A Primer for the Layperson
Let’s face it — most of us give our tires very little thought, especially when it comes to tire pressure. This may be a carryover from our experience with our car tires, when the only time we think about tire pressure is when there isn’t any. A flat on the car may not be too serious, because we can limp over to the shoulder and install the spare. But on a two-wheeler, with only two small patches of rubber separating us from disaster, a flat or blowout will cause serious control problems and possibly a crash.
What Functions Do Our Tires Serve?
We know that our tires keep the wheel rims from rubbing on the road, but how many have considered that they’re actually part of the motorcycle’s steering and suspension systems? The pressurized air in the tire, not the tire itself, is what supports the motorcycle’s weight, helps the tire keep its shape and maintain the handling qualities with which it was designed. When riding mostly vertical, your bike’s forks and shocks handle most of the duties of absorbing bumps and keeping the tires in constant contact with the road surface. But as lean angles increase, upward forces are increasingly shared between the suspension’s springs and the tires’ sidewalls. Motorcycle tires are designed with this in mind and are generally much stiffer than car tires, but we’ll get into more of that later. For now, just remember the tires’ four main functions:
- Support the vehicle and load.
- Transmit drive and braking forces to the road surface.
- Absorb shocks from the road surface.
- Facilitate direction change and maintain desired direction of travel.
A motorcycle tire is made of several components. The casing is what gives the tire its basic shape. It’s made of natural and synthetic rubbers into which nylon or rayon cords have been bonded. For tubeless tires, an inner liner is installed to retard pressure loss. In a bias-ply tire, the cords overlap in an X-shaped pattern. On a radial tire, the cords are applied radially around the casing, 90 degrees to the direction of rotation. Some designs also have belts of steel or other materials applied in the direction that the tire rolls. Make a note of that in case you’re considering mounting a rear tire on the front of a motorcycle and reversing its rotation direction. Tires marked either for rear or front fitment only must never be installed on the “wrong” end, because it’s possible that the belts or tread may separate when encountering forces opposite to those for which they were intended. Also, if the tread sipes (the grooves or channels built into a tire’s contact surface) are reversed, they may not channel water effectively, leading to hydroplaning. Tires not marked as to fitment may be installed either front or rear.
Because a radial tire has a lower aspect ratio (see diagram) than bias-ply tires, it can normally handle less load than a bias-ply designed for large, heavy touring bikes or cruisers. Bias-ply tires normally have taller sidewalls and a more rounded tread profile. But the stiff chassis and cornering requirements of modern sport bikes (and the GL1800) are better suited to the characteristics of radials.
Motorcycle tires are designed differently than car and truck tires.
Before entering this discussion, please understand that it is not off ered for the purpose of recommending motorcycle tires over car tires or vice versa, but merely to examine the design differences and functions of a motorcycle tire as opposed to tires designed for use on four-wheeled vehicles. We off er this information in hopes that educated consumers, regardless of their choice, will be making purchase decisions based on knowledge rather than supposition.
If, after reviewing this information, a person is convinced that automobile tires are best for his/her motorcycle, then I’d encourage further study of the subject. But in the end, that person should do as they see fit. We’re all adults here and capable of making our own decisions. Just don’t tell anyone that you were advised by information contained here that car tires are suitable for fitment on a motorcycle. Nothing in this article implies anything of the sort.
Motorcycle tires are purpose-specific, meaning they’re designed solely for use on single track, two wheeled vehicles. That’s because they serve very different functions and operate under completely different conditions than car or truck tires. Let’s go back to the statement that motorcycle tires generally have much stiff er sidewalls than car tires.
Why is sidewall stiffness such an issue? It’s mainly due to a phenomenon known as camber thrust (see diagram). We know that we must cause our bikes to lean to begin a turn and the tires roll onto a portion of the tread on the side of the turn. Car tires are designed to accommodate a leaning force in exactly the opposite direction — toward the outside of the turn. What’s the difference? The camber thrust developed by a motorcycle tire while turning is what counters centrifugal force. At a lean angle of 20 degrees, a motorcycle tire will produce around twice as much camber thrust as a car tire at the same lean angle, so sidewalls need to be much stiff er to resist that force and keep the tire from deforming. Car tire sidewalls are too soft to produce sufficient camber thrust on a motorcycle. The rigidity of a car tire is the same from the tread to the sidewall, but performance motorcycle tires have stiffened tread shoulders and sidewalls for better cornering stability. One unfortunate consequence of the extra design elements and materials needed to manufacture radial motorcycle tires is that they greatly increase the cost of manufacture over bias-ply bike tires and car tires.
The contact patch is that portion of the tread in contact with the road surface at any given time. Noting the relevant drawings, we see that at zero lean angle, the car tire has a much larger contact patch than the bike tire. This might not be a bad thing, provided we always rode with no need to turn or swerve sharply. And on wet roads, expect loss of traction and hydroplaning due to the reduced weight per square inch of contact. Further, note that the drawings of the motorcycle tire contact patch show a relatively uniform shape and size, regardless of lean angle. This provides fairly constant handling and grip as lean angles increase. By contrast, a car tire used on a motorcycle changes the shape and size of its contact patch suddenly and drastically at only 10 degrees of lean. At 20 degrees or more, the tire is actually riding on its sidewall. This sudden and drastic change in the shape and size of the contact patch decreases grip, changes handling characteristics and makes the bike much harder to control. Motorcycle tires are designed with the tread surface being wider than the sidewalls, making cornering angles of up to 50 degrees possible without the sidewall contacting the road surface. Bear in mind that the tread sipes are designed to channel water out from between the tire and road surface, and motorcycle tires do so at all lean angles. But while cornering using a car tire, half or more of the contact patch may consist of areas with no sipes whatsoever. Even if those sipes were designed to function at lean angles experienced on a motorcycle, they would have a hard time performing that task while not actually in contact with the road surface.
Grip is often referred to in general terms, but it actually consists of two components: adhesion and hysteresis loss. Adhesion is the component we think of as being due to friction between the rubber and the road surface, while hysteresis loss occurs when some of the engine’s power is not transmitted to the road surface due to deformation of the tire’s surface while responding to road irregularities. This hysteresis loss results in generation of grip, which comprises the majority of grip at high speeds.
Grip can be optimized for a particular vehicle by compounding rubber to suit the tire’s use. With car tires, the tread compound is the same from edge to edge, because (when used on a car) the entire tread width is normally in contact with the road surface. Not so with motorcycles. We’d like as much tread life as possible, but also as much grip as possible whether leaned into a corner or riding straight and level. Commuter bikes would use a harder compound for greater wear resistance, while sport bikes would need a softer, faster wearing compound to generate the required grip at speeds and cornering angles they often experience. The compromise for bikes capable of higher speeds and lean angles is to manufacture tires with a harder, longer wearing compound in the center portion of the tread, and a softer compound nearer the edges for greater grip while cornering.
Rim fitment: Is 16 inches really 16 inches?
Rims designed for fitment of motorcycle tires actually have a slightly larger diameter bead seating area than on a comparable diameter car wheel rim. In addition, the bead portion of car tires is shaped slightly different than on a motorcycle tire. These two differences can make it very difficult to get a car tire properly seated on a motorcycle rim without exceeding safe maximum inflation pressures shown on the tire’s sidewall. What’s the big deal here? Depending on the amount of over pressurization, the tire’s steel bead wires can slip, allowing the tire to tear at the bead and explode violently. To say that physical injury or death could result would be a gross understatement. I’d like to stress once again that this is not biased information aimed specifically at discouraging car tire fitment on a motorcycle. It is simply a statement of facts as presented by both the Rubber Manufacturers Association and the Tire and Rim Association, which governs the sizes and specifications of all tires and rims for all types of ground vehicles.
How did these rim sizing variations come about? As motorcycles evolved from bicycles, bicycle tires were often fitted to early motorcycles. But as roads improved and speeds increased, motorcycle wheels and tires became wider. The tires fitted were often the same construction as car tires, but narrower and larger diameter. Real development of specialized motorcycle tires began in the 1950s, as motorcycles became capable of much higher speeds, and the forces they imparted to their tires (and the tires imparted to the bikes) varied greatly from those of car tires. But for various reasons, a lot of riders were still installing car tires. So in the interest of safety, the Tire and Rim Association issued a mandate that all motorcycle wheel rims would be slightly larger than comparable sized car and truck rims. Naturally, tire makers adapted their products to fit the new rim standards.
How can one tell if a wheel rim is designed specifically to fit a motorcycle tire and no other kind? The rim will be embossed showing its width, diameter and service rating — either M/C or MT (sometimes both) indicating motorcycle service. Similarly, a tire designed specifically for motorcycle service will normally display the letters M/C after the size designation.
Moving right along…
Next month, we’ll explore subjects such as how to read a tire date code and why manufacturing date is important to you. We’ll discuss load range and speed index, and how both of those items are affected by inflation pressure. We should also discuss tire repairs, and we’ll weigh the pros and cons of using nitrogen as a tire inflation gas. Stay tuned.
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