Motorcycle Tire Knowledge Tune Up: What’s With All Those Letters and Numbers?

TIRES-CHAPMOTOYou’ve got a lot riding on those two round pieces of rubber at either end of your motorcycle, so it’s best you become familiar with them. There’s plenty to know about your motorcycle tires besides if they have the proper amount of psi. Some of the information you should become familiar with includes what all those numbers and letters on the sidewall means, what tires will work on your bike, how to care for/maintain your tires, and when to repair and when to replace?

Inside the Numbers (and Letters) of a Motorcycle Tire

Taking a look at the side wall of your motorcycle tires, or any tire for that matter you’ll come across a lot of various numbers and letters. All these markings can tell you a wealth of info such as what’s the fastest speed the tire should be ridden up to, how much weight can the tire be subjected to, the direction of rotation (most tires indicate this), when the tire was manufactured, and obviously, the size of the tire.

Measurements:

tire diagramLet’s start with the sizing info first. There are three main measurements you’ll see on a tire: the width, aspect ratio, and rim diameter. The width is the overall dimension as measured from one edge of the tire to the other. The Aspect ratio is a percentage in relation to the height of the sidewall as compared to the width. Rim diameter corresponds to the rim size the tire will fit on and is the measurement of the inside of the tire.

Inches tire measurement, typically found on vintage style tires.

Inches tire measurement, typically found on vintage style tires.

Basically you’ll see the sizing listed in one of three different ways: metric, alphanumeric, and inches. Way back in the day tire widths were measured in inches and while you’ll still find tires with with their widths displayed in inches you’ll typically find it on vintage type tires. This is mostly to keep with the OE look and style of the tires that came on older bikes. In the example here we see that this Avon MKII tire is 5.00-inches wide and fits a 16-inch rim.

Alphanumeric tire measurement

Alphanumeric tire measurement

Metric and alphanumeric tires are pretty much the standard with metric mostly used across the board. You’ll typically see alphanumeric on OE Harley-Davidson tires or replacement Harley tires. Metric and alphanumeric tell you the same information just in slightly different ways. Metric uses all numbers for the tire measurement while alpha numeric uses a combination of letters and numbers. In both instances the width is measured in millimeters (mm).

Metric tire measurement

Metric tire measurement

In this metric designation on the Michelin Commander II we can see that the width of the tire is 170mm the aspect ratio is 70 percent of the width, and the rim diameter is 17 inches.

On some tires after, after the aspect ratio there may be a letter “R” or “B”. This is identifying the manner in which the tire was constructed, radial, bias belted, or standard bias/cross ply. Obviously, “R” stands for radial and the “B” stands for bias-belted. If there is no letter between the aspect ratio and rim size that means the tire is just a standard bias ply tire/cross ply tire. We could spend a couple hundred words talking about the ins and outs of radial and bias play tires, but we’ll save that for another time.

It used to be you would only run either a radial tire front and rear or bias front and rear and never mixed the two. That’s not so much the case anymore. These days there are several motorcycle manufacturers that offer their bikes with mixed radial and bias tires. Ideally though, when it comes to replacing your tires you’ll want to stick with the manufacturer’s recommendation. If radial and bias tires do get mixed and matched you’ll usually see the bias tire up front and the radial at the rear. Swapping this orientation can cause handling and instability issues.

How Much is Too Much?

Load Index Chart

Load Index Chart

Just like your motorcycle, tires have a load carrying capacity, which is called the Load Index. The load index is the maximum amount of weight that the tire can handle at the specified speed rating. Pay attention to your load index when heading out on long trips, you have to take into consideration the weight of the bike, your luggage or any other gear and the weight of you and your passenger (if you’ll be taking one along). It should also be noted that the Load Index rating is taking into account that the tire is in good condition and is properly inflated.

Speed Ratings:

All tires have speed ratings that are represented by a letter or in some cases two letters that correspond to the maximum speed that is recommended for the tire while carrying a load corresponding to its designated load index.

Speed ratings are as follows:

J              62mph
K             68mph
L              75mph
M            81mph
N             87mph
P             93mph
Q             99mph
R             106mph
S              112mph
T              118mph
U             124mph
H             130mph
V             149mph
W            168mph
Y              186mph
Z/ZR       149+mph

How they All Intermingle

The chart below explains how metric, alphanumeric, and inch designations correspond to each other from tire size to tire size. This chart can come in handy when you want to try out a tire from a different manufacturer but they use a different sizing format than what’s currently on your motorcycle.

Street Tires Front:
Metric                  Alpha                    Inch
80/90                   MH90                  2.50 to 2.75
90/90                   MJ90                   2.75 to 3.00
100/90                 MM90                 3.25 to 3.50
110/90                  MN90                  3.75 to 4.00
120/80                                               4.25 to 4.50
120/90                 MR90                   4.25 to 4.50
130/90                 MT90                   5.00 to 5.10

Street Tires Rear:
110/90                  MP85                   4.50 to 4.75
120/90                  MR90                  4.50 to 4.75
130/80                                                5.00 to 5.10
140/90                  MU90                  5.50 to 6.00
150/80                  MV85                   6.00 to 6.25
150/90                  MV85                   6.00 to 6.25

Born on Date

Michelin Born on Date

Michelin Born on Date

One of the most important things people should pay attention to whether they are buying new tires, looking to purchase a used bike, or just haven’t looked at their current motorcycle tires in quite some time, is the date the tires they were manufactured. Tires are just like us, after time they start to show signs of aging in the form of cracks on the surface. It’s these cracks and the breaking down of the tire’s materials and structure that can cause a tire to fail.

Avon born on date

Avon born on date

There are no federal guidelines in regards to tire aging, however, some tire manufacturers suggest replacing tires after five to six years. This doesn’t mean if you have a tire that is a week from turning five years old it’s suddenly going to turn to dust in a week or become completely unrideable. Nor does it mean if you bought a “brand new” tire and it’s manufacture date is showing that its two years old does that mean the tire only has three years of life left. If tires are stored correctly by the reseller and taken care of properly once on your motorcycle they can last longer than five years. Once they do hit that five year mark you should have them regularly inspected by a certified technician. It is possible that you could get more than five, six, even seven years out of them. However, if you’re tires are ten years old then you should definitely replace them.

The “born on date” so to say, can be found at the end of a string of letters and numbers that starts with DOT. There is info that can be gleaned from this string of letters and numbers, but what we are most concerned with here is the code at the end of the string that specifies the manufacture date. Prior to 2000 this was a three digit code, after 2000 it was changed to a four digit code. And that right there should bring up a red flag when looking tires. If that “born on date” is a three digit code, walk away or replace the tires no matter how good they look, because they are nearly 20 years old.

It’s really easy to decipher the four digit manufacture code, basically the first two digits represent the week they were made and the last two represent the year. In the examples below we can see that the Dunlop American Elite tire was made in the 30th week of 2016,   the Michelin Commander II was made in the 38th week of 2016, and the Avon MK II was manufactured in the 9th week of 2017.

Which way? Front or Rear?

Dunlop Rotation Front

Dunlop Rotation Front

A few other indicators you might see on a motorcycle tire side wall is an arrow, and or the words “Front” or “Rear”. If you’re going to take on the task of mounting your own tires then you need to look for an arrow. This will tell you if the tire has a specific direction that it is supposed to run. If there is no arrow then the tire is directionless and it doesn’t matter which way it is mounted. If there is an arrow however, you need to make sure that when the tire is mounted and the wheel is installed the arrow will be rolling/pointing forward. This is important because it will affect handling and the tire’s ability to properly disperse water if not mounted in the correct orientation.

Some tires are marked with “Front” or “Rear” to help indicate at which end of the motorcycle they should be mounted. Some tires are universal and can be run at either the front or rear and in these instances there will most likely be two arrows on the sidewall indicating front and rear. If you have a tire like this that means that if the tire is at the front of the motorcycle it needs to be mounted one way and if the tire is going at the back of the bike it needs to be mounted the other way in order to have the arrow rolling forward.

Handle with Care

Tires are pretty durable they are made to take a lot of abuse, heat, friction, bumps, and jolts. But they can also be susceptible to some pretty unsuspecting things such as harsh chemicals, oils, fuels, weather, improper inflation, and the sun. Taking proper care of your motorcycle tires will go a long way in ensuring that you can get the most mileage and use out of them as possible.

Chemicals, oils, and fuels can cause the rubber material and other components to break down over time. Therefore it’s important to keep an eye on your tires and make sure that none of those things get in contact with them for an extended period of time. Keeping your tires clean not only makes them look good but ensures these harmful items don’t accelerate the aging process or harm your tires. When cleaning, a simple water and dish soap solution often suffices.

IMG_8050When storing your motorcycle for an extended period of time be sure to keep it in a clean and dry area where the temperatures don’t get too extreme. Wet and dusty conditions along with freezing cold and high heat can wreak havoc on motorcycle tires and lead to dry rot, cracking, and accelerate deterioration of the tire’s materials/components. If possible store your motorcycle in a garage or temperature controlled shed. If you have to store your motorcycle outside, do your best to cover it completely up to protect it from the dirt and sun. Just like with your skin, the sun can really do a number on a motorcycle tire.

Another really important factor to keep in mind when storing your motorcycle is to maintain proper air pressure in the tires, underinflated tires can help cracks develop/spread and can cause flat spots in the tire.

When to repair

Getting a flat tire on your motorcycle is a hassle. Besides most likely leaving your stranded on the side of the road, you also have to deal with either removing the wheel and tire yourself or hauling your bike to a shop to get the issue dealt with. If you carry the proper items with you when riding you can potentially repair the tire yourself on the side of the road. When can you repair a motorcycle tire depends on a couple things: where the damage is, what kind of damage is it, and how large is the damaged area? For the most part, a tire can’t be properly repaired if the damage is in the sidewall or too far off to the side of the tread area. Also, it’s extremely tough to fix things like slashes/gashes or gaping wounds. Clean, round or round-ish punctures that are no bigger than a 16D sinker framing nail often result in the most successful repairs. There are a couple of options when it comes to motorcycle tire repair, the most common is to plug up the problem spot, while another route is to slip in some type of sludgy sealant.

IMG_8038Riding around on repaired motorcycle tire isn’t an ideal situation, but if done properly you can get yourself out of unfortunate situation. The Stop & Go Tubeless Tire Plugger Kit With Co2 is a pretty popular item that many riders carry with them for emergency use. The kit consists of a bunch of mushroom shaped rubber plugs, installation tools, and a couple CO2 cartridges to help inflate the tire. The mushroom tip of the plug is inserted through the hole and then the tail end is pulled tight so that the tip of the plug seals tight against the inside of the tire.

While plugging a tire is a good solution (an even better one would be to plug and patch from the inside but that would require pulling the tire off the wheel) it does have its limitations. For instance you can’t plug the sidewall, the inflicted area needs to be in the tread portion of the tire—ideally in the center or at least within the main contact patch of the tire to ensure a good seal. Also there is a limitation as to the size of hole a plug can fix. For example, the Stop & Go kit doesn’t recommend repairing holes that are larger than 5/16th inch (7mm). Once plugged you should monitor your speed and not exceed 50mph (suggested by Stop & Go). Again plugging a tire isn’t a permanent solution and should really only be used to get you to somewhere where the tire can be replaced.

IMG_8036Using a tire sealant like Slime is another solution that motorcyclists have turned to in order to get back on the road. Slime is available in both tube type tire applications and tubeless applications. The deal with a tire sealant is that you fill your tire with the recommended amount of sealant (via the valve stem), air up the tire, and then ride along. Centrifugal force from the rotation of the tire causes the Slime to spread across the inside of the tire as it searches for the escaping air at the puncture site. Once the hole is found, microfibers in the Slime formula build up to form a dam and plug the wound.

Slim in tire

Slim in tire

Just like the Stop & Go Tire Plugs, Slime has its limitations, for example the tubeless formula should only be used on punctures up to 1/4 inch in diameter while the tube solution should only be used for holes no bigger than 1/8 inch in diameter. You should also limit your speed when riding on a tire that has sealant on it. Be forewarned, this stuff makes a mess inside your tire and rim. So don’t be surprised when you finally remove the wounded tire from the rim and the insides look like Slimer from Ghostbusters had a party in there.

When to Replace

Michelin tire wear indicatorHow can you tell if it’s time to replace your motorcycle tire? Unlike the age dilemma where there’s no regulations stipulating how old tire can be, there are regulations for tread wear and therefore all modern motorcycle tires are made with Tread Wear Indicators (TWI) also sometimes called wear bars. These are small bars integrated into the exterior carcass of the tire and sit in the sipes (the grooves in the tread). On a brand new tire they can be hard to see because there aren’t very tall (about 1/32 of an inch/0.8mm) and therefore sit well below the tread.

Michelin tire indicator mark

Michelin tire indicator mark

To help you easily identify them the sides of the tire will either have TWI, a triangle, or some other identifier (Michelin has a small Michelin Man) to let you know where the indicator is located across the tire. The point of these TWIs is for you to replace your tires BEFORE you hit them, basically there’s very little tread left once you get to the wear bar.

IMG_6530Besides looking for the wear indicators, another way to tell when it’s time to replace your tire is to use a penny. Simply take a penny, turn Lincoln’s head so it’s facing you, spin ol’ Abe upside down and then slip him into one of the grooves towards the center of the tire. If the tread is still covering his head you still have a little bit of mileage left.

So we covered quite a bit of info, there’s even more we could have gotten into, but this should help either give you a refresher or shed some light on all those hieroglyphs you’ll find on those rubber hoops on your motorcycle.