While many folks on this forum queeze at the thought of installing aftermaket rims on their beloved Z or not, the information I am presenting is still applicable when purchasing tires and or wheels for your Z (or any performance vehicle).
While this post is intended to enlighten the reader, this is not a all encompassing guide on how to purchase tire and wheel combinations.There is an amazing amount of information available to the purchaser that has been mandated by US and International government regulations and standards that the majority of tire buyers are completely unaware of, and how important this information is to the purchaser.
Aside from the manufacturers name and model or brand names, tire size, load and speed ratings etc.,
Did you know that the manufacturing week and year dates are mandatory as well?
Why is this data important?
When selecting tires for your ride, you should check these dates to ensure the dates are ideally the same (manufactured in the same week), because all tires are mass produced, including non-DOT rated race only tires.
Due to mass production, the tolerances of the manufacturing process tend to expand during the usefull lifetime of the molds and tooling, therefore selecting a set of tires that have a uniform date means that you will have a better chance of your particular set of tires being closer together (tolerance-wise) than a set that has 3, 4 or 12 months or more apart. Additionally, the rubber compounds start oxidizing (i.e. aging) from the moment they are removed from the mold.
Tires that have a large difference in manufacturing dates should always be avoided.
When you are purchasing a set of tires you should check and record this information, prior to allowing the technician installing the tires on the rims. Another important step to have performed is prior to the tires being installed, have the rims checked and record the run-out.
This step should be accomplished regardless of the age of the rim.
Mark and record the run-out and retain this information for your records. If OE or used rims are utilized, additional inspection steps above and beyond a close visual inspection such as Dye Penetrant Inspection should be performed as well, (for alloy rims) or Magna-fluxed (for steel) to ensure the rim is structurally intact.
You should also mark the rim’s installed location on the vehicle as well (a Sharpie works well for this). Additionally, have the rims balanced, PRIOR to any tire being installed, and reject or replace any wheel that has greater than 1 oz of weight used to bring the rim into proper and equal balance (valves installed). Next you should measure the circumference (“stagger”) of each tire in at least three (3) places (Outside, center and inside) and record this information for each tire. Reject any tire that has greater than 1/16” difference in circumference.
Again, check the dates of any tire replaced that was rejected. Once you are satisfied with the stagger, the tires should be mounted following the manufacturer’s instructions. The tires should ALWAYS be mated to the rims to cancel run-out (i.e. the low-side of the rim should be mounted to the high side of the tire).
Ensure the installation tech NEVER exceeds the maximum recommended seating pressure, and NEVER allow a tech to install your tires without the proper bead lubricant!
Check and make sure the bead lubricant bottle is full prior to the tech installing your tires! For consistant tire performance, use compressed nitrogen instead of compressed air to inflate your tires. This process costs a little more but the gains in consistancy and performance are well worth the added expense.
I also use schrader valves instead of the regular tire valves to prevent someone from mistakedly servicing your tires with compressed air. Prior to balancing, again, measure and record the tire circumference (with the tires properly inflated), again rejecting any tire / wheel combination that exceeds 1/16”. After re-checking the inflated stagger, the combination should then be balanced on a spin balancer.
Reject any tire with greater than 1 Oz. difference in weight (between all 4 tires / wheels). If mounting race-only (non-DOT rated) tires, you must maintain heat-cycle records for your tires as well.
While at a track event, you should record each time the tires are brought up to race temperatures, and you should record the tire temperatures to verify your chassis set-up. DO NOT store your car on race tires!
Use a set of street tires or used-up race tires instead. Prolonged storage on race tires will cause flat-spots (at a minimum) from sitting with the weight of the vehicle in one place.
Remove the race tires, bag and seal them with a plastic trash bag, and store the tires in a cool, dry place (out of direct sunlight).preferably, suspended off of the ground. The following information below will assist you in determining tire sizes and explain sidewall markings for your reference:
Additional information can be found at this wesite:
[ http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=33 ]
Determining the Age of a Tire
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 Number (often referred to as the tire’s serial number).
Unlike vehicle identification numbers (VINs) and the serial numbers used on many other consumer goods (which identify one specific item), Tire Identification Numbers are really batch codes that identify the week and year the tire was produced. The U.S. Department of Transportation (DOT) National Highway Traffic Safety Administration (NHTSA) requires that Tire Identification Numbers be a combination of the letters DOT, followed by ten, eleven or twelve letters and/or numbers that identify the manufacturing location, tire size and manufacturer's code, along with the week and year the tire was manufactured.
Tires Manufactured Since 2000
Since 2000, the week and year the tire was produced has been provided by the last four digits of the Tire Identification Number with the 2 digits being used to identify the week immediately preceding the 2 digits used to identify the year.
Example of a tire manufactured since 2000 with the current Tire Identification Number format:
In the example above:
DOT U2LL LMLR 5107
DOT U2LL LMLR 5107 Manufactured during the 51st week of the year
DOT U2LL LMLR 5107 Manufactured during 2007
While the entire Tire Identification Number is required to be branded onto one sidewall of every tire, current regulations also require that DOT and the first digits of the Tire Identification Number must also be branded onto the opposite sidewall. Therefore, it is possible to see a Tire Identification Number that appears incomplete and requires looking at the tire’s other sidewall to find the entire Tire Identification Number
The use of a partial Tire Identification Number on the one sidewall (shown above) reduces the risk of injury to the mold technician that would have to install the weekly date code on the top sidewall portion of a hot tire mold.
Tires Manufactured Before 2000
The Tire Identification Number for tires produced prior to 2000 was based on the assumption that tires would not be in service for ten years.
While they were required to provided the same information as today’s tires, the week and year the tire was produced was 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.
Example of a tire manufactured before 2000 with the earlier Tire Identification Number format:
In the example above:
DOT EJ8J DFM 408
DOT EJ8J DFM 408 Manufactured during the 40th week of the year
DOT EJ8J DFM 408 Manufactured during the 8th year of the decade While the previous Tire Identification Number format identified that a tire was built in the 8th year of a decade, there was no universal identifier that confirmed which decade (tires produced in the 1990s may have a small triangle following the Tire Identification Number to identify the decade).
Match Mounting to Enhance Tire & Wheel Uniformity Tire and wheel manufacturers continually develop new manufacturing methods to enhance the uniformity of their products because radial runout, lateral runout, force variation and imbalance can affect a vehicle's ride quality.
Match mounting tires on wheels is a process where a tire's installed position on the wheel is specifically selected to help minimize the final combination's force variation and/or imbalance.
One match mounting procedure aligns the tire's measured high point of radial force variation with the measured low point of the wheel's radial runout.
The other simply aligns the tire's lightest spot with the wheel's heaviest spot.
Original Equipment (OE) tire suppliers are required to mark the tire's "high point" while OE wheel manufacturers mark the wheel's "low point."
This helps the vehicle manufacturer match mount combinations that maximize new car ride quality while reducing the amount of balancing weight. Today, many vehicle manufacturers specify the use of a temporary tag applied to tires and wheels that are removed before the vehicle is put into service. Unfortunately, this means that there are no permanent marks to reference later. There was a time when the valve stem hole on standard wheels indicated the optimum place to which the tire should be match mounted.
However, with the advent of styled, steel wheels and aluminum alloy wheels, the stem position evolved into an aesthetic issue rather than being a uniformity indicator.
Add to this the probability of wheels retaining their original runout after thousands of miles of use and you can understand that simply mounting the tire so the colored dot is at the valve stem is no longer required practice. We have found that the only way to accurately match mount replacement tires on used original or new aftermarket wheels is to use Hunter tire balancers which have the ability to measure wheel runout and tire force variations under load before the tire and wheel are installed on the vehicle.
Using these machines, a colored dot might be positioned anywhere on the wheel relative to each wheel's runout measurement. In the end, the markers have little, if any, relevance when replacement tires are installed.
Tire & Wheel Package Ride Uniformity Confirmation Smooth-rolling tires and wheels can be the difference between having a vibration-free driving experience or a shaky one.
If the tire and wheel tolerances are not examined, corrected, or are allowed to "stack up" rather than "cancel out" each other, the possibility of a balance or road force induced ride vibration exists. Tires are made up of internal beads, body plies and belts that are encased in rubber, assembled and cured. Alloy wheels are cast, cooled and machined.
And even though they both are built to tight tolerances, there is bound to be some unavoidable weight imbalances during the manufacturing process. When you consider that a typical passenger car tire mounted on an alloy wheel weighs about 40 pounds, it's amazing that the total amount of imbalance is typically no more than three to four ounces, or about one-half of one percent.
So, if the tire and wheel combination isn't balanced with add-on weights, being off even an ounce or two can cause vibration at highway speeds.
Heavier tire and wheel combinations, especially those used on light trucks, require more weights to maintain the same low percentage of imbalance. If the wheel and tire are at the edge of acceptable tolerances, Hunter's patented ForceMatching(TM) feature is used to align the high point of the tire with the low spot of the rim.
This helps maximize ride quality by minimizing the effects of tire force variation and rim runout.
If a wheel or tire is found to be out of tolerance, it is replaced with another wheel or tire that is within tolerance.
Once acceptable uniformity has been confirmed, or corrective actions have been completed, the technician can continue with precision wheel balance. Hunter Engineering GSP9700 Specifications
Roller Force: up to 1,400 lbs. (635 kg)
Capacity:
• Rim Width: 1.5 in. (38mm) to 20 in. (508mm)
• Rim Diameter: 10 in. (254mm) to 24.5 in. (622.3mm)
• Maximum Tire Diameter: 40 in. (1016mm)
• Maximum Tire Width: 20 in. (508mm)
• Maximum Wheel and Tire Weight: 175 lbs. (80kg)
Radial & Lateral Runout Accuracy: .002 in. (0.05mm)
Radial Force Measurement Accuracy: 2 lbs., 10N (1.0 kg)
Imbalance Resolution: +/- 0.01 oz. or 0.5 grams
Placement Accuracy: 512 positions, +/- .35°
Calculating Approximate Tire Dimensions
Most modern passenger car and light truck tires feature size designations that indicate the tire's dimensions in a combination of metric, mathematical and English systems.
While this unusual combination of millimeters, percentages and inches is a byproduct of the evolution of global tire specifications, it also provides the ability to calculate/estimate basic tire dimensions.
Example size: 225/45R17
The first three numbers in a typical size (225/45R17) are the tire's indicated section width in millimeters, measured from sidewall to sidewall.
If you are familiar with measurements in inches, the section width in millimeters can be converted into inches by dividing it by 25.4. For example:
225mm / 25.4 = 8.86"
The second pair of numbers (225/45R17) is the tire's aspect ratio or profile.
This is a ratio of sidewall height to section width. The section height's measurement can be calculated by multiplying the section width by the aspect ratio.
The answer will be the height of one sidewall. For example:
225mm x 0.45 = 101.3mm
8.86" x 0.45 = 3.99"
The last number (225/45R17) is the diameter of the wheel in inches.
If you are familiar with measurements in the metric system, the wheel diameter can be converted into millimeters by multiplying it by 25.4.
For example:
17" x 25.4 = 431.8mm
To calculate the overall diameter of a tire, the sidewall height must be multiplied by 2 (remembering that the tire diameter is made up of 2 sidewalls, the one above the wheel and the one below the wheel touching the ground) and add the diameter of the wheel.
101.3mm + 101.3mm + 431.8mm = 634.4mm
3.99" + 3.99" + 17" = 24.98" WARNING: These calculations (and those provided by all online tire dimension calculators) only reflect nominal tire dimensions, which are often subtly or substantially different than the tire's actual physical size and/or the dimensions provided by the tire manufactures. Drivers working with close fitment tolerances or trying to maintain the original tire diameter of staggered size fitments (similarities or differences) should use the tire manufacturer's specifications for the exact tire(s) they are considering. Actual Tire Section Width is Dependent on Wheel Width
All tire sizes are assigned specific rim widths upon which they are measured (measuring rim), as well as can often be mounted on slightly narrower or wider wheels (rim width range).
Therefore, it is important to note that actual tire section width will depend on the wheel width the tire is mounted on. The rule of thumb is that tire section width changes by 0.2" for every 0.5" change in rim width, being reduced if mounted on narrower then measuring wheel and increased when mounted on wider wheels.
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