Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/126785
the average rider trying to choose a new tire, the only yisible difference is in the tread pallern. Because of this, knowledgeable engineers - people who actually design mOlorcycle tires - admit that there's no way by inspection to know whether a particular tire is superb, average or terrible. The reason why it's so difficult to understand motorcycle tires is actually one of five interrelated reasons: size rating systems, size tolerance, load capaci ty, recommended air pressure, and speed ratings. On examination, it's almost as though the tire industry, as a whole, is playing games with the consumer: stopping just short of deliberate misinformation about motorcycle tires. Let's take a look at these five games, and see if some sense can be made of the tire marketplace. prove important with the introduction of radial motorcycle tires. Curiously, the metric system still uses inch measurements for wheel diameters. This is a result of the fact that currently all motorcycle wheel diameters are listed in inches. Now it's bad enough that the three systems exist to describe motorcycle tires. The real crime is that none of the three systems are directly comparable! The alpha-numeric and inch systems describe tire widths in 14-inch increments, while the metric system makes its jumps 10 millimeters at a time: sligh tl y less than ~- inch J The inch system provides for only one low-profile aspect ratio - 82% - a tire profile that isn't shared by the . other two systems. The load game The tolerance game The size game Step one of purchasing a replacement tire is to determine what size you're after. This isn't as easy as it sounds. There are three different tire sizing systems in current use; none of the three are directly comparable. The oldest method of tire sizing is the alpha-numeric system, which uses two letters of the alphabet to describe the width of the tire. The first leller is always "M" (for "motorcycle"), followed by a second letter which corresponds to the claimed width of the tire. The system is set up to describe widths in 0.25-inch increments from 2.25 inches to 5.50 inches. When contemporary low-profile tires were introduced, a number was added after these two letters to describe the aspect ratio of the tire. For example, the designation MJ90-18 would describe a tire 3.. 25 inches in width, with a 90% a~pect ratio, sized to fit an 18 inch Trying to decode the nightmare created by the three sizing systems becomes absurd when considering the question of size tolerance. Ever wonder why you've never seen a direct size comparison chart for the three systems? The manufacturers don't want to bring up the question of tolerance. Currently, the sale of motorcycle tires for street use in the U.S. is regulated by the National Highway Traffic and Safety Administration (NHTSA), according to standards set forth by Motor Vehicle Safety Standard No. 119: "New Pneumatic Tires for Vehicles Other Than Passenger Cars." According to this standard, the size of the tire must be marked on the tire's sidewall (in lettering not less than 0.078 inches high), and the tiTe manufacturer must make available to the public a listing of rims that may be used with the tire. In addition, all dimensional specifications of the tire must be published by at least one of the international tire and rim associations, or printed in a document made available to retail tire dealers and the general public. nm. It's not· too surprising that the alpha-numeric system proved confusing; while the system is relatively simple, it requires a chart to decode what the letters mean. The inch system was created to make things more straightforward and easier to understand. This system simply uses a number, in inches, to describe the WIdth ofthe tire. Unfortunately, when the new low-profile tires appeared, the curious decision was made to describe the aspect ratio within the numbers. For example, a tire measuring four inches in width would be described as a 4.00-18. A low-profile four-inch tire would carry the designation 4.10-18... the addition of the .10 denoting the profile, not any additional width of the tire. Obviously, when using the inch system, it's easy to select a tire that would seem to be wider - but is actually only shorter due to this peculiar sizing system. The third system is the only one of the three that actually describes the width and height of a modern motorcycle tire: the metric system. Here, the tire's width is first described in millimeters, followed by a slash mark, followed by the aspect ratio - plainly listed as a percentage number. The big advantage of the metric system is the ability to denote any new width or height of tire; a feature which may mounting the tire. Ever tried to return and. exchange a tire that's already been mounted? Those Continental and Dunlop tires aren't isolated examples, either. A Pirelli MT 29, size 100/90V-18, mounted on a 2.5-inch rim, has an operating width of 116mm (4.56 inches). A Michelin A48, same size, same rim, measures 106mm (4.17 inches). A Carlisle Arrow MT90-16 (MT corresponding to five inch width), mounted on a 3-inch rim, measures 5.1 nches across. The same size Continental K112, mounted on the same rim, measures a whopping 6.18 inches - more than an inch wider than the size indicated on the sidewall! All well and good. However, the NHTSA has yet to define the acceptable tolerance for these size designations. Apparently, this is due to the reasoning that the same tire will assume different widths when mounted and inflated on different rims. Under the current system, a tire manufacturer may produce a tire of any width, and mark it with any size he chooses (however far from reality)providing information is provided as to recommended rims for that tirel What does this mean in the real world? Let's try some examples: A Dunlop K127, size 4.00-18, measures 4.45 inches when mounted and inflated on a 2.15 inch rim. A Continental TK44, size 4.00-18, mounted and inflated on the same rim will measure 6.72 inches in width. Neither tire measures four inches, as implied by the size designation. The problem is that some motorcycles, particularly those equipped with shaft drive, are tight on clearance. Should you have the misfortune to remove the Dunlop and install the Continental, your new tire may not fit - something learned only by purchasing and Another item which the NHTSA requires to be imprinted on the tire's sidewall is the load rating. Heavy loads (f'nd to squash tires, which generates destructive heat through sidewall flexing. Heat damages a tire from the inside out, and when carried to extremes can cause the component layers of the tire to separate. Today's crop of V-rated super tires are at a disadvantage when it comes to the load game. Tire designs which are capable of tremendous speeds need thin sidewalls: since the tire is rotating more rapjdly at speed, the tire has less time to cool between revolutions ... and therefore runs hotter. In addition, the tread depth must be shallower; deep tread grooves would enable the tread structure to squirm and shimmy at high speeds. On the other hand touring tires feature a beefier sidewall construction to cope with additional weight, and can make use of a deeper tread pattern for longer tire life. An example of these construction trade-offs is the Dunlop Touring Elite. When introduced, this tire sported a two-ply, two-belt construction, and carried an "H" speed rating (up to and including 130 mph). In January, 1984, Dunlop realized that the Touring Elite was being habitually overloaded by touring riders. Consequently, this tire was redesigned to a sturdier three-ply, two-belt construction - which increased the tire's load capacity by 60 pounds. However, this stronger construction took its toll on the top speed for the tire: the new Touring Elites have been lowered to an "S" rating (up to and including 113 mph). There are companies that take advantage of this relationship of speed and load. Knowing that a tire is designed for the touring market, the speed rating may be omitted entirely: an omission which corresponds to a rating of 95 mph. The reasoning is simple: a tire which is operated at a lower speed can carry more weight, and is therefore more appealing to a touring cyclist. A more realistic approach to the question of load ratings is to list the load rating for the theoretical top speed of the tire, followed by a separate load rating for a more realistic speed. For example, Continental lists a recommended load rating for its motorcycle tires when operated at 65 mph: this load figure is over 50% greater than the capacity for the tire at top speed. For a touring motorcyclist, this difference amounts to sev' eral hundred pounds! The pressure game Most riders don't realize that the air within a motorcycle tire is actually a component of that tire's design: every bit as important as the tread or cord structure. It's air pressure thal supports the motorcycle, and the amount of pressure determines how much the sidewall of the tire will flex when operating. Heavier loads-require higher pressures, as do higher sustained speeds. Depending on the weight and horsepower of a motorcycle, varying amounts of air pressure will be called for - just as varying tread rubber and cord structures will be called for. The problem here is for the latest technology, premium quality replacement tires. New and sophisticated cord materials, as well as different designs for the carcass of the tire will call for varying amounts of air to ,make the tires work. Typically, the original equipment tires sold on motorcycles are not representative of the state-of-th.e-art- yet the bike manufacturers can only recommend air pressures for those stock-equipment tires. And you can bet that the air pressures required for top-grade aftermarket tires will be very different from those of the stock components. The most notable recent example of this problem occurred with the Dunlop Touring Elite. Currently, this tire is the only one on the market that employs fiberglass belts under the tread rubber - a design claimed to improve tread life. Consequently, this design calls for a higher inflation pressure than most other tires: a minimum of 36 psi. When Dunlop had suffered excessive warranty claims the company urged their customers to properl'y inflate their tires, sending out urgent letters to over 360,000 individual owners of registered motorcycles with copies to motorcycle publications. What's the penalty for operating modern'tires at less than optimum pressures? Aside from less than optimal handling due to improper tire contact patch, the problem is the lower pressures "soften" the tire, causing the sidewalls to flex more as the tire rolls forward. This flexing generates heat. Hot rubber is soft rubber, and soft rubber wears rapidly. In addition, heat within the cord structure tends to deteriorate the strength of the cord material; an effect most apparent with rayon-based tires. . • (continued next week) 17