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MOTORCYCLE TIRES

by Phillip Floria
Thanks to:
Dunlop Tires,
Brigstone Tires,
The U.S. Department of Transportation (DOT)
National Highway Traffic Safety Administration (NHTSA)
Motorcycle Maintenance by Mark Zimmerman

Like a lot of things in motorcycling, once you understand the basics; what looks baffling, becomes clear, and tires are a good example of this.

Think of a motorcycle leaned over at a 45 degree angle in a curve. It's putting exactly 1g of lateral force on its tires (and about 40% more weight.) We've seen motorcycle racers lean their bikes about 50 degrees in virtually every curve they encounter on the track.

Ever wonder why the motorcycle manufactures design their bikes with such low pegs making it almost impossible for you to lean farther than 45 degrees without ending up low-siding the bike.

The only thing keeping the tires from sliding out from under you is traction. Street legal tires, if warmed up (but not too hot), will lose traction when a lateral force of 1.1 g's, or less, is applied to them; cold tires or overly hot ones don't have even that much traction potential.

Racers use bikes that allow them to lean at least 50 degrees before any part of the bike other than the tires touch the ground. At a 50 degree lean angle their tires are contending with 1.2 g's of lateral force. Most street legal tires would have lost traction before that happened.

Motorcycle racing tires are built with special compounds designed to provide upwards of 1.3 g's of traction capability. [Formula One Race car tires can handle as much as 4.0 g's - yes, they can make a tighter turn than any motorcycle.]

When you hear your pegs screaming at you because they are dragging they're telling you that any more lean-angle and next you'll be testing the abrasion resistance of your riding gear.

The modern motorcycle tire is designed to do more, and handle different stresses than the tires on your car. The fact that they are black and round is about the only similarity they have. The sidewalls and tread faces are specially made to provide the stiffness and traction a modern motorcycle needs.

Motorcycling is all about traction control.

Today we have more brand choices in tires than ever before. Basically, motorcycle tires are constructed in two types: cross-ply and radial. There are some tire terms that need an explanation before we go on:

  • Beads are the stiff hoop that touches the rim when the tires are mounted.

  • Sidewalls are the strips around the side of the tire between the bead and the tread surface.

  • Plies or belts are stiffeners that are layered under the tread and sidewall during construction.

Cross-Ply Construction

Cross-ply or bias-belted tires are built of plies arranged diagonally, usually at a 45 degree angle, layered one on top of another, running in alternating directions. The plies are not interwoven. If they were the tire would be extremely stiff and the heat created by the tires flexing would destroy it in short order. For the same reason, the manufacturers try to use as few plies as possible. The more plies the tire has the stiffer it is and the more heat it generates while in use.

Tire traction depends to a large extent on the tires footprint, or contact patch - the area of the tire that is in contact with the road. Because the sidewall flexes, it allows the tire to flatten out slightly where it meets the highway. The softer the sidewall, the more flattening occurs. In general the larger the contact patch the more traction the rider will have, all other things being equal.

Thus a six ply tire will likely have a smaller contact patch and thus less traction than a two ply tire when loaded the same weight. But the six ply tire will be able to bear more weight than a two ply tire.

Radial Tires

Radial tires are built with there first two plies set at 90 degrees to the beads. Two reinforcing belts are ten installed under the tread area only. These reinforcing plies are set at a very shallow angle, somewhere near 20 degrees. This make for a very flexible sidewall, which allows for a much larger contact patch than that of an equivalent bias ply tire.

You may be tempted to fit a radial tire to motorcycles that originally came with bias belted tires. In a word, don't. Radial tires require a wider rim to accommodate their wider footprint. If you plan on fitting radials, you'll have to spring for the appropriate rims as well.

Tire Tread

The tread is the outer skin of the tire. It's composed of synthetic rubber which has been compounded to give a calculated compromise between long life and good traction. That compromise is necessary because soft compounds provide better grip at the expense of quick tire wear; harder compounds give longer wear at the expense of grip.

A sportbike will generally come with tires that provide lots of traction but wear out in as few as 3,000 miles. A touring bike generally comes with tires that provide less traction but last 10,000 miles or more.

In a perfect world the ideal tread pattern would be completely smooth, applying maximum amount of rubber to the road surface at all times. Road racing and drag racing motorcycles use smooth tires called slicks. But then they don't ride in the rain; at least not on slicks. In the rain the water gets trapped between the slicks surface and the road with no way to escape.

On a street tire the tread surface is covered in a network of grooves. These grooves are molded into the tire to create a channel for water to escape. The flexing action of the tire also helps pump the water through the tread blocks and out from under the tire. In addition the grooves help the tire run cooler by increasing air flow across the surface of the tire.

Tires and Heat

All tires have an optimum operating temperature. When they reach it the rubber compound is at its stickiest, and the carcass at it's most flexible. Below that temperature, grip is reduced and the tire may have a tendency to slide.

If the tire exceeds its optimum operating temperature things go wrong quickly. Excess heat causes the tire tread to degrade. As the adhesives used to bind the tread compound to percolate to the surface, the tire become very slippery. In severe cases the tire will actually shed chunks of tread.

To reduce heat buildup generated by flex modern tires are built of thin strong plies. Since the carcass runs cooler a softer compound can be used.

 
This image is looking from the top of the tire down CP is the contact patch of the front tire.

Changing tires yourself today can be a major task; because most modern motorcycle tires have a very strong bead. The inside edge of the tire that comes in contact with the rim is called the bead. The bead has steel wires (cable) imbedded in it to seat the tire onto the rim and requires a tool, known amazingly enough, as a bead-breaker, to free the tire's grip from the rim. Of course resourceful riders can come up with a way to break the bead; but be careful not to damage the rim.

There are ways to tell if a tire needs to be replaced. One of the most obvious is the nail sticking out of the tire or some other foreign object, which will let the precious air captured inside escape. Any holes, dry rotting or when the tread is even with the wear bars are reasons to replace a tire. All manufacturers specify a minimum amount of tread that needs to be left in the tread surface of a tire to be safe.

Wear Bars

You can locate the wear indicator by an arrow (note: depending on brand of tire, the arrow may be a little Michelin man as on Michelin tires, the Dunlop logo on Dunlop Tires, a viper head on Avon tires, etc.) on the sidewall or on the extreme edge of the tread area. Follow this arrow onto the tread and look for the raised portion in between the tread groves. If this raised portion becomes flush with the tread, it is time for a new tire.

Excessively worn tires are more susceptible to penetrations. Always replace tires from service before they reach the tread wear indicator bars (1/32 of an inch tread pattern depth remaining). Mixing new and worn tires result in deteriorated handling combinations and worn tires used in wet conditions can result in deteriorated handling.

Sizing and Pressure.

It should be a habit to look at your tires as you approach your motorcycle. Observe the tires for any indication of low air pressure and the condition of the tread surface.

A cheap tire pressure gauge is better than none. Check your tire's air pressure often, if you ride once a-week, do it before you start out. Always check the tire pressure while the tires are cool and follow the tire manufacturer specifications.

Try not to add air to an already hot tire. The air in the tire expands as the tire heats up. Adding air to an already hot tire will over-inflate it. And cause excessive wear in the center of the tire. Tires low on air will run at higher temperatures which can deteriorate the tire's compounds and reduce the tires life; besides degrading your motorcycle's handling. Running tires low on air will cause the outside edges to wear. Tires are not cheap, keeping them properly inflated will make them work better, handle safely and last their intended life.

Tires don't last forever. Front tires seem to last twice as long as rear tires do on motorcycles tread wise. But, exposure to ultra violet light (sun light) attacks the rubber compound. Heat, cold, and use harden the rubber compounds too. The tire may look good but if it is over three years old it should be replaced.

Sometimes it seems as if there should be a codebook just to make sense of all the information on a tire's sidewall. There are three different tire sizing codes in use, although the metric code is becoming more popular today. The inch and alpha codes are still used to identify tires. Using rear tire off a Honda CBR600F4i Bridgestone BT-010 as an example, here's what some of the markings represent. For inch and Alpha tire codes use the conversion charts provided HERE.

The most important label is the sizing designation, which in this case reads "120/70ZR17." This label indicates section width (120mm), aspect ratio (70 percent), speed rating (Z), construction (R), and wheel diameter (17 inches).

This tire has a speed rating of "Z" or more than 149mph, and is a radial (R) construction. Following the size label is an additional marking defining the load rating for the tire. In this case, "58W" indicates the total load carrying capacity is 520 pounds.

This tire is a slight variant of the standard BT-010. While the first "F" denotes this as a front tire, the second "F" indicates the OEM variant. These model-specific tires are subtly modified from the standard units as requested by a specific manufacturer, whether for lighter weight, better mileage, or even a slightly different tread pattern. OEM tires are available through your dealer using the manufacturer's part number.

Also marked on the side of each tire is the number and type of plies in the tread and sidewall construction. For our BT-010, two rayon plies run across the tire from bead to bead, while two Aramid and two rayon plies make up the tread's belt package. Note that this particular BT-010 from the CBR differs in construction from the standard Bridgestones and has more plies in both the tread and sidewall.

Tires must also match the width of the wheel rim. Tire sizes are matched to rim sizes for optimum profile; the shape the properly inflated tire takes when fitted to that rim. You may have heard riders talking about preferring the behavior of a smaller tire (like a 180/55) compared to a 190/50; it's because of its profile on, say, a 6-inch rim. The smaller tire assumes a rounder cross section, offering better turn-in and a bigger contact patch while leaned over.

Correct rim width may be crucial to handling and stability. A tire that is installed on a rim wider than recommended will have a flattened profile, and a rider may easily reach the edge of the tread during cornering. A narrow rim will alter the tire profile, concentrating tire wear in a very small area during cornering, with a smaller contact patch during braking.

There will always be an optimum recommended tire for any given rim, and fitting a bigger tire because you think you'll have more rubber on the road can often mean a trade-off in handling, so unless it's a highly recommended change that improves the handling and road holding, it's not a good idea.

Born on Date

When it comes to determining the age of a tire, it is easy to identify when a tire was manufactured by reading its Tire Identification Code (serial number). Unlike vehicle identification numbers (VINs) and the serial numbers used on many other consumer goods (which identify one specific item), Tire Identification Codes are really batch codes that identify which week and year the tire was produced.

The U.S. Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) requires that Tire Identification Code be a combination of eleven or twelve letters and numbers that identify the manufacturing location, tire size, manufacturer's code, and week and year the tire was manufactured.

Today, the week and year the tire was manufactured is contained in the last four digits of the serial number, with the 2 digits used to identify the week a tire was manufactured immediately preceding the 2 digits used to identify the year.

Examples of tires manufactured since 2000 with this Tire Identification Code format:

XXXXXXXX 0600

 

XXXXXXXX 0600

06 - Manufactured during the 06th week of the year

XXXXXXXX 0600

Manufactured during 2000

And

XXXXXXXX 0604

 

XXXXXXXX 0604

06 - Manufactured during the 06th week of the year

XXXXXXXX 0604

04 - Manufactured during 2004

The Tire Identification Code for tires produced prior to 2000 was based on the assumption that no tire would be in service for ten years. They were required to provided the same information, with the week and year the tire was built contained in the last three digits. The 2 digits used to identify the week a tire was manufactured immediately preceded a single digit used to identify the year.

The first two digits in the Tire Identification Code will indicate the manufacturer and where it was made. See list by clicking here.

 

Remember, correct matching of front and rear tires is important to obtain optimum performance and handling. Follow the Tire Selection guidelines.

  • Mount only tires marked "front wheel" on front and only tires marked "rear wheel" on the rear.

  • A new front tire with a worn rear tire can cause instability

  • Mixing radials, brands, or mixing radials with bias or belted bias tires may adversely affect handling and stability.

Dunlop recommends fitting Sportmax, Sportmax II D204, Sportmax touring D205 and Dunlop D207 high-performance radials in pairs.

It should be noted that there are many factors other than tire incompatibility can affect the handling of a motorcycle, including the weight and height of the rider, mixing worn with new tires, and the addition of luggage or fairings. Consult the motorcycle manufacturer before making modifications to your motorcycle.

Tires offering different load-carrying capacities are available. Consider carefully the weight of the motorcycle, the weight of any optional equipment and whether it will carry passengers. The load- carrying capability of the tires is reduced by under-inflation. It is possible to overload a tire even though it is the correct size specified by the manufacturer. Maximum loads and corresponding pressures are indicated on the sidewalls of all street tires.

  • Never exceed the accessory restrictions and vehicle load capacity found in the motorcycle owner's manual, or the maximum load molded on the tire sidewall. Before a trip, be sure to determine the total weight of luggage, equipment, and rider(s) to be added to the motorcycle.

  • Trailers may contribute to motorcycle instability, grossly exaggerated tire stresses and overload. Such stresses and overload can cause irreversible damage resulting in sudden tire failure and accident.

The next things to look at are compounds. A compound is the blend of rubber used for the tread. A soft compound will generally offer more grip, but wear faster than a harder compound. Some tires are now made with a dual compound using a harder band in the center, where acceleration forces can quickly wear a tire out. And a softer compound for the edges where the tire spends less of its time and where a softer compound offers more grip when the bike is leaned over.

A tire, which appears to be cupping on the sides, can indicate several things, on front tires, worn wheel bearings may be the cause or some other suspension problem. Most cases it's the tires age. Because the tires are made from different compounds of rubber for the center and the edges, as the harder outer rubber wears away the softer rubber becomes exposed and wears much faster. Have your front end checked, and replace the old tire.

Tire mounting

The balance dot on the bead or sidewall area indicates the lightest point of the tire. Tires should be installed with these balance dots at the valve. Street tires also have arrows on the sidewall, which indicate the correct direction of rotation. (Positioning of balance marks and inclusion of directional arrows are not universal among motorcycle tire manufacturers.)

Be sure to align the wheels each time the rear wheel is removed or the chain or belt is adjusted. Each revolution of an incorrectly aligned wheel can scuff off tread rubber, reduce tire mileage, and impair steering and cornering.

It is essential tire/wheel assemblies be balanced before use and rebalanced each time the tire is removed or replaced. Unbalanced tire/wheel assemblies can vibrate at certain speeds, and tire wear will be greatly accelerated.

Wheels may be balanced with spoke nipple weights, lead wire or self-adhesive rim weights. Consult the motorcycle manufacturer for approved wheel weights.

When new tires replace worn out ones (or a differently patterned or constructed tire is used to replace your old tires), they will not react the same. When new tires are fitted, they should not be subjected to maximum power, abrupt lean-angles or hard cornering until you've ridden a reasonable run-in distance of approximately 100 miles. This will permit you the rider, to become accustomed to the feel of the new tires or tire combination, and achieve optimum road grip for a range of speeds, acceleration and handling.

Check and adjust inflation pressure to recommended levels after tire cools for at least three (3) hours following run-in. Remember, new tires will have a very different feel from their increased traction. New tires, mixing a new tire with a worn older tire, and mixing different pattern combinations require careful ride evaluation.

Tire plugging and repair

OK you ride out of the shop take a short hop to a favorite place to eat, only to come out and find your new tire is flat as the pancakes you just finished. What now? Well you really only have two options, replace the tire or; plug or patch it. So with some help here is some information to help you make the decision.

Experts recommend only permanent repairs performed from the inside of the tire, using a combination patch/plug method. Never attempt a repair from the outside, or inject a sealant, or simply use an inner-tube, a patch or a plug as a substitute for a proper repair.

Only a qualified tire repair shop or motorcycle tire dealer should perform repairs. Inspection of the tire and adequacy of repair becomes the responsibility of the person actually performing the repair and most tire manufacturers do not warrant the results of a repair in any way.

Combination patch/plug repair kits for use by the repair shop or dealer are available with instructions from companies such as:

  • 1. Remarco Inc. 200 Paris Ave. Northvale, NJ 07647 (201) 768-8100

  • 2. Technical Rubber Co. P.O. Box 486 Johnstown, OH 43031 (740) 967-9015

  • 3. Tip-Top/Moto Combi Kit

  • 4. Tech Uni-SealŪ Repair Kit (Also has been marketed by Honda, Kawasaki, and Yamaha with their own part numbers.)

NOTE: There may be suitable repair kits and materials provided by manufacturers other than those listed above.

Before any repair should be attempted, however, a tire must be removed from the wheel and thoroughly inspected. The following are minimum guidelines for the repairer:

Tires should not be repaired if any of the following conditions exist:

  • 1. A tire has been previously injected with a sealant/balancer.

  • 2. The puncture is larger than 6mm (1/4") in diameter.

  • 3. The puncture is not perpendicular to the carcass.

  • 4. The puncture is in the tire sidewall.

  • 5. Separation of plies, tread separation, separation of any other components.

  • 6. Cut or broken ply cords.

  • 7. Broken or damaged bead wires.

  • 8. Cut or damaged chafers (bead area).

  • 9. Deterioration of the carcass inside the tire due to "run flat" or underinflation.

  • 10. Cracks or other damage to the integrity of the inner liner.

  • 11. Excessive wear - tire should have at least 1/32 of an inch of tread depth, excluding tread wear indicators.

  • 12. Cracks in sidewall or tread.

  • 13. Impact breaks, cuts, snags or gouges that penetrate the surface.

NOTE:

  • A. There should be no more than one repair in any quarter of the tire and no more than two repairs per tire.

  • B. The wheel itself must be in good condition. Any cracked or bent wheel, however slightly, may allow the loss of air and cause subsequent deflation of the tire.

  • C. Following repair, the valve assembly should be replaced and the tire/wheel rebalanced.

Speed should not exceed 50 mph for the first 24 hours after tire repair and the repaired tire should never be used at speeds over 80 mph. The repairer is solely responsible for instructing the motorcyclist as to the restrictions to be placed on tire use following repair. In summary, NO form of temporary repair should be attempted. Motorcycle tire repairs leave no room for error and any doubt as to inspection or adequacy of repair should be resolved by discarding the tire.

NEVER ATTEMPT TO REPAIR A DAMAGED TIRE WITHOUT THE AIDE OF AN EXPERIENCED TIRE MECHANIC.

Tire Mounting

Danger: Only specially trained persons should mount tires. Improper mounting can cause tire explosion and serious injury.

Follow these mounting precautions:

  • Wear approved eye protection.

  • Clean and lubricate beads and rim.

  • Centralize rim band and tube to prevent pinching if tube-type rim. *Note directional arrows on sidewall where applicable.

  • Lock assembly on mounting machine or place in safety cage before inflating to seat beads.

  • Set air hose relief valve at 40 psi.

  • Use extension gauge and hose with clip-on air chuck. Stand back with no part of your body within the perimeter of the assembled tire and rim.

  • Inflate with core in valve stem.

  • Never inflate above 40 psi to seat beads.*

  • Spin wheel to check bead seating and alignment.

  • *If the beads do not seat by 40 psi, deflate and repeat above procedures. Never use a volatile substance or rubber "donut" to aid bead seating. If the tire is a tube-type, deflate and reinflate after seating to prevent tube wrinkles.

  • For 15-inch motorcycle replacement tires, never mount on a 15-inch diameter passenger car tire rim. Passenger car and motorcycle rims actually differ in diameter.

  • Never mount passenger car tires on motorcycle rims.