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11~~-~'1 II,.,, 'IJIII1~11'' A primer for the layperson BY STU OLTMAN THROTTLE ¢ SAFETY FIRST

et's face it - most of us give our very little 1. Support the vehicle and load. thought, especially when it comes to pressure. 2. Transmit drive and braking forces to LThis may be a carryover from our experience with the road surface. our tires, when the only time we think about tire 3. Absorb shocks from the road surface. pressure is when there isn't any. A flat on the car may 4. Facilitate direction change and maintain not be too serious, because we can limp over to the desired direction of travel. shoulder and install the spare. But on a two-wheeler, with only two small patches of rubber separating us TIRE CONSTRUCTION from disaster, a flat or will cause serious control A tire is made of several components. The problems and possibly a crash. casing is what gives the tire its basic shape. It's made of natural and synthetic rubbers into which nylon or rayon WHAT FUNCTIONS DO OUR TIRES SERVE? cords have been bonded. For tubeless tires, an inner We know that our tires keep the rims from liner is installed to retard pressure loss. In a bias-ply tire, rubbing on the road, but how many have considered the cords overlap in an X-shaped pattern. On a radial that they're actually part of the motorcycle's tire, the cords are applied radially around the casing, and suspension systems? The pressurized air in the 90 degrees to the direction of rotation. Some designs tire, not the tire itself, is what supports the motorcycle's also hav~ belts of steel or other materials applied in the weight, helps the tire keep its shape and maintain the direction that the tire rolls. Make a note of that in case handling qualities with which it was designed. When you're considering mounting a rear tire on the front of riding mostly vertical, your bike's forks and shocks a motorcycle and reversing its rotation direction. Tires handle most of the duties of absorbing bumps and marked either for rear or front fitment only must never keeping the tires in constant contact with the road be installed on the "wrong" end, because it's possible surface. But as lean angles increase, upward forces are that the belts or tread may separate when encountering increasingly shared between the suspension's springs forces opposite to those for which they were intended. Also, and the tires' sidewalls. Motorcycle tires are designed ifthe tread sipes (the grooves or channels built into a tire's with this in mind and are generally much stiffer than car contact surface) are reversed, they may not channel water tires, but we'll get into more of that later. For now, j ust effectively, leading to hydroplaning. Tires not marked as remember the tires' four main functions: to fitment may be installed either front or rear.

WINGWO R LDMA G . CO M 053 THROTTLE ¢ SAFETY FIRST

Because a has a lower aspect ratio (see our own decisions. Just don't tell anyone that you were diagram) than bias-ply tires, it can normally handle less advised by information contained here that car tires load than a bias-ply designed for large, heavy touring are suitable for fitment on a motorcycle. Nothing in this bikes or cruisers. Bias-ply tires normally have taller article implies anything of the sort. sidewalls and a more rounded tread profile. But the stiff and cornering requirements of modern Motorcycle tires are purpose-specific, meaning sport bikes (and the GL1800) are better suited to the they're designed solely for use on single track, two­ characteristics of radials. wheeled vehicles. That's because they serve very different functions and operate under completely MOTORCYCLE TIRES ARE DESIGNED different conditions than car or truck tires. Let's go DIFFERENTLY THAN CAR AND TRUCK TIRES back to the statement that motorcycle tires generally Before entering this discussion, please understand have much stiffer sidewalls than car tires. that it is not offered for the purpose of recommending Why is sidewall stiffness such an issue? It's mainly motorcycle tires over car tires or vice versa, but merely due to a phenomenon known as camber thrust (see to examine the design differences and functions of a diagram). We know that we must cause our bikes to motorcycle tire as opposed to tires designed for use lean to begin a turn and the tires roll onto a portion of on four-wheeled vehicles. We offer this information in the tread on the side of the turn. Car tires are designed hopes that educated consumers, regardless of their to accommodate a leaning force in exactly the opposite choice, will be making purchase decisions based on direction - toward the outside of the turn. What's knowledge rather than supposition. the difference? The camber thrust developed by· a If, after reviewing this information, a person is motorcycle tire while turning is what counters centrifugal convinced that automobile tires are best for his/her force. At a lean angle of 20 degrees, a motorcycle tire motorcycle, then I'd encourage further study of the will produce around twice as much camber thrust as a subject. But in the end, that person should do as they car tire at the same lean angle, so sidewalls need to be see fit. We're all adults here and capable of making much stiffer to resist that force and keep the tire from

Tire type vs. camber thrust

160

120 -l

Camber IhNat

40

Camber angle

0 54 MARCH 2015 TBRO'rTLI ¢ SAFETY FIRS'l'

Motorcycle tire

Passenger car tire (Bias)

Passenger car tire (Radial)

Camber angle 10" 20" 30" 40"

Motorcycle Tire

Operating Range . 4~· ·~ Radial Passenger Car Tire ~~~~ti ~· ~-~,.. ~1 ~~ .., .,~-·.ti-i ,91t§~ J \i1 ~ ,.;.1 Operating Range ( ) Camber angle (degrees) 0 10 20 30 40

When a motorcycle tire goes into a lean, the stays consistent. But when a car tire does the same, the contact patch changes drastically. That's not good on a motorcycle. WINGWORLDMAG . COM 055 THROTTLE (} SAFETY FIRST

deforming. Car tire sidewalls are too soft to produce deformation of the tire's surface while responding sufficient camber thrust on a motorcycle. The rigidity to road irregularities. This hysteresis loss results in of a car tire is the same from the tread to the sidewall, generation of grip, which comprises the majority of but performance motorcycle tires have stiffened tread grip at high speeds. shoulders and sidewalls for better cornering stability. Grip can be optimized for a particular vehicle by One unfortunate consequence of the extra design compounding . ~bber to suit the tire's use. With car tires, elements and materials needed to manufacture radial the tread comfround is the same from edge to edge. motorcycle tires is that they greatly increase the cost because (whemJsed on a car) the entire tread width is of manufacture over bias-ply bike tires and car tires. normally in contaCt with the road surface. Not so with . We'd like as much tread life as possible, CONTACT PATCH but also as much grip as possible whether leaned The contact patch is that portion of the tread in contact into a corner or riding straight and level. Commuter with the road surface at any given time. Noting the bikes would use a harder compound for greater wear relevant drawings. we see that at zero lean angle, the resistance, while sport bikes would need a softer. faster car tire has a much larger contact patch than the bike wearing compound to generate the required grip at tire. This might not be a bad thing. provided we always speeds and cornering angles they often experience. rode with no need to turn or swerve sharply. And on The compromise for bikes capable of higher speeds wet roads. expect loss of and hydroplaning and lean angles is to manufacture tires with a harder, due to the reduced weight per square inch of contact. longer wearing compound in the center portion of the Further, note that the drawings of the motorcycle , and a softer compound nearer the edges for · contact patch show a relatively uniform shape and size, greater grip while cornering. regardless of lean angle. This provides fairly constant handling and grip as lean angles increase. By contrast, FITMENT: IS 16 INCHES REALLY 16 INCHES? a car tire used on a motorcycle changes the shape Rims designed for fitment of motorcycle tires actually and size of its contact patch suddenly and drastically have a slightly larger diameter bead seating area than at only 10 degrees of lean. At 20 degrees or more. the on a comparable diameter car wheel rim. In addition. tire is actually riding on its sidewall. This sudden and the bead portion of car tires is shaped slightly different drastic change in the shape and size of the contact than on a motorcycle tire. These two differences can patch decreases grip, changes handling characteristics make it very difficult to get a car tire properly seated and makes the bike much harder to control. Motorcycle on a motorcycle rim without exceeding safe maximum tires are designed with the tread surface being wider inflation pressures shown on the tire's sidewall. What's than the sidewalls, making cornering angles of up to the big deal here? Depending on the amount of over­ 50 degrees possible without the sidewall contacting pressurization, the tire's steel bead wires can , the road surface. Bear in mind that the tread sipes allowing the tire to tear at the bead and explode violently. are designed to channel water out from between the To say that physical injury or death could result would tire and road surface. and motorcycle tires do so at all be a gross understatement. I'd like to stress once again lean angles. But while cornering using a car tire. half or that this is not biased information aimed specifically more of the contact patch may consist of areas with no at discouraging car tire fitment on a motorcycle. It is sipes whatsoever. Even if those sipes were designed to simply a statement of facts as presented by both the function at lean angles experienced on a motorcycle, Rubber Manufacturers Association and the Tire and Rim they would have a hard time performing that task while Association, which governs the sizes and specifications not actually in contact with the road surface. of all tires and rims for all types of ground vehicles. How did these rim sizing variations come about? MATERIALS As motorcycles evolved from , tires Grip is often referred to in general terms. but it actually were often fitted to early motorcycles. But as roads consists of two components: adhesion and hysteresis improved and Sj:>eeds increased, motorcycle loss. Adhesion is the component we think of as being and tires became wider. The tires fitted were often the due to friction between ~he rubber and the road surface, same construction as car tires, but narrower and larger while hysteresis loss occurs when some of the engine's diameter. Real development of specialized motorcycle power is not transmitted to the road surface due to tires began in the 1950s, as motorcycles became capable

0 56 MARCH 2015 THROTTLE ¢SAFETY FIRST

of much higher speeds, and the forces they imparted motorcycle service. Similarly, a tire designed specifically to their tires (and the tires imparted to the bikes) varied for motorcycle service will normally display the letters greatly from those of car tires. But for various reasons, a M/C after the size designation. lot of riders were still installing car tires. So in the interest of safety, the Tire and Rim Association issued a mandate MOVING RIGHT ALONG ... t that all motorcycle wheel rims would be slightly larger Next month, we'll explore subjects such as how to than comparable sized car and truck rims. Naturally, tire read a tire date code and why manufacturing date is makers adapted their products to fit the new rim standards. important to you. We'll discuss load range and speed How can one tell if a wheel rim is designed specifically index, and how both of those items are affected by to fit a motorcycle tire and no other kind? The rim will inflation pressure. We should also discuss tire repairs, be embossed showing its width, diameter and service and we'll weigh the pros and cons of using nitrogen as rating - either M/C or MT (sometimes both) indicating a tire inflation gas. Stay tuned. [!!!

0 58 MARCH 2015