[~JC~]

How Tires Work

If you're in the market for new tires, all of the variables in tire specifications and the confusing jargon you might hear from tire salesmen or "experts" might make your purchase rather stressful. Or maybe you just want to fully understand the tires you already have, the concepts at work, the significance of all of those sidewall markings. What does all this stuff mean in regular terms?

In this article, we will explore how tires are built and see what's in a tire. We'll find out what all the numbers and markings on the sidewall of a tire mean, and we'll decipher some of that tire jargon. By the end of this article, you'll understand how a tire supports your car, and you'll know why heat can build up in your tires, especially if the pressure is low. You'll also be able to adjust your tire pressure correctly and diagnose some common tire problems!

How Tires are Made

The Bead Bundle
The bead is a loop of high-strength steel cable coated with rubber. It gives the tire the strength it needs to stay seated on the wheel rim and to handle the forces applied by tire mounting machines when the tires are installed on rims.

The Body
The body is made up of several layers of different fabrics, called plies. The most common ply fabric is polyester cord. The cords in a radial tire run perpendicular to the tread. Some older tires used diagonal bias tires, tires in which the fabric ran at an angle to the tread. The plies are coated with rubber to help them bond with the other components and to seal in the air.

A tire's strength is often described by the number of plies it has. Most car tires have two body plies. By comparison, large commercial jetliners often have tires with 30 or more plies.

The Belts
In steel-belted radial tires, belts made from steel are used to reinforce the area under the tread. These belts provide puncture resistance and help the tire stay flat so that it makes the best contact with the road.

Cap Plies
Some tires have cap plies, an extra layer or two of polyester fabric to help hold everything in place. These cap plies are not found on all tires; they are mostly used on tires with higher speed ratings to help all the components stay in place at high speeds.

The Sidewall
The sidewall provides lateral stability for the tire, protects the body plies and helps keep the air from escaping. It may contain additional components to help increase the lateral stability.

The Tread
The tread is made from a mixture of many different kinds of natural and synthetic rubbers. The tread and the sidewalls are extruded and cut to length. The tread is just smooth rubber at this point; it does not have the tread patterns that give the tire traction.

Assembly
All of these components are assembled in the tire-building machine. This machine ensures that all of the components are in the correct location and then forms the tire into a shape and size fairly close to its finished dimensions.

At this point the tire has all of its pieces, but it's not held together very tightly, and it doesn't have any markings or tread patterns. This is called a green tire. The next step is to run the tire into a curing machine, which functions something like a waffle iron, molding in all of the markings and traction patterns. The heat also bonds all of the tire's components together. This is called vulcanizing. After a few finishing and inspection procedures, the tire is finished.


What All the Numbers Mean
Each section of small print on a tire's sidewall means something:

Tire Type
The P designates that the tire is a passenger vehicle tire. Some other designations are LT for light truck, and T for temporary, or spare tires.

Tire Width
The 235 is the width of the tire in millimeters (mm), measured from sidewall to sidewall. Since this measure is affected by the width of the rim, the measurement is for the tire when it is on its intended rim size.

Aspect Ratio
This number tells you the height of the tire, from the bead to the top of the tread. This is described as a percentage of the tire width. In our example, the aspect ratio is 75, so the tire's height is 75 percent of its width, or 176.25 mm ( .75 x 235 = 176.25 mm, or 6.94 in). The smaller the aspect ratio, the wider the tire in relation to its height.

High performance tires usually have a lower aspect ratio than other tires. This is because tires with a lower aspect ratio provide better lateral stability. When a car goes around a turn lateral forces are generated and the tire must resist these forces. Tires with a lower profile have shorter, stiffer sidewalls so they resist cornering forces better.

Tire Construction
The R designates that the tire was made using radial construction. This is the most common type of tire construction. Older tires were made using diagonal bias (D) or bias belted (B) construction. A separate note indicates how many plies make up the sidewall of the tire and the tread.

Rim Diameter
This number specifies, in inches, the wheel rim diameter the tire is designed for.

Uniform Tire Quality Grading
Passenger car tires also have a grade on them as part of the uniform tire quality grading (UTQG) system. You can check the UTQG rating for your tires on this page maintained by the U.S. National Highway Traffic Safety Administration (NHTSA). Your tire's UTQG rating tells you three things:


Tread Wear: This number comes from testing the tire in controlled conditions on a government test track. The higher the number, the longer you can expect the tread to last. Since no one will drive his or her car on exactly the same surfaces and at the same speeds as the government test track, the number is not an accurate indicator of how long your tread will actually last. It's a good relative measure, however: You can expect a tire with a larger number to last longer than one with a smaller number.
Traction: Tire traction is rated AA, A, B or C, with AA at the top of the scale. This rating is based on the tire's ability to stop a car on wet concrete and asphalt. It does not indicate the tire's cornering ability. According to this NHTSA page, the Firestone Wilderness AT and Radial ATX II tires that have been in the news have a traction rating of B.
Temperature: The tire temperature ratings are A, B or C. The rating is a measure of how well the tire dissipates heat and how well it handles the buildup of heat. The temperature grade applies to a properly inflated tire that is not overloaded. Underinflation, overloading or excessive speed can lead to more heat buildup. Excessive heat buildup can cause tires to wear out faster, or could even lead to tire failure. According to this NHTSA page, the Firestone Wilderness AT and Radial ATX II tires have a temperature rating of C.

Service Description
The service description consists of two things:

Load Ratings: The load rating is a number that correlates to the maximum rated load for that tire. A higher number indicates that the tire has a higher load capacity. The rating "105," for example, corresponds to a load capacity of 2039 pounds (924.87 kg). A separate note on the tire indicates the load rating at a given inflation pressure.
Speed Rating: The letter that follows the load rating indicates the maximum speed allowable for this tire (as long as the weight is at or below the rated load). For instance, S indicates that the tire can handle speeds up to 112 mph (180.246 kph). See the chart on this page for all the ratings.
Calculating the Tire Diameter
Now that we know what these numbers mean, we can calculate the overall diameter of a tire. We multiply the tire width by the aspect ratio to get the height of the tire.


Tire height = 235 x 75 percent = 176.25 mm (6.94 in)
Then we add twice the tire height to the rim diameter.


2 x 6.94 in + 15 inches = 28.9 in (733.8 mm)
This is the unloaded diameter; as soon as any weight is put on the tire, the diameter will decrease.


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Traction
There are a lot of different terms used today in the tire industry. Some of them actually mean something and some do not. In this section, we'll try to explain what some of the terms mean.
All-Season Tires with Mud and Snow Designation
If a tire has MS, M+S, M/S or M&S on it, then it meets the Rubber Manufacturers Association (RMA) guidelines for a mud and snow tire. For a tire to receive the Mud and Snow designation, it must meet these geometric requirements (taken from the bulletin "RMA Snow Tire Definitions for Passenger and Light Truck (LT) Tires"):

1. New tire treads shall have multiple pockets or slots in at least one tread edge that meet the following dimensional requirements based on mold dimensions:
a. Extend toward the tread center at least 1/2 inch from the footprint edge, measured perpendicularly to the tread centerline.
b. A minimum cross-sectional width of 1/16 inch.
c. Edges of pockets or slots at angles between 35 and 90 degrees from the direction of travel.
2. The new tire tread contact surface void area will be a minimum of 25 percent based on mold dimensions.

The rough translation of this specification is that the tire must have a row of fairly big grooves that start at the edge of the tread and extend toward the center of the tire. Also, at least 25 percent of the surface area must be grooves.

The rough translation of this specification is that the tire must have a row of fairly big grooves that start at the edge of the tread and extend toward the center of the tire. Also, at least 25 percent of the surface area must be grooves.

The idea is to give the tread pattern enough void space so that it can bite through the snow and get traction. However, as you can see from the specification, there is no testing involved.

To address this shortcoming, the Rubber Manufacturers Association and the tire industry have agreed on a standard that does involve testing. The designation is called Severe Snow Use and has a specific icon (see image at right), which goes next to the M/S designation.

In order to meet this standard, tires must be tested using an American Society for Testing and Materials (ASTM) testing procedure described in "RMA Definition for Passenger and Light Truck Tires for use in Severe Snow Conditions":

Tires designed for use in severe snow conditions are recognized by manufacturers to attain a traction index equal to or greater than 110 compared to the ASTM E-1136 Standard Reference Test Tire when using the ASTM F-1805 snow traction test with equivalent percentage loads.
These tires, in addition to meeting the geometrical requirements for an M/S designation, are tested on snow using a standardized test procedure.They have to do better than the standard reference tire in order to meet the requirements for Severe Snow Use.

Hydroplaning:

Hydroplaning can occur when the car drives through puddles of standing water. If the water cannot squirt out from under the tire quickly enough, the tire will lift off the ground and be supported by only the water. Because the affected tire will have almost no traction, cars can easily go out of control when hydroplaning.

Some tires are designed to help reduce the possibility of hydroplaning. These tires have deep grooves running in the same direction as the tread, giving the water an extra channel to escape from under the tire.

How Tires Support a Car:

You may have wondered how a car tire with 30 pounds per square inch (psi) of pressure can support a car. This is an interesting question, and it is related to several other issues, such as how much force it takes to push a tire down the road and why tires get hot when you drive (and how this can lead to problems).
The next time you get in your car, take a close look at the tires. You will notice that they are not really round. There is a flat spot on the bottom where the tire meets the road. This flat spot is called the contact patch.

If you were looking up at a car through a glass road, you could measure the size of the contact patch. You could also make a pretty good estimate of the weight of your car, if you measured the area of the contact patches of each tire, added them together and then multiplied the sum by the tire pressure.

Since there is a certain amount of pressure per square inch in the tire, say 30 psi, then you need quite a few square inches of contact patch to carry the weight of the car. If you add more weight or decrease the pressure, then you need even more square inches of contact patch, so the flat spot gets bigger.

A properly inflated tire and an underinflated or overloaded tire



You can see that the underinflated/overloaded tire is less round than the properly inflated, properly loaded tire. When the tire is spinning, the contact patch must move around the tire to stay in contact with the road. At the spot where the tire meets the road, the rubber is bent out. It takes force to bend that tire, and the more it has to bend, the more force it takes. The tire is not perfectly elastic, so when it returns to its original shape, it does not return all of the force that it took to bend it. Some of that force is converted to heat in the tire by the friction and work of bending all of the rubber and steel in the tire. Since an underinflated or overloaded tire needs to bend more, it takes more force to push it down the road, so it generates more heat.

Tire manufacturers sometimes publish a coefficient of rolling friction (CRF) for their tires. You can use this number to calculate how much force it takes to push a tire down the road. The CRF has nothing to do with how much traction the tire has; it is used to calculate the amount of drag or rolling resistance caused by the tires. The CRF is just like any other coefficient of friction: The force required to overcome the friction is equal to the CRF multiplied by the weight on the tire. This table lists typical CRFs for several different types of wheels.


Let's figure out how much force a typical car might use to push its tires down the road. Let's say our car weighs 4,000 pounds (1814.369 kg), and the tires have a CRF of 0.015. The force is equal to 4,000 x 0.015, which equals 60 pounds (27.215 kg). Now let's figure out how much power that is. If you've read the HowStuffWorks article How Force, Torque, Power and Energy Work, you know that power is equal to force times speed. So the amount of power used by the tires depends on how fast the car is going. At 75 mph (120.7 kph), the tires are using 12 horsepower, and at 55 mph (88.513 kph) they use 8.8 horsepower. All of that power is turning into heat. Most of it goes into the tires, but some of it goes into the road (the road actually bends a little when the car drives over it).

From these calculations you can see that the three things that affect how much force it takes to push the tire down the road (and therefore how much heat builds up in the tires) are the weight on the tires, the speed you drive and the CRF (which increases if pressure is decreased).

If you drive on softer surfaces, such as sand, more of the heat goes into the ground, and less goes into the tires, but the CRF goes way up.

Problems With Tires
Underinflation can cause tires to wear more on the outside than the inside. It also causes reduced fuel efficiency and increased heat buildup in the tires. It is important to check the tire pressure with a gauge at least once a month.

Overinflation causes tires to wear more in the center of the tread. The tire pressure should never exceed the maximum that is listed on the side of the tire. Car manufacturers often suggest a lower pressure than the maximum because the tires will give a softer ride. But running the tires at a higher pressure will improve mileage.

Misalignment of the wheels causes either the inside or the outside to wear unevenly, or to have a rough, slightly torn appearance.



TO BE CONT:

[~JC~]

Drag Tires

CAN YOU HAVE THE BEST OF BOTH WORLDS?


In the '60s, if you wanted to go fast there wasn't much of a choice in tires. It was either full-blown race slicks or crappy stock bias-plies. Like everything in the performance aftermarket, tires have evolved over the decades, and today's race tires are stickier, softer, and far better than they used to be. Hell, today's street tires are better than most full-race tires from 30 years ago! With the introduction of the BFGoodrich Drag Radial roughly ten years ago, racers have been given the choice of a good compromise between streetability and ultimate traction.

Drag Radials (most of them anyway) allow you to drive the car on the street and still have traction that far surpasses a traditional “street” tire. BFGoodrich and Nitto have long had drag radials, but with the introduction of new tires from Mickey Thompson and M&H, the drag radial world has gotten even more serious. Each tire has its pros and cons, and that's what this story is about: explaining the differences between drag radials so that you can make the right choice for your combination and usage.

BFGoodrich Drag Radial
BFG had the first drag radial on the market, and is by far the most commonly used drag radial in heads-up racing. The BFG combines very good traction with a legitimate lifespan when used on the street. How good does this tire hook? The faster cars in the NMRA's BFG-sponsored Drag Radial class are in the 8.30s at 170mph, and in unlimited form, Dwayne Gutridge has run in the 7's on the 315/60-15 BFG! They won't last as long as the standard Comp T/A on the street, but a few thousand miles is not out of the question.

Under Pressure
We know of some racers that run pretty low tire pressures in their drag radials and sometimes it looks like the tires are flat. We asked Todd Steen from BFG what kind of pressures they recommend, “Well, people that do a 'John Force' burnout on the tires, we tell them somewhere between 16 and 18 pounds. The biggest thing that we tell them is that if they've been launching on another regular street tire, and let's say you've been launching at 4,500-5,000 rpm, do not go out there (on our tires) and air them down to 17-18 pounds and try to launch at that kind of rpm, because you're going to break something. They'll say, ‘no, no…’ but we tell them that's why they're signing the waiver. They'll come back with their axle in their hand, and they're proud of it! We say, ‘we told you!’ ”

Heating the Hides
We know of some racers that absolutely annihilate drag radials in the burnout box, but according to Steen, ”The compound itself works best at somewhere between 160 and 180 degrees. Now you'll see some of these racers doing these huge burnouts on our tires, and it's all for show. Once you get these tires up to 200 degrees, at that point you're just burning rubber. Now, it will take somebody some real time in test and tune to figure out how much of a burnout it takes to get the tire somewhere around 160-180. In unscientific terms, think about getting a stove up to that temperature and putting your hand on it - that's “warm to the touch” for sure. It doesn't flat-out burn you, but if someone was to put their hand on the tire after the burnout, you shouldn't be able to just rest your hand on it. If not, you're leaving grip on the table.”

Tire Additives?
We've heard that VHT and stuff like that work s well with drag radial compounds so we mentioned that to Steen also. He said, “we haven't found anywhere that VHT or any kind of traction compound has worked against us, but it is a street-legal tire, so normal pavement with no additives, it will still work like it's supposed to. For the most part, it should be the same, although it might take some adjustment to the tire pressure to get the most out of the tire. The biggest thing we stress is that because they're street legal, don't leave the track while you're still driving around on 16 or 18 pounds.”

Drag Tires in Street Tire Clothing
If drag radials can be run on the street can they be treated like street tires? We queried Steen. “Absolutely, they can be rotated, and run “against the grain.” You won't get extra traction. Preferably, you'll run them in the direction the tread is pointed, but it won't hurt them. As a matter of fact, there are three guys who swear to me that they got better traction when they turned them around. If it works on your car, it works on your car, but in theory it shouldn't make a difference.'

Mickey Thompson Drag Radial
Mickey Thompson set the tire world on fire with the introduction of their new ET Drag Radial, which at first glance looked to be incredibly sticky. Testing since the introduction has proven that the M/T is a very sticky tire, with traction that's not far removed from their ET Drag slicks. The M/T ET Drag radial is the spec tire for the PSCA's M/T Wild Street class. Unlike the other tires in question, these guys aren't recommended for the street, but they sure can hook! We spoke with drag radial expert Jason Moulton from M/T and got the inside scoop on these new sticky race radials.

Construction
Moulton would not get into specific technical details about the tire, but did say, “It's basically all about how the tire is laid-up and constructed. Our drag radial is a true radial-constructed tire that we would consider an ultra high-performance platform, and they are a DOT-approved tire. We offer two drag radials; we have an E.T Street radial and an E.T Drag radial. The E.T Drag radial is a street tire that is similar to our E.T Street bias ply tires, but we don't recommend it for normal highway use because it will wear out due to reduced tread depth and void in the tire”.

Tire Life
Moulton said, “It's really hard to say how long these tires will last in miles. A lot of it has to do with the type of vehicle the tires are put on and the weight of the vehicle. We haven't really had enough time to tell how long these things are going to last as an average, since most of the tires out there are mostly being used on the strip.” Based on our observation and Mickey Thompson's own verbiage that does not recommend them for street use, you're not going to get very good life out of these tires on the street. But that's the price you pay for the incredible traction they provide.

Compound Differences and Wet Traction
The M/T drag radial offers a ton of off-the-line-performance but are the tire compounds the same between the E.T. Drag and E.T. Street radials? And what about wet traction? Moulton explained, “With these types of tires there's not a big difference in compounds for the E.T Drag and E.T Street. The compounds are in the same family but they are a little different. As far as wet traction goes, the best thing I tell people is use common sense and be aware of what you have on the back of your car. These tires weren't meant for wet traction but if you use common sense under normal driving conditions those tires will get you home, pending any type of hurricane or anything like that.”

Slicks or Radials?
Will these radials hook as good as a full-boogie slick? We ran that by Moulton and he said, “Well again it's all application. I know for a fact that you can try to put these tires (drag radials) on some cars that are used to hooking with a bias ply [slick], and they work. But depending on how they are set up with weight and power levels, a slick might work better. When you get to high power levels there will be a technique to it, so yes this is true.“

Who Needs 'Em?
Since the drag radials are so good, we asked Mickey Thompson if there will still be a market for the E.T Street tire, and they explained, “The market for the E.T. Street drag radial is for the guy that wants more of a user-friendly tire as far as streetability. E.T. Street slicks are for high-horsepower, high-torque applications and manual transmission applications. The way a clutch car will 'hit' the basics of a radial tire, it will not absorb the horsepower like a bias ply tire will, which is good and bad. By not absorbing the power the radial has a potential to be quicker if it can get off the starting line without spinning.”

TO BE CONT.

[~JC~]

PERFORMANCE TIRES

Tires, like all performance automotive components, are constructed to meet very specific requirements. Sport tires are designed to provide maximum performance at the expense of tread life. Conversely, general-use (semi-performance or touring) tires trade some of that handling potential for a bit of extra comfort and longer life. With this in mind, the selection of the right tire requires some careful thought. Tires should be viewed as every bit as important as hard-part upgrades when it comes to performance. Their return on investment cannot only be measured in their ability to allow your suspension components to perform to their maximum potential, but to provide possibly lifesaving traction in extreme conditions, as well.

It is a vehicle's tires that employ friction against the road surface in order to transmit chassis forces, and as a result, the capabilities and limitations of those tires will almost always define the limits of acceleration, braking, and cornering. In other words, you could have the best engineered chassis and suspension components available on your car, and if you're running on crappy tires, the benefit you'll receive from those components will end up falling well short of what those components are actually capable of providing. This is precisely what we'd, up to this point, never fully realized. OK, we'll assume a lot of you knew this already, so we're going to beg your indulgence while we go over some basics that will help bring others up to speed.

There are quite a few levels of performance tires available on the market, from full-on race tires to general use passenger car tires. With a bit of investigation, this assortment of choices allows us to choose the best tire to serve our needs. A DOT-approved competition radial with an asymmetrical, non-directional or directional tread pattern optimized for dry racing... compounds formulated to deliver extreme cornering power for not only fast and consistent track times, but exceptionally impressive handling for aggressive driving, as well.

The main objective of a performance tire is to increase traction. Thicker tread blocks provide more contact area with the road. A softer rubber tread compound and small-channel grooving provide a smaller void ratio contribute to increased grip on the road. With this in mind, there are some trade offs associated with running extreme high-performance tires on a streetcar. Beyond a bit of increased noise and stiffer ride--minor drawbacks for a performance-bred vehicle--the most important is less-than-optimum performance in the wet stuff (true dry erformance tires are terrible in the rain, so use extreme caution). Since performance tires are designed to maximize the contact area with the road, there is less "void area" for water to be directed away from the contact patch, which can lead to hydroplaning in wet conditions. Another consideration is wear. True performance tires are manufactured using softer compounds that wear quicker than the average street tire, so expect to compromise tire life with outstanding handling characteristics.

TO BE CONT

[~JC~]

What kind of Tire is right for you?

An all around (All Weather Tire) works just fine for your common daily driver but doesn't perform well in any one particular area. Just like a drag slick isn't your best choice for driving on snow covered roads. So is it the same with a snow tire at a SCCA track event.

A tire is more than a round black piece of rubber that holds air. There are trade offs in the tire world and those compromises effect how the tire will work... This is something we don't often think about and sometimes only see the dollar figure in the short term. We see a tire that cost a nice bit of change and is geared towards dry road conditions but has a very low milage warranty and we go eeek. If we really stoped to think about it that tire might be just what you should invest in? If your driving in the best of weather and don't drive your car much to begin with? Then a low milage high traction tire might be your best choice? And should that rare once in awhile rain cloud catch you out, don't be afraid that you can't get home. An Extreme Ulta Performance will work in the rain you just have to drive slower and be aware of the limits of that tire. A Tire that is designed more to dry handling will not perform well in the rain and can be extremely dangerous...

A high mileage tire will get you from point A to B but is it exactly what your looking for? Maybe not so with a performance car? High milage tires trade off on grip and ride comfort for longevity (Something you might want on your Park Avenue), Performance tires tend to trade off in mileage for better grip in overall conditions and ride comfort (Something good for a BMW and the like). Extreme Ultra Perfomance tires trade off in grip in certain conditions such as rain and snow for superior grip in drier conditions and there longevity is greatly reduced. 10,000 miles is the best you can expect to see from an Extreme Ultra perfomance tire. You bought a performance car and if you don't drive it as a daily driver then Extreme Ultra Performance tires might be something you should consider upon your next tire purchase.

I hope this info has been helpful and the thread is now open for discussion...

[~JC~]

Uncertain what tires I will purchase when the time comes...

I like the M/T's but can only use them on the rear and they are not really a good street tire... I can't justify the odd looks of two types of tires on the back and the fact you'd be lucky to get 3000 miles out of them if driven on the street... Not really a fan of Nitto's but they do have some advantages that they can be used all the way around and get better service milage out of them... I think I'm leaning towards the BFG Comp T/A's? They don't have as good a grip as the M/T's but are at least as good if not a little better than Nitto's... Plus they can be used all the way around and have a decent service mileage ( If you call 10,000-15,000 miles service life?)

There are other tires out there that have far longer service mileage. However they trade off in grip for stiffness, Mileage, better wet traction, and speed ratings...

Any of you have tire experinces you would like to share?

[~JC~]

here's a nice article on tire reviews

Tire Test: The Quick and the Tread - Features - Car and DriverDecember 2005