fig.)

7 12).The unsmooth ride will be noticeddespite of perfect balancing of 9 Fig. 11 Fig. 12 HeavyspotFlexible spotEgg-shaped rimFig. 10 Heavy spot(unbalance)unflexible spot3. How to remedyunsmooth rideWhat are now the techniques toremedy unsmooth ride on a vehicle?1. BalancingTyre and rim unbalance is determinedduring the measuring run andcompensated for using balance weights(Fig. 13).Mind:Unsmooth ride - due to deformation ofrim and non uniform tyre deflection - isstill possible after MatchingWith this technique nowadays used ingarages and motor vehicle 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 :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 13 Fig.

8 14 Balance weightSmallbalance weightRimunbalanceHeavyrim spot(unbalanceTyre unbalanceHeavytyre spot(unbalance)3. HOFMANN opto ride techniqueThe most important target of thisoptimization procedure is to determinerim deformation, or more precisely thehighest rim spot Fig. this takes three measuringruns - please see can rim deformation asdetermined be compensated for?Rim deformation can only becompensated for by non uniform tyredeflection(unflexible tyre spot - intechnical terms radial force variations)Fig. exact determination of thoseunflexible tyre spots is only possiblewith expensive and highly sophisticatedindustrial follows the lawof probability:In most cases the unflexible tyre spot isalso its heavy spot - Fig. the opto ride technique theunflexible heavy spot of the tyre isopposed to the highest spot on the rimFig.)

9 Tyre and rim unbalance is thencompensated for with balance 15 Fig. 16 Fig. 17 Fig. 18 Rimdeformation egg -shapeFlexible spotHeavy spotHighestrim spotUnflexible spotHeavy unflexible spot of tyreWhat 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 runthe 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 runthe tyre isfitted on the rim and the wholetyre/rim assembly is measured on thebalancing measured result is composed oftyre unbalance and rim deformation(Fig.)

10 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. 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 runthe 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 unbalanceCorrectrim centreActualcentreboreRimdeformationTyre unbalanceTyre unbalanceHighestrim spotOptimizationFirst measuring runSecond measuring runThird measuring run(Electricallycompens-ated)unbalanceof rimHeavy unflexible spot of rimLet us sum up:The patented HOFMANN opto ridesystem does not only determine theunbalances of tyre and rim, but alsorim deformation due to deformation is compensated for byopposing the highest spot on the rim totyre unbalance (where mostly the mostunflexible tyre spot is located).