Front Left Side Tire Wear

Front Left Side Tire WearFRONTLEFT SIDE TIRE WEAR:NOTE:”LEFT”& “RIGHT” are herein referred asone is sitting on the bike.Thanks toRon Fentress who provided me with a ’98 Valkyrie whichexhibited excellent wear patterns after some 7,000 miles ofmostly city driving on the OEM Dunlops D206’s.  Manythanks go out to Marty Rood who first imparted to me thetheory of “left side wear”  the explanation ofwhich and the details of cupping are my own.There has probably been more misinformation and speculationconcerning this common wear pattern on motorcycle tires thanjust about anything else. Why do the front tires show wear onthe left side early? (Of course, that is a USA question as youwill come to see.)One needs really first to understand what causes”side” tire wear to begin with, as this”side” wear is evident on bothsides of a front tire and also to a lesser degree on bothsides of the rear tire. Side wear is a band of wearevident on the sides of the tires at the most common leanangle for that motorcycle. It is caused by theinteraction of the pavement and the tires in a turn. When youturn, your bike really wants to go straight (Newton’sFirst Law of Motion). You apply a force to make it turn inyour wise desire to stay on the curving road. The bike’sdesire to keep going straight is the natural and inertial centrifugalforce in this interaction and the force resisting theinertia making the bike turn to follow the road is the centripetalforce.As one can imagine, there is quite a bit offorce coming into play when your bike makes a turn. You areprobably not a light weight (how about a generous 200lbs withgear?). Your bike likely weighs in at several hundred itself(A loaded Valkyrie goes near 800 or more). Add to that halfton, the actual acceleration of your vehicle (about 45mph in atypical casual turn). So you are running fine up the roaduntil the road presents you with a curve. You pitch your bikeinto a typical lean, that half ton at 45 wants to go straightand you need it to go around the bend. The only thingpreventing a crash is about a 2 inch contact patch betweenyour tires and the pavement where the forces involved in asimple casual turn exceed 1 G in space age parlance. Thescuffing where your tire meets the pavement is what causes the”side wear” bands to appear sooner or later and thisscuffing is the only “force” that iscapable of producing the wear that eventually becomes evident.If one looks at the pictures of the wornfront tire and the wornrear tire, the squared off side band wear is evidentat the typical lean angle for casual riding on typical roads.In comparison with the tread pattern of a newAvon tire, one will see a much deeper tread precisely atthe point where the squared off side band wear will eventuallyappear (might the Avon designers know something about thiswear?).But the question remains – Why is this wearmore evident on the left front inmost cases? Actually, excessive side tire wear is only evidenton the left front in countries whereone rides on the right side of the road.Riding right means that the left side of your tire will havemore (and likely faster) miles on it than the right side. Lefthand turns have a larger radius than right hand turns in rightside driving countries, hence you ride farther (and likelyfaster) turning left than turning right with subsequentincreased side band wear on the tire’s left side. The leftside of your tire has more miles on it (in someextreme cases, twice as many) than the rightside of your tire. And the side of your tire onlygets mileage when you are leaned in a turn, otherwise,this area of your tire does not contact the pavement at all asshown in the photo. European left side drivers find thatthe right side of their front tires will wear out first. Quitethe opposite effect for precisely the same reasons reversed. (Ifyou’re still not convinced, we will re-visit this issue laterwith more reasons)REAR CENTER TIREWEAR:But if it’s only the extra miles that cause the wear, wouldn’tthe center if my tire wear out first since I have more milesupright than leaned? Yes and some upright wear is evident uponinspection of a worn front tire as seen in the photoabove left. Though this wear is not as excessive up frontas the sidewall wear because of one factor… Upright miles ona properly inflated front tire are rollingmiles with little scuffing taking place. If, on the otherhand, youlook at your rear tire, you will indeed see that thecenter wears out first and this wear is often exaggeratedbecause acceleration, engine braking and real braking scuffstuff off the upright rear tire. Each time you downshift,upshift, roll on the throttle or roll off the throttle, youwill scuff the rear tire at the contact patch. Along withthat, the rear is your drive tire and at speed, the rearcontact patch is the only thing that keeps you going (don’tbelieve it? Just let off the throttle and see how quickly yourbike slows to a stop!). Since most acceleration/decelerationand braking occurs when the bike is more or less straight upthis wear is most evident in the center of the rear tire.Drive shaft bikes are the worst offenders since they arenotably “herky jerky” and transfer the shock ofaccel/decel directly to the rear contact patch unbuffered.Belt and chain drives will “buffer” these shocks andlessen this kind of wear. This same scuffing action is minimalon the front tire because the front tire is undrivenand merely rolls while the rear tire is doing all the inertialwork. When brakes are applied, traction at the front tireimproves minimizing scuffing while traction at the rear tiredeteriorates maximizing scuffing.CUPPING:Cupping, which is more accurately described as scalloping(see pictures, but we will use the more commonterm “cupping” here), is a natural wear pattern onmotorcycle tires and it will always follow the tread pattern.It is not a sign that you have bad suspension parts. It merelyshows that your tire is indeed gripping the road when you maketurns (thank you for that Mr. Tire!). This cupping developswithin the side wear bands of a leaned motorcycle. The extremeforces that come in to play when the bike is leaned in a turnare what produce the effect and when the wear becomessufficient, one will experience vibration and noise when onebanks into a turn. Upon examination of the pictures at left ofour samplerear Avon, our dustedfront VTX Dunlop D256, and the pictureof our chalked Dunlop D206 one can see how the cuppingfollows the tread pattern. The leading edge of the tread doesnot flex much as it grips the road and the rubber is scuffedoff the tire in that area causing a depression. As the tirerotates, the pressure moves to the trailing edge of the treadpattern where the tread flexes more causing less scuffing soless material is ground off the tire. The more complex thetread pattern, the more complex the cupping pattern will be.The softer the compound of the tire, the sooner this cuppingwill develop. Radial tires are more prone to cupping than arebias ply because the compound of radials is softer. As one cansee, the simple tread pattern of the Avonpictured produces a simpler scallop pattern while the morecomplex VTXD256 Dunlop is somewhat involved, though still easily seenin our photo. Cupping on the ValkyrieDunlop D206 is very hard to photograph because of thecomplex tread pattern. Low tire pressure will exacerbate thiswear pattern and you will lose many serviceable miles byrunning low. Improper balance has nothing to do with cuppingon a motorcycle tire. Improper balance will merely cause yourbike to vibrate within certain specific speed ranges.The following textual illustration comes from Martin whocontributed to this article by E-mail on June 26, 2006:I was just reading your bit on”cupping” and thought I’d share with you how Idescribe what’s going on.  I usually tell people thatwhat’s happening is that the individual “blocks” or”islands” of tread are squirming and deforming dueto the forces applied to them during cornering and braking.When this deforming takes place, the wear is naturally notevenly distributed across the surface of the tread.  (Idefine a tread block as an area of the tire surface surroundedby a groove.)  I next tell people they can demonstrate tothemselves what’s happening by taking a new pencil with anunused eraser on the end and while holding the pencilperfectly vertical, push down and drag the eraser on a roughsurface in one direction.  Then I tell them to look atthe eraser and note that all the wear is on the leading edgeand not evenly distributed across the end surface of theeraser.  It seems to make the concept easier for many tounderstand.  Cheers!OTHER FACTORS:The frame geometry of the motorcycle can play a majorrole in how early on the “left side” phenomenonmakes itself evident. On the Honda Valkyrie and the Honda VTXmodels, rake is 32° and trail is nearly six full inches. Along trail can cause the wear on the sides of the front tireto show earlier because of the “shear” effect inturns as your front wheel is pointed slightly in the directionof the turn. The front wheel has less a tendency to rollthrough turns and the shear force at the contact patch helpsscuff the tire at the common lean angle.  As a personaltest, I ran an OEM D206 Dunlop on the front of a stockValkyrie and got around 8K miles on it. When replaced, it haddefinite and prominent signs of left side tire wear. Ireplaced the tire with another D206 Dunlop and after about 3kmiles, it began to show signs of left side wear as well. Atthat time, I replaced my stock triple clamp with a TBR tripleclamp which reduced trail. From that time on, the left sidewear was reduced to a great degree. The Avon with which Ireplaced that D206 now has over 15k miles on it and thoughthere is some evidence of left side wear, it is not aspronounced as it is with some I’ve seen using the stock clamp.The Avon profile helps the front tire roll through turns withless shear.Rake is measured as the angle of the steering knuckle offperpendicular with the bike upright, at rest and unloaded on alevel surface.  Trail is measured from where that angleintersects the level surface to the point plumb with the frontaxle. Factors that can affect trail: the angle of the forksrelative to the knuckle; the distance of the forks from theknuckle; raising or lengthening front suspension to increaserake; lowering rear suspension to increase rake; loading thebike with a passenger; apply brakes (dive). Ever notice thatit’s harder to steer your bike when you have a passenger?It’s because the extra weight compresses the rear shocksincreasing rake besides making the bike heavier overall.Rake hence trail changes are also dynamic so should one changethe balance of the rear suspension with the front (the rearflexes more than the front or vice verse in turns and causesthe rake/trail to change while in a turn). Causing the frontto dive less in turns with heavier fork oil or heavier springswill take some weight off the front wheel and lessen side bandwear.Trail is necessary for your bike to be stable goingstraight, but it is also necessary for turns since it causesyour front wheel to steer itself into the turn.Countersteer and trail’s geometry effect resulting in a turnis discussed at Wikipedia in detail.  Clickhere to visit.  A good video of using countersteerfor turns is found here (VIEWMPG).  Note in that film how countersteer is used toinitiate the turn (the bike momentarily turns in the wrongdirection) then once the bike leans shortly thereafter, trailcauses the front wheel to turn itself into theturn which is what actually steers the bike in the desireddirection.OTHER THEORIES DISPROVED:But what about road crown? Plausible and many claim it, but roadcrown plays no part whatsoever in left side tire wear. Thoughit does seem logical and also allows that in European leftside driving countries, the crown is opposite which couldexplain right side wear there – road crown just doesn’thold up under scrutiny. The main reason the theory falls onits face – road crowns are simply not steep enough. If youexamine a picture of the fronttire upright, you can see that even a very steep roadcrown (one inch drop in one foot run) would not evencontact the tire at the necessary angle to produce theevident wear. Typical road crowns are much much less (threeinches drop per twelve foot run).  In fact, if one examinesthe picture of the tire one can see that where a roadcrown would actually contact the tire, there is a PEAKrather than a valley (check our extreme wear pic at the top ofthis page). Plus, as described in the paragraphs above, roadcrown contact is mostly “rolling” contact whichproduces very little wear if any at all. Just sothere is no misunderstanding, road crownplays no part whatsoever in left side tire wear.Road crowns, if they exist at all on a road, are completelyinconsistent and vary greatly as to pitch, vary even moregreatly in turns (road engineers do indeed “bank”turns), and crowns in no way contact the tire at20°off horizontal where the wear occurs. Road crowndoes not cause side tire wear.STILL NOT CONVINCED?If you are still not convinced that increasedmileage is what causes one sided tire wear on the front tireof motorcycles, you’ll have to come up with a theory thatsatisfies all of the evidentiary criteria. A)It will have to explain the fact that when riding upright, thetire’s side wear bands do not contact the pavement (roadcrown, unbalanced/off center bike weight and even wheelmisalignment won’t work). B) It will have toreverse itself in countries where one rides on the left sideof the road rather than the right (road crown still soundsplausible here but it was eliminated in “A” above).In the case of right side driving countries like the USA,one does indeed ride farther on the left side of the tire thanon the right side of the tire. At a simple single laneintersection that is common in most residential neighborhoods,negotiatinga left turn will have you traveling TWICE the distance thatyou do making a right hand turn. That’s at a simple singlelane intersection. A double lane will have you making fourtimes the distance. But even when you are confronted withnothing more than a left curving road, the radius of that leftturner will be larger than if you were coming the other way onthe same road making a right around that same curve. If youdon’t believe this, check out the How Stuff Works web siteabout your car’s DIFFERENTIALand why it’s called a differential (your shaftdriven two wheeled motorcycle does not have a differential, ithas instead a “final drive” which drives only onewheel). Besides the fact that the left radius is larger whichmeans you will probably go faster causing more stress on yourtire than you would going the other way, there is morevisibility when making lefts than rights which will add toyour tendency to make the turn faster as well. Failure tonegotiate a left turn will have you going off the the roadonto the shoulder or into a ditch. Failure to negotiate aright turn will have you crossing into opposing traffic.Though neither scenario is appealing, there is a subliminaladvantage to left turns (riding shoulders and ditches isbetter than crashing into trucks head on) and this will haveyou going a bit faster on lefties too.The increased radius on left turns means more distance istraveled turning left than turning right on the average ridingday. That is plane geometry and plainly undeniable. Because ofthe natural tendency to make left turns faster (admittedlythis is subjective and open to debate, but is plausible forreasons given) there will be more stress placed on your tiresas they travel that longer left distance. Increased left sidetire wear is evident, though, on both the front and rear tiresbut because the front tire shows less evidence of flat bandcenter wear (which disguises the side wear bands on the reartire), side wear is more evident to the eye up front andleaves you to wonder, “Why does the left side† of myfront tire wear out first?” Now you know.† Of course, if you live in Singapore,you’ll say, “Why does the RIGHT side of my front tirewear out first?” And now you know as well![Custom Motorcycle ] [Motorcycle News] [Motorcycle Tours ] [Motorcycle RacingNews] [OffRoad Motorcycles] [ContactUs]Copyright© Steel Horse MotorcycleNews.  May 15, 2012.

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