How to understand Motorcycle Tires:
Sizes, speed & weight ratings, belt technologies, arc-design
PURPOSE:
Although it may seem obvious to you that the tire's purpose is to hold the
motorcycle up, the tires actually perform several different functions. They help
maintain balance of the bike while in motion, they change the steering and
handling geometry, they absorb some of the shocks and vibrations from the road
surface (without involving the rest of the suspension), and the more obvious
transference of energy between the bike and the road (braking, turning forces,
acceleration) -- as well as handling the dissipation of heat from that frictional
transaction of energy transference.
TIRE COMPOUNDS:
Tires come in different compounds for different intended uses. Softer compounds
are designed to give an edge in grip (especially when the bike is leaned over
significantly), while harder compounds are designed to promote longer tread life
and cooler running (for long-haul driving). Most tire manufacturers now
manufacture several different compounds to create a whole range of compounds
available, from a super-soft tire specifically for track purposes, to a semi-soft
street-racing tire, to a only-slightly-soft road touring tire. Additionally, some of the
leading manufacturers also use different compounds within a single tire, such as
the Metzeler Z-series, which uses a softer compound at the tire's outer edges
and somewhat harder compound in the center, for a design that promotes
exceptionally high grip while leaned over and yet still wears slowly in the center
(for longer general lifespan).
TEMPERATURE VS. FRICTION:
As we mentioned previously, one of the features of a tire is to handle the friction
interface between the vehicle (motorcycle in our case) and the road, and then
deal with the heat from that friction. If you've ever seen a semi-truck's tire tread
laying by the side of the road, you've seen what happens to a tire that gets too
hot (the molecular compounds inside the tire break apart, thus splitting or
shredding the tire carcass). As you might imagine, having that happen on a
motorcycle you are riding on is a sure-fire recipe for disaster.
For motorcycle tires, there is a delicate balance going on here, because tire
manufacturers want tires to get warm fast (their traction is dependent upon them
"coming up to temp," or reaching a specific operating temperature -- and staying
there), but at the same time don't want the tire to get too hot (or the tire material
will start to wear out at a very fast pace -- or worse yet, delaminate itself as in the
truck tire example above). The formulation of the tire compound plays a big part
in this equation, since softer materials by their nature flex more (thus come up to
"temp" faster, but also run hotter), while harder compounds flex less (thus take
longer to get warm, but also run cooler). Since tires used on race tracks need to
provide maximum traction, but are not expected to be used for more than a few
races normally (a single race at pro levels), they are made of super-sticky
materials designed to warm up very fast, and to run a bit hotter than normal
street tires (trading life expectancy for superior grip). Touring tires, on the other
hand, are expected to be able take a thousand miles or more in a day, for days
on end, so they are built to heat much less quickly, and to run at a cooler
temperature through-out their use (both for the lifespan of the tire, as well as for
the stability of the tire carcass). Typical street tires fall somewhere inbetween
these two standards, offering a blend that warms up faster than touring tires but
runs slightly hotter than them too.
Your riding style needs to play heavily into tire choices. Running a pure-sports
tire, such as a Pirelli Diablo Corsa or Metzeler M3 or M1, without the constant onbrakes & change of direction input it needs in order to be kept in it's ideal
operating temp range will means a seriously loss of traction compared to running
a sports-touring tire that doesn't need that kind of input... In the same sense,
running a sports-touring tire if you use the bike solely for track day sessions or
are constantly knee-down canyon-carving is insanity. The right tire for the right
task makes a huge difference in everything from stopping distances to outright
grip-on-demand.
The belt design plays into the temperature equation heavily as well, since
adjacent belts can rub against each other, and do generate heat from flexing as
well. Depending on the material the belts are manufactured of, the heat may
spread out more evenly across the tire surface (example: steel belts spread heat
very well, while nylon belts don't, because heat travels better through steel than
nylon).
TREAD DESIGN - LAND AND SEA:
Race tracks are pieces of exceptionally well maintained pothole-free road
surfaces, with special crews that clean up oil spills and wipe off rain water with
squeegies -- and thus tires for racing use often do not have any tread design at
all or one with minimum treading (to maximize the rubber-to-ground interface).
By comparison, a modern street tire has to deal with a long list of possible roadsurface contaminants and irregularities, including water (rain, snow, slush), oil,
automatic transmission fluid (ATF), spilled diesel fuel, sand, dirt, soil, gravel,
loose asphalt, ridged concrete, slab-gaps and expansion-fills, bumps, pits,
potholes, reflectors, cracks (and grass growing from the cracks), tar patches, etc.
The list is almost endless. To help compensate for these irregularities, the
carcass design of street/touring tires are different from those of track tires,
including the inclusion of tire treads (in the USA, DOT-mandated). Please note
that not all treads are designed equal!
In the tire industry, the grooves cut or formed in tires are called "sea" and the
ridges that border the grooves and the raised sections between the grooves are
called "land". Different manufacturers have different land and sea designs, and
even vary the land-sea design between tires in their own line-ups based on the
intended use. Note that some designs have specific advantages over others.
For example, if the front tire has grooves that go around the very center of the
tire for it's entire circumference (center line groove/grooves), then the tire's
center-line groove's sea area will have a tendency to track concrete that has
ridges running in the same direction as traffic, or the metal grating on opengrated bridges (like some draw bridges) -- and can induce a very severe front
end wobble as a result, which is disconcerting to say the least. To avoid this,
many manufacturers redesigned the center line to go back and forth across the
tire slightly (an oscillating center line design), while others avoid a centerline
groove all together.
It may be noted that outside of North America, I've never run into a piece of road
work with concrete ridges running in the same direction as traffic, so it may not
be a design concern in much of the rest of the world. On the other hand,
centerline grooves provide a benefit when riding in heavy rains, slush and slurry,
as they help maintain road contact. It's a trade-off, depending on where you live
and what kinds of roads you tend to face. In the USA, I personally avoid tires with
a straight centerline groove at all costs because of the tracking issue.
Similarly, tires designed specifically for off-road use will have tread designs with
wide seas and very large land-to-sea height differences to compact loose soil
and sand under the tire to maximize traction on that kind of surface. Enduro tires,
which are designed to be used on packed dirt roads and other semi-improved
surfaces will have yet another design over those of conventional street tires,
designed to increase the foot-print of the tire to the road to help offset slippage.
Land and Sea Interactions:
Motorcycle tire manufacturers design their land and sea areas to work together
between the two tires (front and back), especially on street and wet-track tires, so
that in rain or other contaminants, the front tire will create a partial path for the
rear tire to track through -- and land/sea area of the back tire will only have to
move what the front tire's tread didn't. This is the primary reason why no tire
manufacturer recommends mixing different brands and/or designs of tires
together -- because one manufacturer's front tire may not work well with another
manufacturer's rear tire, and thus leave the rear tire with the tendency to
aquaplane, or otherwise act up, thus risking your neck.
BELT TECHNOLOGY:
At one point, it was figured out that for road and racing tires, the inclusion of
belt(s) of nylon tended to provide several benefits over belt-less tires. Nylon
belted tires are called "Bias" tires. As the years went by, new materials (such as
steel, kevlar, etc.) were tried in belt construction and those that were more
effective in a specific use catagory were used as replacements for the older
materials in that type of tire. Tires using such metal belts are called "Radial" tires,
and are now stock and specified for virtually all newer motorcycles. Although
officially bias and radial tires are method of winding the treads, in the motorcycle
industry, bias tires use non-metallic belts and radials use metallic-basis belts.
Older motorcycles can also often switch over to radials to gain the advantages
they offer. A few of the benefits of radial tires gain by use of belts in motorcycle
tire design are:
Impact absorbtion: The tire spinning at speed throws the belt outwards (centrifical
force), and this helps keep the tire's circumferance truely round, as well as
helping it suck up minor bumps and dips without excessively stressing the rubber
compounds, resulting in cooler running tires (and higher tire life).
Temperature Distribution: Most metallic belting materials (such as steel) help
distribute heat away from the point of friction (the place where the tire is
contacting the road surface), across the entire surface of the tire, helping it both
come up to temperature faster, and simultaneously run cooler at the point of
contact, thus improving both performance and expected lifespan of the rubbermix compounds used in the carcass.
Reduced Rolling Resistance: Most belt materials help reduce the rolling
resistance of tires by helping them hold their shape and giving them a certain
amount of rotational mass.
Lateral Stiffening: The presence of belts in a motorcycle tire's construction helps
take the lateral stress load of the tire when it's running off-center (i.e. - when you
are leaned over), so the rubber compounds don't have to handle as much of the
force.
Pressure-Loss Rate & Deformation Reduction: Radial tires have another distinct
and major advatange here -- when they get compromised (punctured by a nail,
screw, etc), they normally leak much slower than bias tires, which gives you
more time to come to a controlled stop. Additionally, they tend to hold their shape
much better while deflating at speed, which is crucial if the tire gets compromised
while you are negotiating a turn, especially a high-speed turn. It can make the
difference between walking away and leaving the scene in an ambulance,
helicopter or hearse.
Belts these days are woven in different patterns for different purposes. For
example, a zero-degree pattern (with outer layer of threads running the same
direction as the tire's rotation) provides a very high degree of high-speed stability.
A diagonal belt pattern by comparison, provides a higher degree of lateral
stiffness against tire deformation during cornering. Generally, tires designed for
your specific kind of application (supersport/racing, sports/street-performance,
sports-touring, pure-touring, enduro, etc) will have a belt design optimized for the
conditions the tire is expected to see. Additionally, belt density and winding
patterns are critical factors in the wieght handling characteristics of the tire -- a
tire designed for a 700 lb bike capable of 150 MPH will traditionally have a
denser spread of belting than a tire built to handle a 400 lb bike with a 100 mph
top speed.
Steel vs. Nylon:
Although nylon is, by volume, a lighter material than steel, tires using steel belts
may weigh less than similar designs using nylon, even with the same amount of
other materials. Why? Because for the same total belt strength, a much thinner
steel wire can be used in place of a thicker nylon strand in the belt construction.
CHEMICAL COMPOSITION:
Motorcycle tires were originally built out of pure vulcanized natural rubber. The
industry has come a long, long way since then, and have added various bits of
chemistry into their tires to change the characteristics of the tires. Exact tire
compound formulations are now closely guarded trade secrets for the most part,
but that doesn't mean that we can't look at the basic compounds and explain how
they change the tires.
The first compound is always the rubber base-compound everything else is
added to, and these days it's usually synthetic rubber (which is consistant in it's
strength and uniform in it's purity). Some manufacturers still add a small
percentage of real rubber to their tires (usually for cost-effectiveness reasons),
and a couple actually use significant amounts of natural rubber (the cheapest
tires on the market).
After that, there are two other major chemical compounds in use: carbon black
and silica. Carbon black gives the compound it's black color and it's strength,
while silica improves grip under all conditions (but especially wet ones). Then
come the minor chemicals -- anywhere from 8 to 45 of them, including cobalt
salts, various artificial resins (both designed to improve adhesion of various
layers to each other), anti-oxidants (to improve the shelf-life of the tire), plus
various other chemical compounds to improve different aspects of the tires. Most
manufacturers pre-mix their rubber compounds before they hit the tire forms, but
some of the newest motorcycle tire forming systems actually compose the
chemicals on the spot at the tire forming machines.
Now, let's go back to those two major chemical additives (carbon black and
silica). By varying the percentages of these two chemicals in the formation of the
tire's compounds, manufacturers can vary the hardness or softness of the tire, as
well as the tackiness or grippiness of the tire. Replacing most the carbon black
with silica reduces the hardness of the tire but promotes strength between the
rubber molecules at the same time (and silica runs cooler than carbon black).
But, removing most of the carbon black also reduces the lifespan of the tire, and
may prompt the tire manufacturer to make a thicker tire to offset the removal of
carbon black. Track tires are traditionally high in silica and low in carbon black
compared to street tires, because grip is of paramount importance in track
environments (and lifespan isn't, since most races are limited to 80 to 150 miles
in length).
Lately, a number of manufacturers have also been varying the initial bead size of
these particles to improve the evenness of the mixture. Most tire manufacturing
processes start with the raw mixtures preformed as small beads which will be
joined in a heated press to form the tire layers in the press. R&D at these firms
have found that they get new material matrix formulations by stepping to a
smaller bead size, which can provide benefits in seeming contradictory values -both better grip and longer life than prior generations of tires that used the same
chemical basis in a larger bead size.
PROFILE DESIGN:
Although we've talked about the grooves cut into a tire, we haven't discussed the
actual profile of a tire (the outer curve of a tire when you look at it head-on).
Different tires have different profiles, and generally speaking, a more triangular
radius (think pointier) tire will handle faster, while a tire with a wide slow arc will
handle more slowly but provide more straight-line stability (because of a larger
center section to track flatly on).
Manufacturers have been tweaking these parameters for decades to get benefits.
Personally, I like Metzeler's solution (also implimented on some Pirelli tires as
well) of a triple-radius design, which instead of having a single arc shape,
actually has three arcs, each at a different curvature (so you get the benefit of the
stability in the very center, with progressively faster handling as you lean further
over).
FITMENT SIZE:
Your rims are a certain size across from edge to edge (the short way), and
specific diameter. The tires rated for your bike are designed to fit into this wheel
(think "tire retainer" instead of the term wheel), while maintaining proper contact
at the edges to keep the tire just rigid enough, not distorting the profile design (by
rebending it to fit a narrower or wider gap at the base), and still clear everything
else that the tire might rub on (such as the swing arms, the chain, etc).
How to read tire fitment charts and info:
Typical complete tire description: 120/70ZR17J (58W)
Now, let's break that down into pieces that make sense:
The first number (120) represents the widest point of the tire's width between the
left and right sides, called the "section width", and measured in millimeters. In
this example, the width of the tire is 120 millimeters.
The slash ( / ) is there to differenciate the first number from the second number,
and serves no other purpose.
The second number (70) represents the sidewall height as a percentage of the
first number (in this example, it tell us the tire sidewall height is 70 percent of 120
millimeters, or 84 millimeters tall).
Then comes either one or two letters (in our example, ZR). Sometimes these
letters are placed elsewhere in the description, but traditionally, they should be
listed this point. The first letter is always the speed rating (seeSpeed Rating
chart), and the second letter, if it is there, is an "R" (radial tire) or a "B" (bias tire).
Lack of a second letter means it is a bias or non-belted tire.
The next numbers (17) represent the wheel's diameter, measured in inches. In
this example, the wheel is 17 inches across.
A letter at the end of the first part of the designation (the Jin our example) is not
required, but if present means that the tire has a special characteristics to match
some manufacturer's specific motorcycle model. For example, Metzeler makes
(made at the time I first wrote this portion) a Z4 radial as a 150/70ZR17, as a
150/70ZR17B, and as a 150/70ZR17J, where the "B" model was intended
specifically the OEM factory tire for BMW 1150GS models (slightly different tread
pattern), and the "J" model was intended for Yamaha FJR1300's (again a minor
tread pattern variation from what I could tell).
A two digit number follows, which is sometimes omitted but never should be (58).
This number represents the load capacity or weight rating of the tire (in terms of
how much weight, including both the rider and the motorcycle itself, the tire is
designed to handle as it's maximum). In our example, the 58 can be looked up in
a cross-referenced chart, and represents a maximum load rating of 520 lbs.
There is no simple way to directly convert the number to the weight that I have
been able to figure out.
Finally, a letter may follow the load capacity (W). This is a manufacturer
"modifying descriptor" and means that the standard for the tire is modified by
some means (W generally means higher speed rated than a standard Z rated
tire). We haven't been able to find a cross-reference for the various modifiers (yet
-- they do vary by manufacturer), so if you are concerned about the rating
modifier, contact the actual manufacturer's rep for the tire.
Additionally, all motorcycle tires are normally marked with three or four digit code
on their sidewalls, which represent the date of manufacture (the first two digits
are the week of the year, the last digit is the year of the decade it was
manufacturered in). Since modern cycle tires are only good for about five years
from the date of their manufacture (the time period it takes the various
compounds that keep the tire pliable and strong to evaporate out), knowing which
year of a decade it was manufacturered is normally enough. Example: 011 would
be a tire manufactured in January (1st week) of 2001. 118 would be a tire
manufacturerd in the 11th week (between the 13th and 19th of March) of 1998
(since 2008 hasn't arrived yet, as I write this). As a general rule, newer tires are
better, and we recommend you buy tires manufactured within the past 12 months
whenever possible (obviously, if you have a flat away from home, you will take
whatever you can get). Do not buy a four or five year old tire -- and if you buy an
old bike, immediately check the tire age before riding it!
SPEED RATING:
Speed Rating vs. Used Tires
Tire's speed ratings are established with new production tires. Tires that have
been repaired (patched), abused (dyno-tested, used at a track day), or are worn
down somewhat no longer qualify for their original speed rating. Keep that in
mind when you try to figure out why the motorcycle's manufacturer specified a
149+ mph rated tire for a bike that only goes 130 mph max. It's for your own
safety!
RATING RATED TOP SPEED MPH / KPH
J 62 mph / 100 kph
N 87 mph / 140 kph
P 94 mph / 150 kph
S 112 mph / 180 kph
H 130 mph / 210 kph
V 149 mph / 240 kph
Z 149+ mph / 240+ kph
SPECIAL NOTE: Tires with a 2" to 2.5" nominal section width (think choppers)
are rated at 75 mph.
LOAD RATING:
The importance of the load rating on a tire can not be overstressed. A tire that is
overloaded beyond it's design limit will run hot, wear fast and may quite literally
blow out on you. The load the tire is designed to bear also affects it's shape and
construction -- if you think about it, a GoldWing tire is not going to look like a
GSXR600 tire. The load rating the manufacturer sets as the spec for your bike
takes into account extra loading for weight-transfer (loading up the front wheel
during braking, loading up the back wheel under acceleration), so you can't
simply go by the curb weight of the bike. Always check the specs for your bike
before ordering tires, and make sure the tires meet or exceed the load rating
requirements for that wheel on that particular bike. For example, if the load
requirement for the front wheel is "58" (520 lbs max), then you can fit a 59 or 60
weight-rated tire in the right size on there with no issue, but should never fit a 57
or below!
TIRE LOAD-RATING CHART
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
176 lbs / 80 Kg
182 lbs / 82.5 Kg
187 lbs / 85 Kg
193 lbs / 87.5 Kg
198 lbs / 90 Kg
204 lbs / 92.5 Kg
209 lbs / 95 Kg
215 lbs / 97.5 Kg
220 lbs / 100 Kg
227 lbs / 103 Kg
234 lbs / 106 Kg
240 lbs / 109 Kg
247 lbs / 112 Kg
254 lbs / 115 Kg
260 lbs / 118 Kg
267 lbs / 121 Kg
273 lbs / 124 Kg
282 lbs / 128 Kg
291 lbs / 132 Kg
300 lbs / 136 Kg
309 lbs / 140 Kg
320 lbs / 145 Kg
331 lbs / 150 Kg
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
454 lbs / 206 Kg
467 lbs / 212 Kg
481 lbs / 218 Kg
494 lbs / 224 Kg
507 lbs / 230 Kg
520 lbs / 236 Kg
536 lbs / 243 Kg
551 lbs / 250 Kg
567 lbs / 257 Kg
583 lbs / 264.5 Kg
600 lbs / 272 Kg
617 lbs / 280 Kg
639 lbs / 290 Kg
661 lbs / 300 Kg
677 lbs / 307 Kg
694 lbs / 315 Kg
716 lbs / 325 Kg
736 lbs / 334 Kg
761 lbs / 345 Kg
783 lbs / 355 Kg
805 lbs / 365 Kg
827 lbs / 375 Kg
853 lbs / 387 Kg
43
44
45
46
47
48
49
50
51
52
342 lbs / 155 Kg
353 lbs / 160 Kg
364 lbs / 165 Kg
375 lbs / 170 Kg
386 lbs / 175 Kg
397 lbs / 180 Kg
408 lbs / 185 Kg
419 lbs / 190 Kg
430 lbs / 195 Kg
441 lbs / 200 Kg
76
77
78
79
80
81
82
83
84
882 lbs / 400 Kg
908 lbs / 412 Kg
937 lbs / 425 Kg
963 lbs / 437 Kg
992 lbs / 450 Kg
1019 lbs / 462 Kg
1047 lbs / 475 Kg
1074 lbs / 487 Kg
1102 lbs / 500 Kg
WHAT ELSE TO KNOW:
NARROW VS WIDE: Although wide tires are all the rage now, narrower tires
handle better under most real world conditions.
TIRE GROWTH: All tires will expand to some degree over time once they are
mounted, both due to the pressure on them, and due to heating-cooling cycles.
Street tires normally expand 3 to 8%, while race tires can expand up to 22%. Be
cautious in mounting a tire that will barely fit your clearances; the tire may easily
grow to exceed the available space once it gets up to temp.
UNEVEN TIRE WEAR (#1): Because roads are normally crowned to permit
rainwater drainage, and you consistantly ride on the same side of the crown
(right side of the road in the most countries; left side in the UK, Japan, and
certain former British colonies), under normal street riding, tires wear unevenly to
one side across the peak of their center apex.
UNEVEN TIRE WEAR (#2): Depending on a variety of factors, including the size
of the land-sea boundaries, the compound composition, the ambient temperature
and the road surface temps, tires may wear unevenly from the front to rear of the
individual tread blocks. Tire pressure and how it relates to tire surface
temperature can have a drastic affect on whether this kind of scalloping/cupping
at the land/see boundaries occurs, although certain tires types do it more than
others.
HIDDEN LEAKING: Your tires leak. Period. Under pressure, tires leak out some
of their pressure directly through the rubber compound of the tire each and every
day, very slowly. Check your tire pressure before you ride anywhere for the day
(usually referred to as measured "cold") and add air as required.
PRESSURE DIFFERENCIALS: A ten degree (F) change in the ambient temp will
result in a significant pressure change in the tires. If the weather cooled or got
hotter since yesterday, check your tire pressure again before you ride.
NAILS, SCREWS, TACKS: If you get a nail, screw or tack in your tire miles from
home or the closest shop, you have a few choices. The wisest of them is to call a
ride to tow you & the bike to somewhere where you can get a new tire, then pull
the object once you get there and hope it didn't go all the way through. The
second is to cut it off flush (if it isn't already) and ride it to a safe place slowly -very very slowly (think walking pace), ready for a possible leak or blow-out. The
third is to pull the item out and pray it didn't go through -- and if it did, you're not
going anywhere without a tow. And the final way is to carry an emergency
patch/plug kit, pull the item, plug it as necessary, then proceed directly to
somewhere that sells replacement tires.
TIRE AGE: Motorcycle tires should not be used after five years from date of
manufacture. Tires contain Volitile Organic Compounds (VOC's) which help keep
the rubber flexible and grippy; as tires age, the VOC's leach out. Certain
conditions can accelerate this process, including extremely hot and excessively
humid or dry environments, parking on certain forms of man-made carpeting,
certain molds & mildew forms (which may accelerate or even cause what is
commonly known as "dry rot"), as well as overheating the tires due to low tire
pressure or hard use.