Rubber Overview

The Early Days

Rubber was being used by South American natives at the time the Spanish explorers made their way to the New World in the 1600's. By the 1700's there are reports that the native culture had been using clay molds to cast rubber waterproof bottles for storing liquids. Natural rubber is derived chiefly from the "Hevea braziliensis" tree, though a number of other trees produce a sap capable of making rubber compounds from, including the guayule, a shrub which grows exuberantly in America's south-west states. Rubber got it's name from the English chemist Joseph Priestly who found that the crude gum rubber was capable of "rubbing out" pencil marks, coining the name "rubber" for his new found erasing process. The tree has diagonal cuts made on it's trunk, which exude a milky white tree sap known as Latex. The latex sap has a number of components to it, there are proteins, water soluble material, acetone soluble material, esthers insoluble in acetone, and mostly, about 92%, rubber hydrocarbon. The rubber molecule has 5 Carbon atoms and 8 Hydrogen atoms (C5H8)n suspended within other compounds that give its noncrystalline matrix form. How trees synthesize the rubber hydrocarbon isn't known.

The Birth of Synthetic Rubber

Raw natural rubber (called NR in the industry) softens and stretches with heat, while it hardens and contracts in cold, which wouldn't be useful for tires and tubes. What the bicycle and tire industry use is a synthesized version of rubber called "butyl rubber" for inner tubes and "Buna" rubber for tires. The first synthetic rubber was developed by Grenville Williams, in England in 1860. Using heat he changed rubber from a solid to a liquid, calling the result "isoprene". By 1887 chemists had discovered a way to turn it back into a rubber-like substance. The rubber shortage that developed during World War II caused world governments to look for a natural rubber substitute. Butyl rubber was the result of the U.S. team of "stretch detectives". In 1940, the US team, announced GR-I which we now call butyl rubber. It's made by forcing molecules to combine (polymerizing), making much larger more complex molecules (creating a polymer) of isobutylene from petroleum with a small amount of isoprene at a temperature of -150 degrees F (-100 degrees C). Butyl rubber is outstanding for inner tube purposes. Because of its low permeability to gases, it retains air 10 times longer than natural rubber, which is why some makers of latex tubes line the inside of the tube with butyl rubber. In 1935 German researchers announced a product they called Buna synthetic rubber. Buna's name was derived from the first two letters of its chief ingredients "butadiene" and "natrium" (which is used as the catalyst in polymerization). Two types of Buna rubber were developed Buna N and Buna S. Buna S is made of Butadiene and styrene. The US research team designated to this compound GR-S, and was used for general purpose rubber applications including tires during WW II. The GR-S compound is now referred in the industry as "SBR" for Styrene Butadiene Rubber and it's what bicycle tires are made from. SBR has very similar properties to natural rubber. Though not oil resistant, and has a poor chemical resistance, it has excellent abrasion and impact resistance. SBR rubber in its liquid form still must under go more manufacturing steps. Several materials, fillers, reinforcing agents, antioxidants and pigments are compounded with it before final processing or vulcanization.

Hi-Tech Rubber

In the case of SBR the filler and reinforcing agents amount to largely one substance. Carbon black is used by everyone. Carbon black is any chiefly or wholly carbon substance, like Lamp black, MT black, HAF black or Coal Black. Carbon black adds strength, stiffness and resistance to wear. More carbon black makes the rubber harder, less makes it softer and thicker. The finer and rounder the particle of carbon black, the greater the strength and stiffness it provides to the rubber. Molecular cross-links are established between the carbon and rubber molecules providing the reinforcing and tear resistance quality rather like the crystalline structure of metal alloys. Adding more carbon black to the rubber compound increases its tensile strength up to a maximum point where the addition of more fails to bring further strength. What determines this maximum point for each rubber type is unknown. The tread stock of automobile and bicycle tires use between about 45 parts of carbon black to 100 parts SBS rubber. Exposure to oxygen, heat, light, ultra-violet rays and ozone damage rubber so a number of specific antioxidant additives have been developed to help give the rubber longer life. As a protective agent against UV damage tire makers use additives that inhibit, but don't prevent UV damage, which act as light absorbers such as Tinuvin P and the 2-hydroxybenzophenones. At time of vulcanization the zinc salt of mercaptobenzimidazole (ZMBI) is introduced as an ingredient to increase the resistance to heat degradation. In the same process, to inhibit the effect of sunlight, 2% of nickel dibutyldithiocarbamate may be used. Atmospheric oxygen attacks rubber slowly, which causes it to become hard and brittle, so relatively insoluble waxes are incorporated into the tire compound which migrates or blooms to the rubber surface, protecting it until the wax surface is broken. The incorporated wax produces a whitish tinge on a new tire when it is rolled tightly between the fingers or possibly pinched.

Ozone Matters

By far ozone exposure, is the most damaging to rubber, even the small amounts that are normally present in the atmosphere (a few parts per hundred million), which are sufficient to cause severe cracking within a few weeks or months. Its damage raises considerably with higher concentration, at higher altitudes, or proximity to ozone emitting electrical devices. Ozone molecules act like a chemical scissors which cut through the mass of cross-linked molecules that otherwise impede cracking gaining the energy required to break the molecules almost wholly from direct chemical interaction. Ozone has no visible effect on unstretched rubber, but microscopic examinations have demonstrated that a reaction does occur at the surface. Inflating the tire, stretches the rubber. With just a 5% tensile strain, cracks form on the surface and grow perpendicular to the direction of the stretching, growing greater in number the tighter the stretching. This is because the molecular movement induced by the stretching with surface ozone damage leads to rupture of the surface layer permitting further ozone penetration and reaction on a repeated basis until cracks result. In practice, many tire makers rely on the same waxes used to impede oxygenation to prevent ozone damage, which makes a "tough transparent coating", but the wax is short lived and so is its protection. However, two classes of protective chemicals exist, known as "antiozonants", one type reduces the rate of crack growth, the other increases the amount of energy needed for crack growth to occur. They function by forming an intermediate molecular structure that requires a greater ozone concentration to complete the rubber's degradation.

Tire Construction

Tires are made using what is known as the flat band method. The foundation of a tire is it's casing. The casing is made of a flat strip of rubberized cord fabric which are cut on the bias, meaning the strip is cut so the fabric's cord moves diagonally left to right. The cord may be cotton, nylon or another fiber. This strip of rubberized fabric is applied to a collapsible drum. On each side of this strip on the drum, a "bead" which is a hoop of steel wire is placed and the rubber fabric is folded over the beads. Because the strip of fabric was cut at a diagonal, when it's folded over the bead each of the two upper layers has the cord running at right angles to the previous layer adding strength and torsional rigidity to the casing structure. The bead keeps the finished tire on the rim. Each of the now three layers of fabric is referred to as a "ply", because the fabric is cut on the bias, it is also known as a "bias ply".

Bike Tire Construction

Bicycle tires are usually made just this, way with only three fabric layers, but there are of few makers, with some tire models that have one or two extra layers on top of the three plies. These extra rubberized fabric strips called "belts" are only as wide as the upper tread part of the tire known as the "crown". The added fabric of the belts makes them more resistant to failure or punctures. Chafing strips are placed over the outer side of the bead for protection against chafing wear against the rim. What will be the tread and sidewall are made as a separate round yet flat piece, which is now set over the plies and belts. The separate layers of the tire are rolled under pressure to produce the near finished tire. The drum is collapsed and the tire is removed. It's taken to an expanding machine that takes this flat round object and forms it the tires round hollow curved-like shape. They are then put inside a steel mold, with the interior engraved with a negative relief of the tires finished tread, and side wall. An air bag is expanded inside the tire casing to provide outward pressure on the tire during vulcanization. Air, steam, hot water or various combinations are introduced to the bag to heat the tire carcass and make the rubber soft and flow into all the areas of the tire mold. The vulcanization process involves the heating of the mold, through tubes that pass through it permitting circulation of the steam, hot water or hot air through the mold. The heat and pressure in the mold during this vulcanization process set and fix the rubber elements of the tire to one another and stabilize the rubber itself against deterioration and the effects of heat and cold found in raw rubber.

Tires - An Overview

There are several parts to a tire, let's define our terms. The "crown" is the uppermost part of the tread which includes all the tread and tread blocks that come in contact with the ground. The "tread" includes the uppermost crown and the rubber of added thickness that extends part way down the sides. From the end of the tread on the sides to the chafing strip is the "sidewall". On mountain tires the raised rubber knobs on the tread are called tread blocks. For uniform measurement purposes all the mountain tire measurements are taken with the tire mounted on a Mavic 231 rim with the tire inflated to exactly 40 PSI (2.4 BAP). The road tire measurements are taken with the tire mounted on a Mavic Open 4 CD rim with the tire inflated to their nominal pressure range. These measurements are taken with the tire "unloaded" meaning there is no weight on the tire. The measurements we will give you are section height, section width, and crown width. "Section height" is the measurement from the top of the chafing strip mounted in the rim to the uppermost tread surface. "Section width" is the measurement is the outer dimension from one sidewall to the other, the maximum width, in cross-section, of the tire including side tread blocks and decorations if any. Section width is useful in determining whether the new wider tires will even fit within your fork blades or chainstays. There are cases now where some tires are too wide to fit usefully on some bikes. "Crown width" is the distance across the actual tread that comes in contact with the ground. We measured the depth of the tread using a Michelin metric tread depth gauge. This will let you know how tall the tread blocks are, or how deep the tread is, to give a sense of useful life of the tire. In the case of tire sipings it will give you a sense volumetrically of how much air of water it will contain. Several tires are being marketed with soft or hard rubber properties for use in specific riding conditions. The tires for riding on hard ground conditions are said to be made using soft rubber so there is greater stickiness in the tire's traction with the ground. The tires for riding on soft conditions (sand, loose dirt, small rocks) are said to be made of stiffer or harder rubber so the tire will penetrate through the loose material, trying to get a grip by volume on the loose material. Remember, the chief component in tire hardness is the amount of carbon black in it's composition.

Current Trade Trends

One of the latest buzz words in recent mountain bike tire ads is "low density" used in describing a tire as being made of soft rubber. Not wanting to take any ads too seriously, we have chosen to scientifically measure each tires hardness. To measure the hardness of rubber compounds an instrument called a "Durometer" is used. The Durometer is the tire industry tool for hardness measurement. Our Durometer is the Pacific Transducer Corporation's 408L. It is certified to meet or exceed the current ASTM D2240 specifications for "A" type durometric measurements in the 10/A to 90/A range. The hardness test on rubber reflects such qualities as resilience, durability, uniformity, tensile strength, and abrasion resistance. A 10/A reading is very soft and a 90/A reading is very hard. We found none of the tires we tested had a hardness below 58/A, while none had a hardness above 72/A. The Durometer uses a highly accurate spring to measure how deep a blunted needle can penetrate into the rubber or polymer substance. The amount of pressure is measured as "load grams". The lowest 10/A reading requires 130 load grams, while a 45/A requires 400 load grams and the highest for our instrument 90/A requires 746 load grams. As the reading gets higher the amount of added force or pressure decreases relative to the increase in the instrument reading. For our tire range the 58/A bottom requires 500 load grams and the 72/A high requires 600 load grams which is a fairly narrow range of hardness in which to measure the entire genus of tires. The results we print of this test are the average of 5 separate tests. Though the size of the tread blocks varies by tire, this isn't a real factor for the test, and we objectively performed them to the best of our ability in a controlled temperature environment.

Materials of Choice

One of the considerations in the quality of a tire is the material used for the fabric in the tire casing plies. Nylon for example, is considered more durable than cotton. Very few manufacturers volunteer the fabric type, because of this, in many cases, we can't report it. The other factor the number of threads per inch of the fabric. The higher the thread count, generally the better the tire casing and thereby the tire. Thread count is expressed "60 TPI" meaning 60 threads per inch. Again many manufacturers don't volunteer the thread count, so we can't report it in every case. Tire beads come in two types, those made of steel and those made of Kevlar. The steel or "wire" bead tire is the more traditional, less expensive and weighs slightly more than the Kevlar beaded tires. Kevlar has tensile strength 7 times greater than steel for the same weight. Kevlar beads were originally introduced as a "foldable" tire because the Kevlar bead is so flexible the tire could be folded or rolled and carried on a ride in case of catastrophic tire failure. Wire bead tires can be twisted, with no damage, into thirds so they can also be carried on a ride, maybe a little less conveniently. Recently, Kevlar beaded tires have become more popular because they weigh less. Some of the greatest improvement in the rider's "sense of feel" and agility on their bike comes from reducing the outer spinning mass of their wheel. This includes the tube, tire, rimstrip and rim. It takes more effort to turn a wheel in motion with a heavier outer mass than a lighter one. Those of you who have ever played with a gyroscope know that the centrifugal force of the moving flywheel holds it in a standing, stable, rotating orbit, and takes some effort to turn the gyroscope away from its standing rotation. A bicycle wheel functions like a gyroscope on its side.

Treads and Sidewalls

To contrast the above remark, we should now talk about mounted tire widths on mountain bikes. The front wheel is used to turn, while the rear wheel follows where the front has been. The custom for mountain riders is to put a wider tire on the front wheel and a narrower one on the rear. A wider front tire with any side tread, permits surface traction for turns and getting through ruts. A wider front tire also contains a greater volume of air to cushion landings which occur mostly on the front tire. The narrow rear tire is the drive tire and added width there will only increase the rolling resistance of the tire because the bigger tire patch will introduce more friction. Tire sidewalls come in three types, Skin sidewall, Black sidewall, or Gum sidewall. Sidewalls on tires have some benefits, because they prevent sun damage of the tire casing underneath. They also add some rigidity to the tire structure by adding torsional support across the tire. Sidewalls could also prevent the majority of mountain bike flats which are generally "snake bite" flats caused by the rim incising the tube through the tire casing on hard landings. Sidewalls add thickness and depth to help prevent this type of "pinch" flat. What they gain in structure they take back in suppleness and the tire has a more difficult time conforming to new shapes. However the best minds in the industry seem to agree that a sidewall for mountain bike riding is highly desirable. Skin sidewalls are really the lack of a sidewall. They are made by just eliminating the sidewall part of the tread. Skin sidewalls are the two rubberized fabric plies of the tire casing left exposed and are a light brown "skin" color, hence the name. Gum sidewalls are used only occasionally on after market tires. The rubber is pigmented to a light brown color to infer the use of a softer rubber compound. Black sidewalls are made by extending the width of the tread stock so it protectively encloses the casing to the chafing strip

Sales and Scams

Important, new scam, very few mountain bike tires are made with a real Black sidewall. Historically the bias cut casing fabric was rubberized with a light brown tinted rubber which is why Skin side tires are "skin" colored. The current practice in making "black sidewalls" is to rubberize the casing fabric with Black tinted rubber so in essence you have an exposed tire casing that's Black in color being called a "Black sidewall". In almost every instance of mountain bike tires having a "Black sidewall" we've found it's really exposed Black colored tire casing and the gain from an actual sidewall doesn't exist. We'll tell you in each case where this has been done. Opinions about tread and tread block design are like body organs, everybody has a least one. While every tire maker and tread designer has a Promethean theory or explanation of why their tread block design is life giving, there are always a certain number of riders who find or believe the designer's work to range from uncreative all the way up to dangerous. Rather than get involved in a shoot-out of theory which will alienate readers, or "experts" we will describe each specific tread in the body of our work for you to evaluate against the type of riding you do. Remember, whatever characteristics the tread is said to have, disappear the moment anything foreign, like mud, sticks to it, because its shape and traction depth changes. Some tires have "sipes". Sipes are fine channels which are indented into a tire tread or into the top of a tread block. The purpose of siping is to allow a channel for water under the tire to be squeezed out from under it. The thinking is this reduces aquaplaning, and evacuates the water to help maintain a larger tire patch, or surface contact. The tires chosen for review are based on customer interest, with a known rate of sale. As always the published weights are from actual production tires, accurate to the 1/2 gram. Specifics about each tire are found in the tables. As a rule Japanese tires are of better quality than those made in Taiwan.

Good Advice !

Lastly, we haven't found another bicycle part category with ads and ad material filled with such a gross amount of hype, crappy information (or none at all), and just misleading Bull. Remember all those champion riders in ads aren't given just parts to ride with for free. Many, many of the more winning riders are paid in 5 figures per manufacturer to have their name linked in print, and use the makers parts because they are paid handsomely to do so. Within the industry there are many stories of riders who use other than their sponsors equipment because they don't want to lose, but you'll never see these stories in print. The minute your inclined blindly believe, without your own research, any tire maker or marketing organization's ads ... be careful to not bend over for the soap.

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