What’s In A Tire?

Story by Pat Morrison//
April 1 2012

Buying tires for your bike can be confusing. We won’t be recommending brands or models here; much of that choice is personal, or strictly based on cost. Instead, we can give you an idea of what to look for in a tire when it comes time to replace the old ones on your bike.

Let’s begin with proper selection. Out of a multitude of tire makes and models, you’ve got to choose the right ones for your motorcycle. Even though you can find the proper size of supersport tire to fit onto the wheels of a BMW K1600GT, it would be silly, and possibly dangerous, to fit such specialized racing tires onto this touring bike. Aside from their extremely soft rubber compound designed to grip within a narrow temperature range, they’d probably wear out by the time you reached the end of your driveway on such a heavy machine. These tires also have very soft sidewalls, which would have a detrimental effect on handling such a big bike.

Likewise, if you ride a dual-sport machine, using aggressively knobby dirt tires, whether they have a street-legal DOT rating or are slated for off-highway use only, might not be the best choice if you ride mostly on pavement. Using such tires on a dual-sport machine that spends most of its time on the road will wear them out prematurely, as well as unnecessarily hinder handling and braking on asphalt. This is easily avoidable with such a wide variety of dual-sport rubber on the market, ranging from 90/10-percent on/off road to 10/90-percent on/off road.

Most tires are rated tubeless; that is, they will hold air when mounted onto an appropriate, tubeless rim. The bead (the innermost part of the tire, where it meets the rim) of this type of tire is specially designed to provide an airtight seal on the rim so an inner tube is not needed. This type of tire can be mounted on a tube type-rim (usually a laced wheel) if an inner tube is also used.

Tube-type tires (mostly motocross knobbies and some vintage bike tires) are meant to be used with inner tubes no matter what type of rim they are mounted on.

The large numbers on the side of the tire indicate its size. Most tires now use the metric sizing system. An example of this sizing is 100/90H-19. The “100” indicates the width of the tire in millimetres; the “90” indicates the aspect ratio of the tire, or the height from the bead to the top of the tread. In this case, the tire’s aspect ratio is 90 percent of its width, which works out to 90 mm. Finally, the “19” is the diameter of the wheel it’s to be mounted on, in inches.

The letter H indicates the speed rating of the tire, or its capability to sustain high speeds, which in this case would be 210 km/h. Most sportbike tires have a Z speed rating and can sustain speeds above 240 km/h. If the letter R follows the speed-rating letter (180/55ZR-17), it indicates a radial tire.

Some manufacturers still use the Alpha sizing system, which appears like this: MU90H-16. Here, the “MU” denotes the width of the tire, which in this case is 5.5 inches. The “90” is again the aspect ratio, the “H” is the speed rating and the “16” is the diameter of the wheel. This system is still used on cruisers; Harley’s Blackline Softail uses an MH90-21 front (3 inches wide, 21-inch diameter) and an MU85-16 rear (5.5 inches wide, 16-inch diameter). All major tire manufacturers offer equivalent metric replacement tires, and cross-reference charts can be found on their websites (listed below).

When selecting tires, it is best to stick to the original sizes. Motorcycles are designed to handle best using the tire sizes they left the factory with. Many people believe going to wider tires will improve grip and handling, but this is not true. A wider rear tire will give a bike more of a tendency to stand up in turns, which increases steering effort, and a wider front tire only slows down steering. Grip on the road is almost never an issue as far as tire width is concerned; it’s more affected by rubber compound and tread design.

This isn’t to say you cannot improve on the handling by switching to a different tire brand, but those differences are more related to each tire’s rubber compound and carcass design than to size.

You should note that there is some size discrepancy between tires from different manufacturers; a tire from one maker might be slightly wider or taller than one from another maker, even though the numbers on the sidewalls are identical, and this may affect handling, for better or worse. Again, you can visit each tire maker’s website for specific information – it’s all there and it’s useful.

Also, don’t change tire types. If your bike came with radial tires, stick to radials; ditto for bias-ply tires. And don’t mix types; use the same type of tire front and rear. Mixing brands should be avoided, mostly because different construction techniques might cause unusual handling issues.

Once you’ve selected your tires, make sure they are installed properly (check that the directional arrow is pointing in the proper direction), verify that they are properly balanced (this makes the bike ride smoother and extends the life of the tires), and check that they are properly inflated, either according to the motorcycle manufacturer’s recommendations, or those specified by the tire maker for the model you selected for your bike.
Tire pressure is the single most influential factor affecting both tire wear and handling. It’s more likely that tires will be under-inflated than over-inflated (tires naturally lose pressure, as air molecules are smaller than rubber molecules and air will seep through a tire over time). This makes steering heavy if the front tire is low, and makes the bike weave and wallow if the rear is low. A low tire will also deflect more, causing more friction and heat buildup, which not only wears the tire faster, but can also cause a blowout at high speeds. Check your pressures once a week during the riding season to avoid these problems.

Manufacturer Websites:


Technical articles are written purely as reference only and your motorcycle may require different procedures. You should be mechanically inclined to carry out your own maintenance and we recommend you contact your mechanic prior to performing any type of work on your bike.


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