The Official Tech Thread from The Tire Rack - i-Club



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Old 04-08-2004, 05:43 AM   #1
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The Official Tech Thread from The Tire Rack

This thread is to offer basic tire info

Tire Tech:
what do the side wall markings mean ???
There is a lot of information on the sidewall of a tire. Typically, you'll find UTQG ratings for treadwear, traction and temperature, the size of the tire, the load rating index number with a speed rating index, the construction type (bias or radial), the D.O.T. compliance code, construction details, and of course, the make and model of the tire. On some tires used as original equipment, you may also find a marking that indicates its OE status. Porsche uses an N-0 or N-1 designation, BMW uses a star on some O.E. tires and General Motors uses a "TPC" code. Light Truck tires are sometimes marked with an LT for "Light Truck" before the size, passenger tires are often marked with the letter P for "Passenger" before the size. Passenger tires of the same size with or without the P are virtually interchangable.
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Old 04-08-2004, 05:44 AM   #2
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Air Pressure:
Your tires support the weight of your vehicle, right? Well, they don’t! It’s the air pressure inside them that actually supports the weight. Maintaining sufficient air pressure is required if your tires are to provide all of the handling, traction and durability of which they are capable.

However, you can't set tire pressure...and then forget about it! Tire pressure has to be checked periodically to assure that the influences of time, changes in ambient temperatures or that a small tread puncture has caused it to change.

The tire pressure recommended in your vehicle's owner's manual or tire information placard is the vehicle’s recommended "cold" tire inflation pressure. This means that it should be checked in the morning before you drive more than a few miles, or before rising ambient temperatures or the sun’s radiant heat affects it.

Since air is a gas, it expands when heated and contracts when cooled. In most parts of North America, this makes fall and early winter months the most critical times to check inflation pressures...days are getting shorter...ambient temperatures are getting colder...and your tires' inflation pressure is going down!

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower).

In most parts of North America, the difference between average summer and winter temperatures is about -50° Fahrenheit...which results in a potential "loss" of about 5 psi as winter’s temperatures set in. And a 5 psi "loss" is enough to sacrifice handling, traction, and durability!

Additionally, the difference between cold nighttime temperatures and hot daytime temperatures in most parts of the country is about 20° Fahrenheit. This means that after setting tire pressures first thing in the morning, the vehicle’s tire pressures will be almost 2 psi higher when measured in the afternoon (if the vehicle was parked in the shade). While that is expected, the problem is when you set your vehicle’s tire pressures in the heat of the day, their cold pressures will probably be 2 psi low the following morning.

And finally, if the vehicle is parked in the sun, the sun’s radiant heat will artificially and temporarily increase tire pressures.

we even evaluated the effects of heat generated by the tire’s flexing during use. We tried to eliminate the variable conditions we might encounter on the road by conducting this test using our "competition tire heat cycling service" that rolls the tires under load against the machine’s rollers to simulate real world driving. We monitored the changes in tire pressure in 5-minute intervals. The test tires were inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the same load, the air pressure in all of the tires went up about 1 psi during every 5 minutes of use for the first 20 minutes of operation. Then the air pressures stabilized, typically gaining no more than 1 psi of additional pressure during the next 20 minutes. This means that even a short drive to inflate your tires will result in tires that will probably be "underinflated" by a few psi the following morning.

Add all of these together, and you can understand why the conditions in which you set your vehicle’s tire pressures are almost as important as the fact that you do set it.

It’s important to remember that your vehicle's recommended tire pressure is its "cold" tire inflation pressure. It should be checked in the morning before you drive more than a few miles, or rising ambient temperatures or sun’s radiant heat affects it.

And by the way, if you live in the North and park in an attached or heated garage you will "lose" pressure when you leave its warmth and venture into the real world outside during winter. Add 1 psi "cold" pressure tire pressure to compensate for each 10° Fahrenheit temperature difference between the temperature in the garage and outside.
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Old 04-08-2004, 05:45 AM   #3
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Checking your air Pressure:
When vehicle manufacturers select a tire size for a vehicle, they evaluate the vehicle’sgross axle weights, the anticipated use of the tire, and the tire diameter and width. Adjustments to these factors give the manufacturer a way to improve handling and appearance. This is especially true for performance tire sizes. The size selected is rarely limited to only one capability (i.e. carrying the vehicle’s weight). The tire usually needs to have additional load capacity as well. This extra capacity is important because without it all of the tire’s performance would be used up just carrying the weight of the vehicle and little would be left for durability at high speeds or responsive handling. For all vehicles produced since 1968, the original tires sizes and inflation pressures (including the spare) are listed on a vehicle placard. This placard can be located on:

The driver-side door or door jamb (Ford vehicles on the rear passenger door jamb)
Glove box or counsel door
Fuel filler door
The engine compartment
Additionally, some manufacturers also list the original tire pressure in the vehicle’s owner’s manual.If a car’s inflation pressure has varied from that which was recommended by the manufacturer, it’s likely that the tire’s wear and performance characteristics have also changed.

Underinflation

If your vehicle’s tires are underinflated by only 6 psi it could lead to tire failure. Additionally, the tire’s tread life could be reduced by as much as 25%. Lower inflation pressure will allow the tire to deflect (bend) more as it rolls. This will build up internal heat, increase rolling resistance and cause a reduction in fuel economy of up to 5%. You would find a significant loss of steering precision and cornering stability. While 6 psi doesn’t seem excessively low, remember, it usually represents about 20% of the tire’s recommended pressure.

Overinflation

If your tires are overinflated by 6 psi, they could be damaged more easily when running over potholes or debris in the road. Higher inflated tires cannot isolate road irregularities well causing them to ride harsher. However, higher inflation pressures usually provide an improvement in steering response and cornering stability up to a point. This is why participants who use street tires in autocrosses, track events and road races, run higher than normal inflation pressures.

The pressure must be checked with a quality air gauge as the inflation pressure can not be accurately estimated through visual inspection.
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Old 04-08-2004, 05:45 AM   #4
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Tire Dimensions:
The first number is the width of the tire in millimeters, measured from sidewall to sidewall. To convert to inches, divide by 25.4 In the example above, the width is 185mm or 7.28".

The second number is the aspect ratio. This is a ratio of sidewall height to width. In the example above, the tire is 7.28" wide, multiply that by the aspect ratio to find the height of one sidewall. In this case, 185x0.60=111mm or 7.28"x0.60=4.36".

The last number is the diameter of the wheel in inches.

To figure the outside diameter of a tire, take the sidewall height and multiply by 2,(remember that the diameter is made up of 2 sidewalls, the one above the wheel, and the one below the wheel) and add the diameter of the wheel to get your answer.

Example...185/60R14 85H or 185/60HR14

185mm x .60=111mm x 2=222mm + 355.6mm(14")= 577.6mm or 22.74"

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Old 04-08-2004, 05:46 AM   #5
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Speed Ratings:
It is important to note that speed ratings only apply to tires that have not been damaged, altered, under-inflated or overloaded. Additionally, most tire manufacturers maintain that a tire that has been cut or punctured no longer retains the tire manufacturer’s original speed rating, even after being repaired.

In Europe, where selected highways do not have speed limits and high speed driving is permitted, speed ratings were established to match the speed capability of tires with the top speed capabilities of the vehicles to which they are applied. Speed ratings are established in kilometers per hour and subsequently converted to miles per hour (which explains why speed ratings appear established at “odd” mile per hour increments). Despite the tire manufacturer’s ability to manufacturer tires capable of high speeds, none of them recommend the use of their products in excess of legal speed limits.

Speed ratings are based on laboratory tests where the tire is pressed (to reflect its required load) against a large diameter metal drum and run at ever increasing speeds (in 6.2 mph steps in 10 minute increments) until the tire’s required speed has been met.

Speed Rating - Miles/Hour - Kilometers/Hour - Typical Use
N=87 MPH, 140km/h, Spare Tires
P=93 MPH, 150km/h
Q=99 MPH, 160km/h, Winter, LT Tires
R=106 MPH, 170km/h, LT Tires
S=112 MPH, 180km/h
T=118 MPH, 190km/h
U=124 MPH, 200km/h
H=130 MPH, 210km/h, Sport Sedans
V=149 MPH, 240km/h, Sports Cars
Z=149 MPH, 240km/h and over, Sports Cars
W=168 MPH, 270km/h, Exotic Sport Cars
Y=186 MPH, 300km/h, Exotic Sport Cars

*Today, the Z-speed rating is the only speed rating that is still branded “within” the tire size, as in P225/50ZR16. All other speed ratings are shown in the tire’s service description.

When Z-speed rated tires were first introduced, they were thought to reflect the highest speed rating that would ever be required. Since that time the automotive industry has found it necessary to add W- and Y-speed ratings (indicated in the tire’s service description) to identify the tires that meet the needs of new vehicles that have extremely high, top speed capabilities.

While all Z-speed rated tires are capable of speeds of 149 mph and above, prior to the W- and Y-speed ratings were identified in the service, how far above 149 mph was not identified.

Prior to 1991, the most popular speed ratings were “S,” “H” and “V.” However, while the speed capabilities of S- and H-rated tires still indicate the same speeds as before, the V-speed rating has been modified. Previously a V-speed rated tire with the “V” branded “within” the tire size indicated that the tire was capable of 130+ miles per hour. Today’s new V-speed rated tires are always identified in the tires service description
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Old 04-08-2004, 05:46 AM   #6
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Tire Rotation:
Tire rotation can be beneficial in several ways. When done at the recommended times, it can preserve balanced handling and traction of the tires and even out tire wear. It can even provide performance advantages. When should tires be rotated ? We recommend that high performance tires be rotated every 3,000 to 5,000 miles, even if they don't show signs of wear. Tire rotation can often be done with oil change intervals while the vehicle is off the ground anyway. Tire rotation helps even out tire wear by allowing each tire to serve in as many of the vehicle's wheel positions as possible. Remember, tire rotation can't correct wear problems due to worn mechanical parts or incorrect inflation pressures. It's also important to check your owner's manual for specific details on what method of tire rotation the vehicle's manufacturer recommends.

While every vehicle is equipped with four tires, usually the tires on the front need to accomplish very different tasks than the rear tires. And the tasks encountered on a front wheel drive car are considerably different than those of a rear wheel drive car. Tire wear experienced on a performance vehicle will usually be more severe than those on a family sedan. Each wheel position can cause different wear rates and different type of tire wear.

While no one likes their tires to wear out, it is actually an advantage when all of the tires on a vehicle wear at the same rate throughout their life. As tire wear reduces tread depth, it allows the tires to respond to the driver's input more quickly and increases dry road performance. Since tire rotation will help all of the vehicle’s tires wear at the same rate, it will keep the tires performing equally on all four corners.

When your tires wear out together you can get a new set of tires, without being forced to buy pairs. If you replace tires in sets you will maintain the original handling balance. And our suppliers are constantly introducing new tires, each of which improves upon their past product's performance. If you replace your tires in sets, it allows you to experience today's technology, instead of being forced to match yesterdays.

On front wheel drive cars, rotate the tires in a forward cross pattern (fig. A) or the alternative X pattern (fig. B)
On rear wheel or four wheel drive vehicles, rotate the tires in a rearward cross pattern (fig. C) or the alternative X pattern (fig. B)
If you car has directional wheels or tires, rotate them as shown in fig D.
If you car has non-directional tires that are a different size from front to rear, rotate them as shown in fig. E.




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Old 04-08-2004, 05:49 AM   #7
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Here is some technical info about after market wheels

Performance benefits:
While many people choose alloy wheels for their beauty, there are equally important performance benefits to be derived including...

Reduced Unsprung Weight Compared to Steel Wheels
This is one of the most critical factors affecting a vehicle's road holding ability. Unsprung weight is that portion of a vehicle that is not supported by the suspension (i.e. wheels, tires and brakes) and therefore most susceptible to road shock and cornering forces. By reducing unsprung weight, alloy wheels provide more precise steering input and improved "turning in" characteristics.

Improved Acceleration and Braking
By reducing the weight of the vehicle's rotational mass, alloy wheels provide more responsive acceleration and braking.

Added Rigidity
The added strength of a quality alloy wheel can significantly reduce wheel/tire deflection in cornering. This is particularly critical with an automobile equipped with high performance tires where lateral forces may approach 1.0g.

Increased Brake Cooling
The metals in alloy wheels are excellent conductors of heat - improving heat dissipation from the brakes - reducing risk of brake fade under demanding conditions. Additionally, alloy wheels can be designed to allow more cooling air to flow over the brakes.

What determines quality ?
The Tire Rack’s quality standard for wheels is very high and the manufacturers that we represent in this market understand that we constantly monitor products to ensure that quality products are sold to our customers. But what determines quality?

Manufacturing Process
Manufacturing processes and levels of testing are critical to a wheel's structural integrity. (Read more in "Wheel Construction.") International quality standards such as ISO9001, QS9000, TUV of Germany or VIA of Japan, establish important production and quality standards that manufacturers must follow. In addition, dimensional tolerances based on strict, original equipment market standards versus the more “casual” standards allowed for many aftermarket products should be met. Even durability standards for finish are different between the original equipment market and the aftermarket.

Proper Fit
An accurate fitment is the difference between good, better and best. Critical wheel dimensions such as width, diameter, offset, center bore, brake clearance, as well as load factor and lug hardware are the basics when it comes to properly fitting aftermarket wheels. Installation also requires a high level of sophistication. Many new vehicles are available with features such as ABS, traction control and other features that create a more difficult environment for installing aftermarket wheels. Stability control systems, run-flat tires, large high performance brake systems and staggered wheel and tire sizes are also factors to be considered when establishing accurate fitments. Wheel manufacturers with product design, research and development teams work to determine proper fitment as part of the manufacturing process.

Protective Finish
The type and quality of protective finish on your wheel (as well as proper maintenance) will determine how your wheels look years from now. Check for finish warranties backed by manufacturers with outstanding reputations for quality.

Reputation and Heritage
The reputation of a manufacturer is a strong indicator of quality since it is quality upon which a distinguished reputation is built. It takes time to build a positive reputation and a commitment to maintain it is important. And know a wheel company's roots. Many wheel manufacturers first established themselves in the motorsports arena and apply that technological and philosophical foundation to their production of wheels for use on the street.

Country of Origin
Certain areas of the world bring to mind certain products and characteristics. For example, Germany is at the top in automotive technology and precision. Italy is the front-runner in design and exotic cars. Know where your wheels are manufactured.

The Tire Rack's Commitment to Quality
The Tire Rack constantly reviews wheel data from new vehicles to be sure that we are aware of the original equipment sizes and packages offered. We physically inspect many of today’s new vehicles and often supply technical data to some of the manufacturers outside the U.S. that may not have access to certain vehicles in our market. For many wheels that we import or represent, we specify certain dimensions that we require to ensure wheel fit and maintain our high quality standards.
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Old 04-08-2004, 05:49 AM   #8
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Offset:


The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types.

Zero Offset
The hub mounting surface is even with the centerline of the wheel.

Positive
The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative
The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.

If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.

Torquing lug hardware:
Proper installation requires that the wheel lug torque be set to the recommended specification for your vehicle. These torque specifications can be found in your vehicle’s shop manual or obtained from your vehicle dealer. Finish tightening the lugs down with an accurate torque wrench. Use a crisscross sequence (shown below) until they have reached their proper torque value. Be careful because if you over torque a wheel, you can strip a lug nut, stretch or break a wheel stud, and cause the wheel, brake rotor and/or brake drum to distort.

NOTE: When installing new wheels you should re-torque them after traveling the first 50 to 100 miles. This is necessary because as the wheels are “breaking in” they may compress slightly allowing their lugs to lose some of their torque. Simply repeat the same torque procedure listed above.
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Old 04-08-2004, 05:53 AM   #9
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Load capacity issues:

So, let's look into the issue of shorter tires for AutoX ......

The Facts:

A P-Metric tires max load is achieved at 35 psi and a Euro metric tires max load is achieved at 36 psi adding more air pressure will not increase the tires load capacity but, lowering it will reduce the load carrying ablility. When increasing pressure all you are accomplishing is a lower rolling resistance, quicker steering response, stiffer ride and contact patch reduction.

Based on a WRX:
the OE tire is a 205/55-16 89V
max load capacity is 1279 lbs.
load capacity at recommended pressure (which we will call PLC for placard load capacity at 32psi) = 1213 lbs.

so the minimum load capacity for a WRX is 1213 lbs. according to the PLC

now if you look at the optional tire "OK'd" by Subaru
215/45-17 87W
max load is 1201
PLC = 1135

so in reality the minimum load capacity is 1135 lbs.

now lets consider the idea of running a capacity that is too low .... what happens:
the tire is put into a condition where it can support the car at a static rest point but, when driven aggressively the sidewalls are so over worked they can not properly control the contact patch of the tire allowing for excessive sidewall roll-over, irregular wear tread block and contact patch flex which robs the tires performance ability and grip. also the added flexing of the tire will increase the heat it generates which can make a tire feel really slippery .... better gear ratio is achieved but if you can't utilize the power it doesn't matter ....

A good way to understand this is: let's say I just put a 100lb. back pack on you .... you can stand there for quite a while (that's your car sitting in a parking lot) now go run up and down some stairs with that back pack .... that's normal driving and you can't do that very well, nearly as fast or for very long before your legs give out .... after you've recovered enough to walk (let the car rest once you arrive at the venue for your autoX), take a 500 meter sprint through deep sand, that's your autoX and you will most likely not make it to the finish without falling atleast once if you can finish at all.

there are possibilities for a shorter tire though

alternate tire sizes that would work on the WRX while maintaining load capacity (these may or may not be all of the sizes that could work and some of these may not even fit)

235/45-17/1135 PLC
225/45-17/1250 PLC
245/45-16/1235
215/45-16/1168 **** minimum 36 psi
195/55-16/1168
225/50-15/1246 PLC
215/50-15/1157 PLC
225/45-14/1168

all of these tires would be acceptable as long as at least 35 psi was run in them except where noted
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