Describing a motorcycle tire as a round, black piece of rubber that’s about a two-foot diameter circle won’t give you the information you need to make a smart buying decision, will it? When it comes to tires, particularly the ones you plan to use on your motorcycle, information is a very handy thing to have. This is especially true if you’re interested in replacing those tires and even more so if you plan to replace them with something other than the OEM tires.
To make it a little easier, the tire manufacturers have come up with nomenclature that describes everything you need to know about your tire and printed it all on the tire sidewall. Actually they embossed that info on the sidewall to comply with DOT regulations. On the downside the information is printed in what appears to be some sort of code, but fortunately it’s one that’s easy to decipher, if you know what to look for. This brief overview won’t provide all the information on all the tires, in large part because while certain information is required, the tire manufacturer is free to add anything they think is pertinent. For example a race tire will always have a compound designation on it, but that information isn’t generally included on a street bike tire, though there’s no reason it couldn’t be. By the same token some of the information is a bit arcane. Case in point, while the DOT or NHTSA may need to know where a particular tire was built in the event of recall; consumers generally don’t have that same compelling interest. Consequently it’s enough to know that it’s there, (it is) without knowing how to decipher it. What follows are the high notes.
All motorcycle tires will have certain information printed or embossed on the sidewalls. Some of the information, like the tire size will be bold and printed on both sides of the tire, other information will be in small print and can, depending on the manufacturers whim, be found on both sides of the tire or more often on only one side.
There are three general ways of describing a tire: the inch designation, which is not as popular as it once was, the alpha/numeric system which was never particularly popular and the metric system that is currently the standard of the industry.
The inch designation uses the most basic description. What you see on these tires (think of a Dunlop Goldseal K70 from circa 1970 in this instance) is a short series of letters and numbers. For example 4.00H-18 4PR provides only the most rudimentary details. In this example, the 4.00 stands for the tire’s width in inches, the H is its speed rating, and 18 means the tire is designed to fit an 18 inch rim. The 4PR means the tire has four plies. Additionally the tire may have a directional arrow indicating which way the tire is meant to spin, or it may not. A lot of those old tire designs could be mounted in either direction. These types of tires are still available. Vintage motorcycle enthusiasts use them, in addition to some modern tires that are now available in those sizes.
Other nomenclature will also be present, but we’ll get to that in a bit. By the way, that H rating means the tire is good for sustained speeds of 130 mph, which is pretty impressive when you figure that the bikes these tires originally came on were lucky to make 50 horsepower.
As far as motorcycles are concerned the alpha-numeric designation was never wildly popular, it was really more of a car tire thing, but the designation is still in use on cruiser tires used by Harley Davidson and others. In alpha-numeric, the tire codes look like this: MM90 18 Load Range B. The first M means the tire is intended for motorcycle use. The second M represents the width, in this case four inches. 90 is the tire’s aspect ratio, which is the sidewall height expressed as a percentage of the width. Since this tire is four inches wide, the side wall height (90% X 4.0) is 3.60 inches. As with our first example the 18 means this tire fits an 18-inch rim. The load range designation is a curveball. Alpha-numeric tires used an alphabetic load range system rather than a speed rating. A was the lightest, B slightly heavier and so on. As with the inch designation other nomenclature will be included.
The metric designation is currently the most popular and most complex, but it also provides the most information. Let’s breakdown a tire stamped 170/60 R 18 73 H, which is a fairly common size. The 170/60 means the tire is 170 mm wide, with an aspect ratio of 60, making the sidewall height 102 mm. (60% X 170 = 102) the R means this tire is a radial. As you may have guessed by now the 18 is the rim diameter, which oddly is always given in the inch format. The 73 is the tire’s load rating, which won’t mean a thing unless you have a load chart to tell you that a tire so designated can carry a load of 805 pounds or look for the fine print that describes the load the tire can bear at the maximum inflation pressure, which will be stamped somewhere on the tire. H is the speed designation and as was the case with our first example, H means the tire can be used at sustained speeds of 130 miles per hour.
If you’ve figured out a pattern here, you’re right on the money: all three tires are nominally of the same size and notwithstanding the fact the last one is a radial, all three are more or less interchangeable.
Additionally all tires will have the following information (or in some cases not):
Balance Dot – Many, but not all, new tires will have a dab of paint indicating their light spot. This should be aligned with the valve stem during installation. The balance dot isn’t required by law, nor is the convention always observed, particularly in the automotive world, where paint dots sometimes indicate the tire’s high spot, which is aligned with the wheel’s low spot during installation. Bottom line, if there’s any doubt as to where the dot goes, consult the tire manufacturer’s tech specs.
Directional Arrows – Many motorcycle tires are designed to rotate in a particular direction. Arrows molded into the sidewall make sure the tires are properly oriented and they are generally accompanied by the words “Front Rotation” or Rear Rotation” to remove any doubt.
Mounting Instructions – This is a warning not to over inflate the tire during installation. To protect the tire and installer it generally recommends using 40 psi or less to bead the tire.
Maximum Pressure – The maximum pressure stamp indicates the safe maximum pressure the tire can bear. It is NOT the recommended tire pressure. That information can be found in your owner’s manual or stamped on a sticker somewhere, but if you subtract four psi from the maximum pressure you’ll generally be in the ball park as far as normal riding goes.
Maximum Load – This is the amount of weight the tire can bear when inflated to the maximum pressure. The load your bike can bear may be different, so always consult your manual or the bike’s GVWR (Gross Vehicle Weight Rating) before packing for that round the world trip.
Low Tread Indicators – These are small guides embossed on the sidewall usually in the form of a triangle or the letters TWI that indicate where the low tread warning blocks can be found on the tire.
The Date Code – Tires harden with age and depending who you talk to have a shelf life of between 5-8 years. So, knowing when they were built is handy information to have, especially when somebody offers you a bargain basement price on set of NOS (New Old Stock) skins for your superbike.
To find the Date Code look for a raised block with four digits. It’s normally next to the DOT tire I.D. number and will be found on only one of the sidewalls. The first two digits represent the week of manufacture, the following two indicate the year so if the tire reads “2002” it would mean the tire was manufactured in the twentieth week of the year 2002.
As we mentioned the tire sidewall provides a ton of information. Armed with it you can make an informed choice when it comes to your next tire purchase.
Example of a good Tire Nomenclature Image/layout:
Tire Size & Speed rating: http://p.vadstorage.com/images/motorcycle.com/SuperStore/motorcycle-tire-size-conversion-chart.jpg