A Motorcyclist’s Guide to Motorcycle Tires, Sizing and More

Have you ever wondered what all the letters and numbers on a motorcycle tire mean? What is an “aspect ratio” anyway? How does age affect a tire’s safety, and how do you determine a tire’s age? If not for age, why else do tires need replacement?

These are all great questions, and if you’ve asked them before you’re not alone! We’ve put together this motorcyclist’s guide to motorcycle tires to shed some light into the mysterious world of tires.

Image Courtesy of Yamaha Motor Corporation, U.S.A.

Keep reading to learn more.

Key points:

• Motorcycle tire lifespan and replacement indicators: Motorcycle tires typically last up to five years, but signs like worn tread, uneven wear patterns (such as tire cupping and flat spots) and visible dry rot are critical indicators that a tire needs to be replaced. Checking the tire's DOT date stamp helps determine its age, which is crucial since tires can become unsafe over time even if the tread appears minimally worn.
• Understanding tire wear patterns: Tire cupping, a common uneven wear pattern, significantly affects performance by causing handling pulses and stability issues. It's essential to replace cupped tires regardless of tread depth. Other wear patterns, like flat spots and squared-off patterns, also indicate the need for tire replacement and can result from suspension issues or consistent straight-road riding.
• Tire construction components: A motorcycle tire comprises three main parts: the carcass (the base structure), the compound (a blend of rubber and chemicals for grip and wear resistance) and the tread pattern (which affects water displacement and grip). The construction of these components determines the tire's grip, wear and handling characteristics.
• Types of motorcycle tires and their uses: Tires are designed for specific motorcycle types and riding conditions, including cruiser and touring tires for heavy, long-distance bikes, dirt bike and dual-purpose tires for off-road and mixed-use and sport bike and track tires for performance riding.
• Motorcycle tire sizing and technician considerations: Tire sizing formats include metric (most common) and inch systems, with numbers indicating width, aspect ratio and rim size. Motorcycle technicians frequently deal with tire replacements and must navigate customer requests for non-OEM specification tires, assess tire safety and manage liability concerns related to tire repairs and customer refusal to replace unsafe tires.

How Long Do Motorcycle Tires Last?

As a motorcyclist, it’s very important to know how long motorcycle tires last — both for performance and safety.

As a general rule of thumb, the lifespan of motorcycle tires typically is not more than five years. However, it’s more important to look out for the following signs when it comes to evaluating when to replace them.

A quick visual inspection can often determine if a motorcycle tire needs replacement, as a worn tread is easy to spot. Unfortunately, some riders ignore their tires until the belts are visible!

Other visual clues used to determine a tire’s health include an uneven wear pattern, dry rot and checking the tire’s DOT date stamp.

Uneven wear patterns

Tire cupping is a specific type of uneven tire wear and common on all motorcycles. It’s a condition that occurs from normal braking, particularly on the front tire on street bikes and rear tire on cruisers, which are often used as the primary braking wheel.

As braking pressure is applied, the tire deforms and rolls backwards. This wears heavily on the leading edge of the tire tread, like an eraser. The result is a low spot on the leading edge of the tread and a high spot on the trailing edge. The wear resembles a subtle shape of a saw blade.

You can feel this condition by running a hand both directions along the tire circumference—it will have a smooth, yet ramped transition as it rolls up and over to the next tread in one direction. Feeling the other direction will be more resistant as the hand hits the face of the sharp higher edges.

Cupped tires should be replaced regardless of whether the tread depth is good or not. As cupping increases, performance decreases. It causes a pulse in handling and can lead to stability problems and loss of traction, which is important on two-wheeled vehicles.

Flat spots can also form on tires, which are caused by suspension (damping) problems. This occurs when the wheel bounces in a timed pulse from natural suspension (spring) frequencies, which causes increased pressure and wear on the same spot over and over — resulting in a flat spot.

A squared-off pattern (in the center of the tire) can also form as the result of a rider who cruises many miles on straight roads.

Determining a tire’s age

Tools exist to measure tread depth, and if you don’t have one, most tires are manufactured with tread wear indicators to aid in evaluating how much life remains in the tire. A visual inspection can reveal cracks and dry rot, but how do you determine tire age?

Department of Transportation (DOT) information is included on the sidewall, and the last four digits of the DOT information indicate when the tire was manufactured. For example, if the last four digits are “2517,” we know that tire was made during the 25th week of 2017.

So why does this matter? Tires can become unsafe, even if they show very little tread wear. So, it’s important to know their manufacturer date and keep a close eye on their appearance. An experienced technician can determine the best course of action for the motorcycle owner to take.

Motorcycle tire construction

To understand what makes a tire grip, wear or handle, it helps to understand how they are put together. There are three major parts: carcass, compound and tread pattern.

The carcass is the base of the tire. A weave of belts encased in rubber keeps the overall shape. Belt design allows more or less tire flex and contact area.

Image Courtesy of Yamaha Motor Corporation U.S.A.

Compound is the blending of rubber and chemicals to create a stickiness and wear resistance. Usually, softer compounds provide better grip but wear quicker, and stiffer compounds provide less grip but last longer.

Tread patterns consist of gaps between contact areas, which provide passages for water displacement and open areas for dirt grip. Larger contact areas and less gaps provide better grip for solid road. Larger gaps, such as on knobby dirt bike tires, are best for off-road.

Types of Motorcycle Tires

Motorcycle tires vary in their construction (bias ply, radial, etc.) and in their intended use. Naturally, there is a degree of overlap between a few categories. However, the following tire types each have their specific use:

• Cruiser and touring tires
• Dirt bike tires and dual-sport tires
• Sport bike tires (track tires)

A tire’s shape and rubber compound will influence how it performs and its longevity. For example, you could put a dirt bike tire on a cruiser and vice versa, but neither is a good idea. Let’s take a deeper look into each category.

Cruiser and touring tires

Heavier motorcycles need a tire designed to support a lot of weight. Many cruisers and tourers weigh between 700 and 1,000 pounds, before rider and gear. Additionally, cruisers often travel long distances, so their shape, rubber compound and tire construction work together to hold up in these conditions.

Dirt bike and dual-purpose tires

Knobby dirt bike tires, aka knobbies, are for 100% off-road use where grip and durability are the main goal. Dirt bike tires need to flex and conform to the terrain. Knobbies “swim” through loose terrain and the knobs perform much like a paddle. Taller, softer and thinner knobs provide better grip, but can tear off easier.

Additionally, knobs wrap onto the tire’s shoulder to offer traction in turns and when ridden on angled trails or courses. Tire life, decreased pavement traction and road noise make these tires unsuitable for road use.

Dual-sport tires, or “street legal knobbies,” have a mildly aggressive tread pattern for gravel roads, fire roads and some easier trails. They aren’t as aggressive as pure dirt bike tires, but they are capable off road and allow for many miles of on-road riding. These tires improve road stability by using shorter and wider knobs.

Over the past decade, dual-sport and “adventure” riding has become one of the most popular forms of motorcycle riding. Manufacturers offer a variety of dual-sport tires that range from mostly on-road to 50/50 to mostly off-road focused riders.

How to read dirt bike tire sizes

Understanding how to read dirt bike tire sizes is crucial for selecting the right tires for your ride. Tire sizes are typically displayed on the sidewall in a format such as 110/90-18.

• The first number (110) represents the tire's width in millimeters.
• The second number (90) is the aspect ratio, indicating the height of the tire's sidewall as a percentage of the width. In this example, the sidewall height is 90% of 110 mm.
• The final number (18) denotes the diameter of the wheel rim in inches.
• Additionally, you may find a letter code like "R" or "B," which stands for radial or bias-ply construction, respectively. Understanding these numbers and letters helps ensure you choose tires that provide the right fit, performance and safety for your dirt bike.

Sport bike and track tires

The longevity (mileage) of a sport tire depends on the hardness of the compound. Dual and tri-compound tires provide different bands of material, strategically placed at different lean angles on the tire tread. These hybrid tires provide softer compounds on the sides of the tires for improved grip at extreme lean angles, while the center of the tire is made of harder compounds for durability during straight up distance riding.

Compound tires can provide the best of both worlds for grip and mileage, though must run softer carcasses to allow a wider contact area. Because of this, sport bike tires will grip better than touring tires but will not last nearly as long.

For race and track day riding, even dual and tri-compound tires are made extremely soft and flexible. It is not uncommon for an aggressive rider to completely wear out a set of track tires in a single race day or club track-day. Non-DOT road-race tires, such as slicks, are not legal for road use and typically wear out very quickly.

Heat cycles are another issue for race tires. When racing, tires can see temperatures of 250 degrees Fahrenheit! Each time they cool down, the compound hardens and reduces performance. While DOT-approved race tires can be used on the street, this hardening process dramatically reduces their durability. A high grip street tire will eventually outperform the race tire and last much longer.

Motorcycle Tire Sizing Formats

There are a variety of motorcycle tire sizing formats, including metric and inch tire sizing systems.

Understanding motorcycle tire sizes

Here is how to read motorcycle tire sizes. Tire sizes are typically indicated on the sidewall in a format such as 180/55ZR17.

• The first number (180) represents the tire's width in millimeters. The second number (55) is the aspect ratio, showing the height of the tire's sidewall as a percentage of its width—in this case, 55% of 180 mm.
• The letter "Z" indicates the tire's speed rating, denoting its capability to handle speeds above 149 mph. The "R" stands for radial construction, which is preferred for its enhanced performance and handling.
• Finally, the number (17) specifies the diameter of the wheel rim in inches. Understanding these details helps in selecting the right tire, ensuring a proper fit, improved handling and safety on the road.

Metric

Metric is the most commonly found tire sizing system.

If 180/55/17 is listed on a tire, this would indicate that the tire has a width of 180 mm, a sidewall height of 55% of the width and fits a rim diameter of 17 inches. When numbers are formatted like this, they represent width/aspect ratio/rim size.

Occasionally, letters will be included to indicate a tire’s speed rating. Z or W correspond to the maximum speed in which a tire will still perform as designed. Motorcycle tires may also have the letter R mixed in the size code, indicating it is a radial tire.

Inch

While not common, the inch tire sizing system is still in place today on some vintage motorcycles and dirt bikes. The inch system assumes an aspect ratio of 100%, meaning the tire sidewall and tread width is equal.

A sub-category of this system is the low-profile inch system. If the tire’s sidewall is less than the tread width, an additional number will be present. An example of the inch system would be 4.0 -19 and an example of the low-profile inch system would be 4.10 - 19 (the sidewall height is 10% less than the width).

As with the metric and alpha sizing systems, speed and/or load rating may be present.

What Does This Mean for Motorcycle Techs?

Motorcycle technicians replace tires — a lot.

When it comes to changing tires on a customer’s motorcycle, there is much more that goes into it than the physical act of replacing the tire. Potential areas of concern include:

Plugging or patching a good tire that has gone flat due to puncture

As tires are a critical safety item on a motorcycle, a shop may be liable for damages caused after a motorcycle leaves the shop. For this reason, some shops will turn you down when it comes to plugging a tubeless motorcycle tire or patching a tubed tire.

Technicians working for the shop or dealership will not set the policy, however, if your goal is to be a business owner, you will decide if repairing tires is a job you’ll do. The actual act of plugging or patching a tire is easy, however the liability can be significant.

Customer refuses to replace tires despite technician suggestion

Part of a technician’s job is to ensure that a motorcycle is safe when it leaves the shop. Even when motorcycles come in for work that doesn’t involve tires, skilled techs will make note of thread depth, tire age and overall tire condition.

If a customer acknowledges that their tires need to be replaced, but doesn’t agree to change them, most shops will have a liability release signed by the customer. In cases like this, technicians are called upon to educate the customer on how tires may look acceptable but be unsafe to ride on.

Customer request non-OEM specification size or tire type

The size and shape of a motorcycle tire impacts how it performs when ridden. A seemingly innocent customer request for a non-OEM specification tire could have dangerous consequences out on the road.

Shops will have a policy in place for handling situations such as this, but as with other tire issues, if your goal is to work for yourself, be prepared for this ahead of time. It is possible for street bike owners to request dual-sport tires for light off-road usage. This is generally safe, but educating the customer is key to both their satisfaction with their purchase and their safety.

Train for a Career in the Motorcycle Industry

Passionate about motorcycles? Train to pursue a career as a motorcycle technician at Motorcycle Mechanics Institute (MMI).^1.1 In addition to course topics like tire construction, tire balancing, wheel removal and replacement, students get hands-on practice troubleshooting engines, denoising electrical components, maintaining powersports equipment and more!¹⁸

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