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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. Keep reading to learn more.
Image Courtesy of Yamaha Motor Corporation, U.S.A.
As a motorcyclist, it’s very important to know how long motorcycle tires last—both for your performance and safety.
As a general rule of thumb, the lifespan of your motorcycle tires will typically not last beyond five years. However, it’s more important to look out for the following signs when it comes to evaluating when to replace them.
Oftentimes, a quick visual inspection can determine if a motorcycle tire is in need of 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 uneven wear pattern, dry rot and checking the tire’s DOT date stamp.
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 training 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 infinitely more important on 2-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 the same spot to be hit and wear 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.
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 4 digits of the DOT information indicate when the tire was manufactured. For example, if the last 4 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.
In order to understand what makes a tire grip, wear or handle, it helps to understand how they are put together. There are 3 major parts: carcass, compound and tread pattern.
The carcass is the base of the tire. A weave of belts incased 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 dirt bike knobbies, are best for off-road.
Motorcycle tires vary in their construction (bias ply, radial, etc.), and they also vary in their intended use. Naturally, there is a degree in overlap between a few categories, however, the following tire types each have their specific use:
A tire’s shape and rubber compound will influence how it performs as well as 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.
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 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 like a paddle, as much as grip. Taller, softer and thinner knobs grip better, though, tear off knobs easier.
Additionally, raised lugs (knobs) wrap onto the tire’s shoulder to offer traction in turns and when ridden on angled trails or courses. Tire life, 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 miles and miles of on road riding. These tires improve road stability by using shorter and wider knobbies.
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.
The longevity of a sport tire (mileage) 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 extreme lean angles and grip, while the center of the tire is made of harder compounds for 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˚ F! 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 ability to last. A high grip street tire will eventually outperform the race tire and last much longer.
There are a variety of motorcycle tire sizing formats that exist today, including metric and inch tire sizing systems.
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.
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.
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:
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.
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 actually be unsafe to ride on.
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.
In Motorcycle Mechanics Institute’s 42-week Motorcycle Technician Training Program, you’ll learn the foundations of motorcycle, ATV, side-by-side and personal watercraft to prepare for a career as a motorcycle technician.
MMI grad Jim Drew is the owner of Hingham Cycle in Massachusetts. Click here to learn his tips for running a successful motorcycle repair shop.
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