An Analysis of Variables Affecting Tire Wear And The Disparity In Tire Longevity Observed In Automobiles And Motorcyles
This article identifies the major variables affecting tire wear in general, applying these variables to automobiles and motorcycles. In so doing, it enumerates the differing factors applied to these variables that influence observed tire longevity between these vehicles.
How It All Got Started
Conversations with several individuals have stimulated much thought on the application of automobile tires to motorcycle use. Many folks use automobile tires for the rear wheel of their motorcycles. The motivation for this practice is extended tire life. This practice is known as riding on the dark side!
Motorcycle tires typically have a much shorter life span than automobile tires. The vehicles which use these tires are different which implies this is all the explanation necessary. But if one considers that on average a rear motorcycle tire has about 1/3 the longevity of a front motorcycle tire while a rear car tire used on a motorcycle will last between 2 to 5 times longer than the motorcycle tire would, wouldn't one ponder the factors influencing such longevity disparity between these two classes of tires when used on a motorcycle?
Most motorcycle riders in temperate regions typically log most miles over the summer months. It is a recreational vehicle to them and they may ride between 5,000 and 7000 miles annually. This would be replacing one rear motorcycle tire annually and one front tire roughly every three years. For these folks it may not be objectionable to replace a tire once a year.
But what about the person riding 3 times or more the miles during a year, logging 15,000+ miles? This person will probably be replacing 3 rear tires on average per year and one front tire annually. Motorcycle tires are typically more expensive than automobile tires and they cost much more to have installed. There is no Discount Tire for motorcycles, $60 being the average service fee to replace one motorcycle tire. This fee includes pulling the wheel, dismounting the tire, mounting a new one, balancing and reinstalling the wheel. The new tire will cost a minimum of $100, but $150 is closer to the average cost. You can shop around a find them somewhat cheaper by mail order but shipping is typically extra so it is necessary to shop for care when bargain hunting.
A suitable automobile tire may be had for under $60, but will probably not exceed $100, unless of course you want a 215mm run flat snow tire. Folks do use these with much success. You can buy an ATV jack for $!00 if you shop around and. If you pull your own wheels and find an automobile tire shop that will dismount a motorcycle tire and mount a automobile tire they may do this for as little as $11.50. You have some work to do, but the very first time you do this you are saving money and the jack is a one time fee. The shop that mounted your auto tire may not be able to balance it, but if you can purchase a static balancer for about $30. Harbor Freight is a good place to find inexpensive tools of this type. You can also use dyna beeds which may cost about $15 per tire.
Factors Affecting Tire Wear In Motorcycles and Automobiles
Numerous variables are considered during tire design. A complete discussion of the topic is beyond the scope of this article. Consideration will be restricted to those variables identified as germane to the topic or peripheral to central issues.
Various factors influence general vehicle tire wear. While the following list is not complete it does identify the major factors.
- Outside temperature
- Inflation pressure
- Frequency of rotation
- Appropriate pairing of tires on same axle (new opposite new, old opposite old)
- Proper or improper loading of the vehicle
- Rating of the tire itself
- Front end misalignment
- Unbalanced wheels
- High speed driving
Of these variables, many do not apply for our consideration. We will assume that operational parameters such as temperature, inflation, loading, alignment, balance and speed are held constant under the scope of consideration and therefore not issues affecting the comparison. Further, pairing tires does not apply to motorcycles and we will assume it is use on automobiles is appropriate to the application. The only factor that we must reflect upon but essentially negate is tire rotation. The practice in automobiles if properly performed, stabilizes the rate of wear across all four tires. This can not be done for motorcycles as the front tire is typically of different design than the rear tire. If we could rotate motorcycle tires it would minimize the differing rate of wear between the two tires, but we would be rotating at an annoying frequency and would be impractical. An automobile tire used on the rear wheel would typically provide at least 20,000 miles of service as opposed to roughly 5,000 miles for a motorcycle tire. With this consideration in mind we will restrict out focus to examining the variables that affect the rate of rear motorcycle tire wear. Without rotation your average automobile tire when used on an automobile will get at least 20,000 miles of wear but 40,000 is typical.
For all practical purposes the only factor left is the rating of the tire as specified by the manufacturer. There are several variables buried in the manufacturers specifications. Of these we will briefly examine a subset that includes user controlled and maintained values. These include:
- Air pressure
- Tire size
It should be noted that the above information can be obtained in any vehicle owners manual or by inspecting the side wall of any given tire.
It has already been stated that air pressure, load and speed will be considered to be in compliance, but we will revisit load limits for other purposes later in the discussion. Some components of tire size will be relevant later in the discussion as well. There also exist ratings for specific applications such as winter tires, high mileage tire, racing tires, etc. These ratings imply various aspects of tire design that may or may not be made available. They may include implications with respect to tire rubber harness, tread design and additional special features. Tire rubber hardness will be a factor of concern later in the discussion.
Dunlop Motorcycle Tires has specifically addressed the disparity between motorcycle tire wear and automobile tire wear in a FAQ entitled Why do motorcycle tires wear out so quickly compared to car tires? The explanation follows:
"In answering this question, we must first make sure that we are comparing apples to apples. When you think of it, the vast majority of motorcycles are high performance vehicles compared to most cars. Consider that you rarely see a motorcycle with less than ''H'' speed-rated tires. By comparison, the vast majority of cars ride on tires with lower speed ratings. Power-to-weight ratio and speed are big factors with respect to tire wear. Unfortunately, when many people compare mileage, they do so with a family sedan and a much higher-performance motorcycle.
Another significant factor in this comparison is the size of the tire contact footprint. In the case of a car, the footprint is much larger and there are four, not two tires contacting the pavement. You must keep in mind that virtually the entire width of the relatively flat tread of a car is in contact with the road all of the time. A car remains upright, even when cornering. This results in a much smaller contact footprint for a motorcycle tire. High power-to-weight ratio, speed, size of footprint and other aspects explain why motorcycle tires wear out quicker than car tires.
To obtain the best mileage from your motorcycle tires, observe the following guidelines: obey the speed limit; avoid quick acceleration and hard braking; maintain recommended tire pressures; and do not overload your bike or tow a trailer."To itemize Dunlop's statement. The following factors are primarily responsible for the disparity of tire wear in motorcycles compared to automobiles:
- High power to weight ratio
- Size of footprint
They also mention "other aspects". This is stated last in the list of factors but never elaborated upon. We can assume the three items listed first are the primary variables influencing tire wear disparity in motorcycles. It has been suggested by other parties that tire rubber hardness would be the dominant issue in consideration of the topic of motorcycle tire wear disparity. Certainly, "other aspects" could include rubber hardness, but from Dunlop's perspective it is not one of the three factores specifically enumerated.
Dunlop continues, making the following recommendations to extend tire life:
- Don't speed
- Avoid rapid acceleration
- Avoid hard breaking
- Maintain proper tire pressure
- Do not overload your motorcycle
- Do not tow a trailer
Of these items it is assumed items 1- 4 are understood.
Items 5 and 6 will be considered in detail shortly.
We now consider the hardness of the rubber as a factor in tire design. Tires made of harder rubber last longer, but they do not provide as much traction as softer rubber. It is an inverse relationship, harder rubber->poorer traction, softer rubber->greater traction. Tire design is a trade off between longevity and traction to produce an acceptable product. Other factors also affect traction including tread design but consideration will be restricted to rubber hardness as there are other aspect of rubber hardness to consider with respect to the topic under discussion.
Tire traction can be thought of primarily as a product of the frictional coefficient of sliding rubber. An object sitting on the ground will have frictional contact with the ground. If it is too heavy to pick up we may decide to push it. To start sliding the object we must exert enough force to overcome the frictional force of an object at rest. There are two frictional coefficients, one for an object at rest and another for the same object in motion. The frictional coefficient for an object in motion is less than that of an object at rest. Also, different substances have different frictional coefficients. How many folks performing this particular task have said, "if I can just get it moving?"
When a wheel breaks traction and skids, it is operating with the coefficient of an object already in motion. It has significantly less traction than before it started to skid, when its frictional coefficient was greater. It is important to consider that harder rubber has a lower frictional coefficient than softer rubber.
We now consider tire wear with respect to rubber hardness. Softer rubber wears faster than harder rubber. As a side issue, rubber hardness is controlled by the addition of oil. Softer rubber contains more oil in the formulation.
As previously stated. softer rubber wears faster than harder rubber in tire applications. Exactly how does this affect motorcycle tires and does it have significant affect with respect to the high degree of disparity between motorcycle tire and automobile tire wear? Dunlop suggests it does not, but central to previous discussions with some parties was the assertion that it played a very large part. This assertion claims that automobile tires in general are made from harder rubber than motorcycle tires and produce vastly greater longevity. This will be referred to as assertion 1. Other sources have stated in contradiction that automobile tires are in general made from softer rubber than motorcycle tires. Assertion 2 offers no explanation as to why automotive tires last longer than motorcycle tire. We will refer to this as assertion 2. Central to assertion 1 is the greater need for improved traction. Those who claim assertion 2 do not suggest a rational for that assertion.
We are left with the need to formulate an argument that proves assertion 1, assertion 2, or determines that neither assertion applies to motorcycle tire wear disparity. Our task is to consider the general case only. We arrive at a thesis statement:
In general, are car tires made with softer rubber than automobile tires and is it a significant factor with respect to disparity in motorcycle tire wear compared to automotive tire wear?
In an initial attempt to answer this questions a professional in a business that sells tires for a living was consulted, and employee of Budget Tire in Holt Michigan. He emphatically stated that automobile tires were made of softer rubber and that winter tires were made of the softest rubber.
This statement was rejected by the party asserting claim 1 with the suggestion that the tire professional was not qualified to answer the question.
Another tire professional was consulted at Discount Tire in Lansing Michingan. He stated that he did not know, but he made a phone call in an attempt to produce an answer. He was told that depended on the application. This query was inconclusive.
Other research revealed anecdotal testing on various internet forums including numerous comparison durometer readings of automobile and motorcycle tires. These tests revealed higher motorcycle readings in every case. Of course these would also be rejected by the party in question due to the fact that they were anecdotal in nature. They certainly were recorded though. It should be noted that there was only 2 attestations to assertion 1 found in all web research.
Finally we arrive at consideration with respect to tire wear that does not provide any actual hardness data. This analysis will consider the following factors influencing tire wear:
- Tire rubber hardness
- Tire width
- Contact patch
Force of Friction = frictional coefficient of material X Normal Force
While the frictional coefficient is a simple number, Force of Friction and Normal Force are vectors which are a number with a direction. For our purposes we can disregard direction. The Normal Force must be further calculated. It is the force gravity exerts on a mass, i.e. the mass X the force of gravity. The force of gravity is a constant number, 9.8 m/s^2.
This is usually the total mass resting on the contact patch of the tire. Softer rubber has a higher frictional coefficient and produces more friction under any load than a tire made of rubber with a lower frictional coefficient, i.e. harder rubber. Unfortunately the softer the tire rubber is, the more poorly it wears for any given tire design, load and pressure.Tire width is varied primarily for two reasons, for any given tire design and rubber hardness the tire may last longer and softer rubber may be used with wider tires than would be practical with narrower tires. There is a reason racing slicks are designed the way they are. They are made of very soft rubber that will provide great traction. Traction ultimately sets the limit of rate of acceleration. These tires need their width to provide side wall support to compensate for soft
rubber which is structurally weaker.
Wide tires tend to increase rolling resistance and the possibility of aquaplaning. Narrow tires on the other hand offer reduced rolling resistance and increased fuel efficiency. Also, the probability of aquaplaning is reduced.
We now consider the contact patch, that variable considered third in the Dunlop Motorcycle article discussing factors affecting motorcycle tire wear disparity in comparison to automobile tires. First consider the contact patch, that portion of a tire which is in direct contact with the road, typically extends across the entire width of the face of an automobile tire. With respect to a motorcycle tire the situation is vastly different. More narrow tires are more responsive and maneuverable. Tires with a wider contact are more durable. Motorcycle tires have been cleverly designed to provide a narrow contact patch when the tire is vertical and a wider contact patch when the tire is at an obtuse angle. This requires a curved working surface as opposed to a flat one and the contact patch is only a small portion of the width of this curved surface.
If one considers the case of a 150 mm wide automotive tire compared to a motorcycle tire of identical width, it would be obvious that the contact patch of the motorcycle tire will be of significantly smaller length. Virtually all of a 150 mm wide automotive tire will constitute the length of its contact patch while possibly as little as 50 mm of the width of a motorcycle tire will constitute the length of the contact patch.
A formula is frequently used to estimate the Surface Area (SA) of a contact patch:
SA = weight of car/pressure of tires
This formula implies several things, one of which is that increasing the width of the tires does not increase the surface area of the contact patch!
This implication suggests that decreases in wear experienced when tire width is increased are explicitly due to the increase in width. Let us examine this momentarily. If we increase the width of a tire we also increase the length of the contact patch. If the above formula is accurate the width of the contact patch has also been proportionately reduced. This is the only way to hold contact patch SA constant.
If increasing contact patch length decreases contact patch width it also reduces the time the contact patch is in contact with the road at any given speed, load and pressure! Reducing tire contact time with the road at any given speed should decrease tire wear.To complicate issues though, Avon Tire Company Conducted Comprehensive testing on tires including pressure, load and tire face displacement. One user analyzed this data and concluded the contact patch formula did not accurately predict contact patch SA with respect to changes in load and pressure. We have no good way of calculating contact patch width reduction for any given length increase. It is assumed the only way to accurately obtain the contact path SA is by direct measurement under each condition for which data is needed.
Making Sense Of It All
The purpose of this discussion was to consider various factors that could explain the wear disparity between automotive and motorcycle tires. With automobiles the front steering tires typically wear faster than the rear tires. If the front tires are also the driving tires they wear even faster. Rotating auto tires will normalize the wear between them.
Motorcycle tires can not be rotated. The rear tire, being the driving tire and the contact patch being small, is typically subject to much greater wear than the front tire. It Is not uncommon to replace three rear tires before the front tire must be replaced. 5,000 to 7,000 miles on a rear tire is typical and 20,000 before replacement of a front tire is good. With this consideration in mind, we compare front motorcycle tire life to automotive tire life and conclude that automotive tires last on the average at least 3X longer.
For any given tire design and operation conditions, wider tires last longer than narrower tires. Widening a tire may permit softer rubber to be used to improve traction while providing acceptable longevity.
Motorcycle tires are typically much more narrow than automotive tires. Further, their contact patch is even more greatly narrow than their width. Discounting the disparity of rear tire wear due to greater power to weight ratios and acceleration, it is obvious that motorcycle tires don't last as long.
Making the average motorcycle tire from harder rubber could reduce the disparity between the two classes of tires some what, but can motorcycle tire design keep traction levels high while using harder rubber?
Using an automotive tire on the rear wheel of a motorcycle can extend rear tire life to the same term as the front tire. In so doing, a much longer contact patch is gained. But we are not comparing apples to apples in more than one respect. Most motorcycle tires are still bias tires while the automotive tire that is stuck on the back is probably a radial tire. And you know what, we did not get a car tires worth of use out of that rear tire. It only lasted 20,000 miles, not 40,000 or 60,000 miles! This comparison clearly demonstrates the wear factors that power to weight ratio and acceleration present to the rear tire. If automobile tire hardness plays a factor in wear with respect to increasing rear tire longevity it is not obvious here.
As rubber becomes colder it loses elasticity. The frictional quality of rubber is also affected by temperature. As rubber becomes warmer its frictional properties increase. Harder rubber will experience great loss of frictional properties as it cools below a certain threshold. Softer rubber less so. Winter automotive tires are made from softer rubber than all weather tires and high mileage tires are made from harder rubber. An all weather tire would provide acceptable performance in cold weather but not nearly as good as a winter tire. A high mileage tire would not be good for winter use as the rubber would in general be so hard as to provide very poor traction. A winter tire would experience a fairly short life when used as a summer tire and it would provide poor fuel economy due to higher rolling resistance. Most folks use a general purpose all weather tire due to the expense of purchasing multiple sets of tires for their vehicle and also due to the storage space required to manage multiple sets of tires
A coworker mentioned during a conversation that his motorcycle tires did not work when the temperature dropped below 40 degrees Farenheit. He had a sports bike with wide hard tires designed to optimize longevity. This may actually represent the general case for this motorcycle tire application. If you were racing a bike of this type you would use a much softer tire on the track to optimize traction for cornering and acceleration. By comparison all weather automotive tires work acceptably at 40 degrees Fahrenheit and below.
With respect to cruiser application, the situation may be different. These bikes are typically heavier than sports motorcycles and the heaviest motorcycles of all are the fully dressed touring bikes. These can weigh over 900 pounds.While they are not as powerful as some of the sports motorcycles, they typically accelerate more quickly and in general have higher top end speeds than your average automobile. These motorcycles may be used in many different types of weather and require acceptable traction. This weather includes wet roads and possibly usage at temperatures below freezing. Under these conditions the rubber used in the tires can not be too hard. This author has on occasion operated a cruiser motorcycle on icy roads for short distances of 200 feet. It has been done with both a motorcycle tire on the rear wheel and a car tire on the rear wheel. In both cases the ability to maintain traction at slow speeds was impressive. The performance was superior to a rear or front wheel drive automobile in each situation with negligible tire slippage. This is a practice the author does not advocate and is quite complicated. It requires placing both feet on the ground and using them as ski runners, keeping the bike vertical and creeping out of the icy area at not more than 1 or 2 miles per hour. It is an unnerving process requiring much concentration. If the motorcycle is dropped under these conditions it may be impossible to pick it up due to the lack of traction the ice covered road offers!
The anwser to the questions of motorcycle tire rubber hardness compared to automotive tire rubber hardness can not be definitively answered but in all probability there is insignificant difference in these rubber hardnesses. It is also entirely possible that general use automotive tires are made with softer rubber.
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