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FRONT
LEFT SIDE TIRE WEAR:
NOTE:
"LEFT"& "RIGHT" are herein referred as
one is sitting on the bike.
Thanks to
Ron Fentress who provided me with a '98 Valkyrie which
exhibited excellent wear patterns after some 7,000 miles of
mostly city driving on the OEM Dunlops D206's. Many
thanks go out to Marty Rood who first imparted to me the
theory of "left side wear" the explanation of
which and the details of cupping are my own.
There has probably been more misinformation and speculation
concerning this common wear pattern on motorcycle tires than
just about anything else. Why do the front tires show wear on
the left side early? (Of course, that is a USA question as you
will come to see.)
One needs really first to understand what causes
"side" tire wear to begin with, as this
"side" wear is evident on both
sides of a front tire and also to a lesser degree on both
sides of the rear tire. Side wear is a band of wear
evident on the sides of the tires at the most common lean
angle for that motorcycle. It is caused by the
interaction of the pavement and the tires in a turn. When you
turn, your bike really wants to go straight (Newton's
First Law of Motion). You apply a force to make it turn in
your wise desire to stay on the curving road. The bike's
desire to keep going straight is the natural and inertial centrifugal
force in this interaction and the force resisting the
inertia making the bike turn to follow the road is the centripetal
force.
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As one can imagine, there is quite a bit of
force coming into play when your bike makes a turn. You are
probably not a light weight (how about a generous 200lbs with
gear?). Your bike likely weighs in at several hundred itself
(A loaded Valkyrie goes near 800 or more). Add to that half
ton, the actual acceleration of your vehicle (about 45mph in a
typical casual turn). So you are running fine up the road
until the road presents you with a curve. You pitch your bike
into a typical lean, that half ton at 45 wants to go straight
and you need it to go around the bend. The only thing
preventing a crash is about a 2 inch contact patch between
your tires and the pavement where the forces involved in a
simple casual turn exceed 1 G in space age parlance. The
scuffing where your tire meets the pavement is what causes the
"side wear" bands to appear sooner or later and this
scuffing is the only "force" that is
capable of producing the wear that eventually becomes evident.
If one looks at the pictures of the worn
front tire and the worn
rear tire, the squared off side band wear is evident
at the typical lean angle for casual riding on typical roads.
In comparison with the tread pattern of a new
Avon tire, one will see a much deeper tread precisely at
the point where the squared off side band wear will eventually
appear (might the Avon designers know something about this
wear?). |
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But the question remains - Why is this wear
more evident on the left front in
most cases? Actually, excessive side tire wear is only evident
on the left front in countries where
one rides on the right side of the road.
Riding right means that the left side of your tire will have
more (and likely faster) miles on it than the right side. Left
hand turns have a larger radius than right hand turns in right
side driving countries, hence you ride farther (and likely
faster) turning left than turning right with subsequent
increased side band wear on the tire's left side. The left
side of your tire has more miles on it (in some
extreme cases, twice as many) than the right
side of your tire. And the side of your tire only
gets mileage when you are leaned in a turn, otherwise,
this area of your tire does not contact the pavement at all as
shown in the photo. European left side drivers find that
the right side of their front tires will wear out first. Quite
the opposite effect for precisely the same reasons reversed. (If
you're still not convinced, we will re-visit this issue later
with more reasons) |
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REAR CENTER TIRE
WEAR:
But if it's only the extra miles that cause the wear, wouldn't
the center if my tire wear out first since I have more miles
upright than leaned? Yes and some upright wear is evident upon
inspection of a worn front tire as seen in the photo
above left. Though this wear is not as excessive up front
as the sidewall wear because of one factor... Upright miles on
a properly inflated front tire are rolling
miles with little scuffing taking place. If, on the other
hand, you
look at your rear tire, you will indeed see that the
center wears out first and this wear is often exaggerated
because acceleration, engine braking and real braking scuff
stuff off the upright rear tire. Each time you downshift,
upshift, roll on the throttle or roll off the throttle, you
will scuff the rear tire at the contact patch. Along with
that, the rear is your drive tire and at speed, the rear
contact patch is the only thing that keeps you going (don't
believe it? Just let off the throttle and see how quickly your
bike slows to a stop!). Since most acceleration/deceleration
and braking occurs when the bike is more or less straight up
this wear is most evident in the center of the rear tire.
Drive shaft bikes are the worst offenders since they are
notably "herky jerky" and transfer the shock of
accel/decel directly to the rear contact patch unbuffered.
Belt and chain drives will "buffer" these shocks and
lessen this kind of wear. This same scuffing action is minimal
on the front tire because the front tire is undriven
and merely rolls while the rear tire is doing all the inertial
work. When brakes are applied, traction at the front tire
improves minimizing scuffing while traction at the rear tire
deteriorates maximizing scuffing. |
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CUPPING:
Cupping, which is more accurately described as scalloping
(see pictures, but we will use the more common
term "cupping" here), is a natural wear pattern on
motorcycle tires and it will always follow the tread pattern.
It is not a sign that you have bad suspension parts. It merely
shows that your tire is indeed gripping the road when you make
turns (thank you for that Mr. Tire!). This cupping develops
within the side wear bands of a leaned motorcycle. The extreme
forces that come in to play when the bike is leaned in a turn
are what produce the effect and when the wear becomes
sufficient, one will experience vibration and noise when one
banks into a turn. Upon examination of the pictures at left of
our sample
rear Avon, our dusted
front VTX Dunlop D256, and the picture
of our chalked Dunlop D206 one can see how the cupping
follows the tread pattern. The leading edge of the tread does
not flex much as it grips the road and the rubber is scuffed
off the tire in that area causing a depression. As the tire
rotates, the pressure moves to the trailing edge of the tread
pattern where the tread flexes more causing less scuffing so
less material is ground off the tire. The more complex the
tread pattern, the more complex the cupping pattern will be.
The softer the compound of the tire, the sooner this cupping
will develop. Radial tires are more prone to cupping than are
bias ply because the compound of radials is softer. As one can
see, the simple tread pattern of the Avon
pictured produces a simpler scallop pattern while the more
complex VTX
D256 Dunlop is somewhat involved, though still easily seen
in our photo. Cupping on the Valkyrie
Dunlop D206 is very hard to photograph because of the
complex tread pattern. Low tire pressure will exacerbate this
wear pattern and you will lose many serviceable miles by
running low. Improper balance has nothing to do with cupping
on a motorcycle tire. Improper balance will merely cause your
bike to vibrate within certain specific speed ranges.
The following textual illustration comes from Martin who
contributed 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 I
describe what's going on. I usually tell people that
what's happening is that the individual "blocks" or
"islands" of tread are squirming and deforming due
to the forces applied to them during cornering and braking.
When this deforming takes place, the wear is naturally not
evenly distributed across the surface of the tread. (I
define a tread block as an area of the tire surface surrounded
by a groove.) I next tell people they can demonstrate to
themselves what's happening by taking a new pencil with an
unused eraser on the end and while holding the pencil
perfectly vertical, push down and drag the eraser on a rough
surface in one direction. Then I tell them to look at
the eraser and note that all the wear is on the leading edge
and not evenly distributed across the end surface of the
eraser. It seems to make the concept easier for many to
understand. Cheers!
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OTHER FACTORS:
The frame geometry of the motorcycle can play a major
role in how early on the "left side" phenomenon
makes itself evident. On the Honda Valkyrie and the Honda VTX
models, rake is 32° and trail is nearly six full inches. A
long trail can cause the wear on the sides of the front tire
to show earlier because of the "shear" effect in
turns as your front wheel is pointed slightly in the direction
of the turn. The front wheel has less a tendency to roll
through turns and the shear force at the contact patch helps
scuff the tire at the common lean angle. As a personal
test, I ran an OEM D206 Dunlop on the front of a stock
Valkyrie and got around 8K miles on it. When replaced, it had
definite and prominent signs of left side tire wear. I
replaced the tire with another D206 Dunlop and after about 3k
miles, it began to show signs of left side wear as well. At
that time, I replaced my stock triple clamp with a TBR triple
clamp which reduced trail. From that time on, the left side
wear was reduced to a great degree. The Avon with which I
replaced that D206 now has over 15k miles on it and though
there is some evidence of left side wear, it is not as
pronounced as it is with some I've seen using the stock clamp.
The Avon profile helps the front tire roll through turns with
less shear.
Rake is measured as the angle of the steering knuckle off
perpendicular with the bike upright, at rest and unloaded on a
level surface. Trail is measured from where that angle
intersects the level surface to the point plumb with the front
axle. Factors that can affect trail: the angle of the forks
relative to the knuckle; the distance of the forks from the
knuckle; raising or lengthening front suspension to increase
rake; lowering rear suspension to increase rake; loading the
bike with a passenger; apply brakes (dive). Ever notice that
it's harder to steer your bike when you have a passenger?
It's because the extra weight compresses the rear shocks
increasing rake besides making the bike heavier overall.
Rake hence trail changes are also dynamic so should one change
the balance of the rear suspension with the front (the rear
flexes more than the front or vice verse in turns and causes
the rake/trail to change while in a turn). Causing the front
to dive less in turns with heavier fork oil or heavier springs
will take some weight off the front wheel and lessen side band
wear.
Trail is necessary for your bike to be stable going
straight, but it is also necessary for turns since it causes
your front wheel to steer itself into the turn.
Countersteer and trail's geometry effect resulting in a turn
is discussed at Wikipedia in detail. Click
here to visit. A good video of using countersteer
for turns is found here (VIEW
MPG). Note in that film how countersteer is used to
initiate the turn (the bike momentarily turns in the wrong
direction) then once the bike leans shortly thereafter, trail
causes the front wheel to turn itself into the
turn which is what actually steers the bike in the desired
direction.
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OTHER THEORIES DISPROVED:
But what about road crown? Plausible and many claim it, but road
crown plays no part whatsoever in left side tire wear. Though
it does seem logical and also allows that in European left
side driving countries, the crown is opposite which could
explain right side wear there - road crown just doesn't
hold up under scrutiny. The main reason the theory falls on
its face - road crowns are simply not steep enough. If you
examine a picture of the front
tire upright, you can see that even a very steep road
crown (one inch drop in one foot run) would not even
contact the tire at the necessary angle to produce the
evident wear. Typical road crowns are much much less (three
inches drop per twelve foot run). In fact, if one examines
the picture of the tire one can see that where a road
crown would actually contact the tire, there is a PEAK
rather than a valley (check our extreme wear pic at the top of
this page). Plus, as described in the paragraphs above, road
crown contact is mostly "rolling" contact which
produces very little wear if any at all. Just so
there is no misunderstanding, road crown
plays no part whatsoever in left side tire wear.
Road crowns, if they exist at all on a road, are completely
inconsistent and vary greatly as to pitch, vary even more
greatly in turns (road engineers do indeed "bank"
turns), and crowns in no way contact the tire at
20°
off horizontal where the wear occurs. Road crown
does not cause side tire wear. |
STILL NOT CONVINCED?
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If you are still not convinced that increased
mileage is what causes one sided tire wear on the front tire
of motorcycles, you'll have to come up with a theory that
satisfies all of the evidentiary criteria. A)
It will have to explain the fact that when riding upright, the
tire's side wear bands do not contact the pavement (road
crown, unbalanced/off center bike weight and even wheel
misalignment won't work). B) It will have to
reverse itself in countries where one rides on the left side
of the road rather than the right (road crown still sounds
plausible 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 than
on the right side of the tire. At a simple single lane
intersection that is common in most residential neighborhoods,
negotiating
a left turn will have you traveling TWICE the distance that
you do making a right hand turn. That's at a simple single
lane intersection. A double lane will have you making four
times the distance. But even when you are confronted with
nothing more than a left curving road, the radius of that left
turner will be larger than if you were coming the other way on
the same road making a right around that same curve. If you
don't believe this, check out the How Stuff Works web site
about your car's DIFFERENTIAL
and why it's called a differential (your shaft
driven two wheeled motorcycle does not have a differential, it
has instead a "final drive" which drives only one
wheel). Besides the fact that the left radius is larger which
means you will probably go faster causing more stress on your
tire than you would going the other way, there is more
visibility when making lefts than rights which will add to
your tendency to make the turn faster as well. Failure to
negotiate a left turn will have you going off the the road
onto the shoulder or into a ditch. Failure to negotiate a
right turn will have you crossing into opposing traffic.
Though neither scenario is appealing, there is a subliminal
advantage to left turns (riding shoulders and ditches is
better than crashing into trucks head on) and this will have
you going a bit faster on lefties too.
The increased radius on left turns means more distance is
traveled turning left than turning right on the average riding
day. That is plane geometry and plainly undeniable. Because of
the natural tendency to make left turns faster (admittedly
this is subjective and open to debate, but is plausible for
reasons given) there will be more stress placed on your tires
as they travel that longer left distance. Increased left side
tire wear is evident, though, on both the front and rear tires
but because the front tire shows less evidence of flat band
center wear (which disguises the side wear bands on the rear
tire), side wear is more evident to the eye up front and
leaves you to wonder, "Why does the left side† of my
front 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 tire
wear out first?" And now you know as well!
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