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New England
Riders

 


Other Riding-Related Info:
Tires

 

Deciphering Your Tire A Method to the Madness!

The rear tire from a Duece motorcycle

Have you ever looked at a tire and wondered what all of the numbers and letters listed on the sidewall mean? Have you ever thought that all those letters and numbers look kind of random and meaningless? Of course you have!

At first glance, you would be forgiven for thinking that tire information looks like something that a team of drunken monkeys banging away at typewriters would crank out. However, after a little bit of time and effort you will be able to decode all of the wonderful information on your tire yourself amazing your friends, impressing your co-workers, and mystifying onlookers.

This series of letters indicates the tire manufacturer and model number. It is important to make sure that your front and rear tires match; that they are the same model made by the same manufacturer.

Tires vary in construction strength, material composition, warm-up requirements, and ride quality. Due to this, most modern tires are designed to be used as a matching set. It is potentially dangerous to ride on mismatched tires, as these tires will behave differently under different road stresses.

160/70

This first set of numbers, the designation 160/70, is a metric description for the size of the tire. 160 is the width of the tire in millimeters. Simple enough, right? The number 70 is a little more complex, however. To understand what it means, you first have to understand how the height of tires is measured.

The height of a tire is not how tall a tire is; instead, the height of a tire is determined by measuring from the inside diameter of the tire (the big hole where the rim fits) to the top of the tire's tread. 70 is the aspect ratio that describes the height of a tire as a ratio of the tire's width. Confused yet? Put simply, the number 70 means that the tire is 70% as tall as it is wide. Thus, if your tire is 160 mm wide, and the number behind the slash is 70, your tire height is 70% of 160, or 112 mm.

What if your tire doesn't have a number that looks like 160/70? What if your tire has something that looks like MT90 instead? This letter designation still tells you the width and height of your tire, just in a slightly different format. In this form, the letter M means that the tire is designed for use on motorcycles. The letter T tells you the tire width code (see table below), which can be used to determine the width of the tire. 90 is the aspect ratio of the height of the tire in terms of the tire's width (see above).

Front Tires

Letter Designation

Metric Conversion

MH90

80/90

MJ90

90/90

MM90

100/90

MN90

110/90

MR90

120/90

MT90

130/90

 

Rear Tires

Letter Designation

Metric Conversion

MN90

110/90

MP85

120/90

MT90

130/90

MU90

140/90

MV85

150/90

Knowing your tire's height and width will tell you a lot about the handling characteristics of a particular tire. For instance, a narrow tire is great for low-speed maneuverability, but its narrow dimensions make it relatively unstable at high speeds. Wider tires are great for high-speed stability, but their width makes them difficult to maneuver in tight spaces (parking lots, city streets, and that pesky driver's license test).

The height of a tire can also tell you a lot about a tire's ride quality. Tires with a high aspect ratio usually translates into a softer, more comfortable ride that is well suited to long-distance hauls. However, because the sidewalls of this type of tire flex a fair amount (this is what gives a tire its cushy feel) they are not well suited to high speed cornering. Tires with a small aspect ratio typically offer a firmer ride and also transmit a lot of feedback from the road. While this translates into a rougher ride, it also means that they are more steady and manageable especially in high speed turns.

B

This letter tells you a little bit about the process used in the construction of the tire. B means that the tire is a belted bias tire. An R would mean that the tire is a radial tire. While bias tires and radial tires both do a good job of supporting your motorcycle, many of their ride characteristics are radically different. For this reason, bias tires and radial tires should not be used on the same motorcycle unless specifically recommended by the manufacturer.

17

This number tells you the diameter of the rim that the tire fits on, in inches. So the 17 listed here means that the rim the tire fits is 17 inches in diameter.

V

This is the speed rating of the tire (see chart below). The speed rating is the maximum speed a tire should be ridden at under its recommended load capacity. This speed should not be exceeded; doing so will not only dramatically reduce the life of the tire, but can even cause the tire to fail while you are riding.

Tire Speed Ratings

Rating

Speed

Q

Up to 99 mph

S

Up to 112 mph

T

Up to 118 mph

U

Up to 124 mph

H

Up to 130 mph

V

Up to 149 mph

W

Up to 168 mph

Y

Up to 186 mph

Z

149 mph and over

       Max. Load 365 kg (805 lbs)

 

This is the maximum weight that the tire is capable of handling when properly inflated, including the weight of the motorcycle, rider, passenger, and gear. Each tire will have a maximum load rating. The combination of these load ratings is the total amount of weight that the tires are capable of supporting.

Keep in mind that this total is likely to be greater than the GVWR of the motorcycle, the total amount of weight that the motorcycle itself can support (including motorcycle, rider, passenger, and gear). The GVWR for a motorcycle can be found in the owner's manual, the service manual, or on a label located on the motorcycle's front frame down tube. Listed with the GVWR will also be listings for the maximum amount of weight that each wheel's axle can support. Make sure that your motorcycle's load is properly balanced so as not to exceed each axle's weight limit or each tire's load rating, and never load your motorcycle with more gear than either the GVWR or tire load rating recommends.

290 kpa (42 psi) Cold

 

This indicates the maximum amount of air pressure that the tire should hold when the motorcycle is fully loaded. Take note that this is the maximum air pressure for the tire, not the recommended pressure for the tire. It is usually recommended by the manufacturer that a tire is inflated to a few psi less than the maximum pressure rating how much less will depend on the tire and the manufacturer (see Avon, Bridgestone, Continental, Dunlop, Metzler, Michelin, Pirelli, and other tire manufacturers for more information). For instance, the tire in our example has a maximum tire pressure of 42 psi; however, the recommended tire pressure (according to the Dunlop website) is between 36 and 40 psi.

Tire pressure should always be checked when the tire is cold, before you ride your motorcycle. As a tire is ridden the tire, and the air inside the tire, heats up. Because warm air occupies more space than cold air, and because a tire is a sealed system, a tire that has been heated from the friction of rolling over the road will swell slightly as a result of the newly warmed (and so more voluminous) air inside of it. This means that the air pressure inside a warm tire will exhibit higher air pressure than the same tire when it is cool, and that as a tire warms more and more from contact with the road, the air pressure inside the tire will become greater and greater. Air pressure readings must be taken when the tire is cool because tire manufacturers use an unheated tire as the "base reading" when determining the amount of pressure that their tires must have and the amount of pressure that their tires can withstand. If tire pressure is checked when a tire is partially warmed up (after a few miles of riding), it will give a higher reading than what it should. If the same tire is checked after a few dozen miles it will give an even greater air pressure reading. Because there is no way for the consumer to easily tell whether their tire is fully warmed up or not, checking the tire pressure on anything but a cold tire will likely introduce inaccuracies into the pressure reading and could cause the rider to under-inflate his tires (see below this is a very common, and potentially dangerous, problem).
Tire pressure should be checked several times a month to ensure that you have enough air pressure in your tires. You absolutely must check your tire pressure before riding if you have not ridden your motorcycle more than one week. We estimate that between 3 and 7 psi of air pressure can be lost per week when a motorcycle is not ridden.

This is due to the fact that while the tire and rim form an almost air-tight seal, the key word is almost. Cooler temperatures, due to weather or inactivity, cause both the rim and the tire to shrink imperceptibly leaking small amounts of air in the process.

Over 80% of the motorcycles that come into our serivce department are markedly underinflated (having 30 psi of air pressure or less). Many have only 5 psi less than 15% of what they should have. These tires are so underinflated that their lifespans are shortened by several thousand miles, and riding on them is potentially dangerous. Given that new tires are typically several hundred dollars in cost, maintaining the proper air pressure in your tires can save you a lot of money as well as a trip to the emergency room.

Born on date, and wear's the tread?

Other useful information on the tire's sidewall includes its manufacturing date. Look on the side for a raised block with four digits; it's usually next to the U.S. DOT tire identification number. The first two indicate the week of its manufacture, and the last two are for the year. For example, 1702 would indicate the tire was manufactured in April, 2002. Prior to 2000, there were only three digits, with the last one indicating the year.

Some tires may have raised triangles, or the letters TWI, to show where the tire wear indicators are in the tread. When these marks are equal to the tread, it's time for new tires.