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Tire Data Calculator

(updated MAR2013)

This Tire Data Calculator converts the tire-geometric data of various RADIAL passenger car tire sizes from theETRTO standard (European Tire and Rim Technical Organization) into useful data which can be used insuspension calculations and spreadsheets. It does not include calculations for commercial or light-truck tires.Most of the calculations provided here are based upon the equations found in The Automotive Chassis:Engineering Principles 2nd ed. , Reimpell Stoll Betzler.

Your Browser must be able to run Java to be able to use this program.

An explanation for the Input and Output values can be found below after clicking 'Calculate'.

To calculate the tire-geometric data:Input the following Tire specific data: Rim Width; Rim Diameter; Tire Aspect Ratio; Tire Section Width;Select: Tire Load Index Number; Tire Speed Index LetterInput the following Vehicle specific data: Tire Air Pressure; Vehicle Speed; Vehicle Corner Loads

Static Input Data Value Tire 1 Value Tire 2

Chosen Rim Width in inches(e.g. 8" as in 8Jx17)

Rim Diameter in inches(e.g. 17" as in 8Jx17)

Aspect Ratio in %(e.g. 45% as in 245/45 R 17 95W) **

Tire Section Width in mm(e.g. 245 mm as in 245/45 R 17 95W)

Tire Load Index Number(e.g. 95 as in 245/45 R 17 95W)

Tire Speed Index letter(e.g. W as in 245/45 R 17 95W)

Output Spec. Data Result Tire 1 Result Tire 2

Spec. Design Rim Width(manufacturers measuring rim)

Spec. Tire Outer Diameter(at spec. air pressure and no load)

Spec. Tire Section Width(at spec. air pressure and no load)

Spec. Tire Section Height(at spec. air pressure and no load)

Spec. Static Load Radius(@ spec. maximum load and spec. air pressure)

Spec. Tire Rolling Circumference(@ 60 km/h, spec. load and spec. air pressure)

Spec. Dynamic Rolling Radius(@ 60 km/h, spec. load and spec. air pressure)

Spec. Tire Stiffness Rate(@ spec. load and air pressure)

Tire Air Volume(@ spec. load and air pressure)

Dynamic Input Data Value Tire 1 Value Tire 2

Air Pressure in bar(See the chart below)

Vehicle Speed in km/h

Vehicle Corner Load in kg(input weights right/left or front/rear)

Output Vehicle Data Result Tire 1 Result Tire 2

Vehicle Tire Section Width(@ vehicle load, speed and air pressure)

Vehicle Tire Section Height(@ vehicle load, speed and air pressure)

Vehicle Tire Deflection(@ vehicle load, speed and air pressure)

Vehicle Tire Rolling Circumference(@ vehicle load, speed and air pressure)

Vehicle Dynamic Rolling Radius(@ vehicle load, speed and air pressure)

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Vehicle Tire Stiffness Rate(@ vehicle load, speed and air pressure)

Contact Patch Width(@ vehicle load, speed and air pressure)

Contact Patch Length(@ vehicle load, speed and air pressure)

Contact Patch Area(@ vehicle load, speed and air pressure)

Speedometer Reading(Tire 1 is assumed accurate)

Tire Suspension Comfort Value

Length Conversion Calculator

Millimeters Inches Centimeters Feet

Area Conversion Calculator

Square Centimeters Square Inches

Speed Conversion Calculator

Km/h MPH

Weight Conversion Calculator

Kilograms Pounds

Pressure Conversion Calculator

Bar Kpa Psi

Rate Conversion Calculator

kg/mm N/mm lb/in

Volume Conversion Calculator

Liters Cubic Inches

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Explanation of the Input Data

The chosen rim width is usually in inches. A rim size of 8Jx17 has a width of 8" or 203.2 mm.As a rule-of-thumb, you may choose a narrower or wider rim by inch, but its wise to check with thespecific tire manufacturer for their approved rim sizes. Choosing a width near the middle of the approvedrange will give a balance between ride quality and handling. A wider rim will improve handling at the expenseof ride quality, while a narrower rim will improve ride quality at the expense of handling. Consider thesecompromises when selecting rims.

The rim diameter is usually in inches. A rim size of 8Jx17 has a diameter of 17" or 431.8 mm.

The aspect ratio or tire height/width ratio is measured as the section height of tire (SH) divided by thesection width of tire (SW) and this gives the result as a percentage. A tire with the size 245/45 R 17 95W hasan Aspect Ratio of 45%.** If no value is given, for instance as in 165 R 15 S, then the Aspect Ratio is 80%.

The tire section width is in mm and is shown as the first set of numbers on the tire wall, for example; 245mm as on a tire size 245/45 R 17 95W. This is specified in the ETRTO standards as the width of an inflatedtire mounted on its design rim, excluding elevations due to labeling, marking, decorations, or protective bandsor ribs.

The tire load index number is a number shown in the last set of digits on the tire wall, for example; 95ason a tire size 245/45 R 17 95W. See the tables below for more information regarding tire loads, speedratings, and air pressure.

The maximum tire load is in kg and can be found on the chart below based on the tire load index number.The maximum tire load is used along with the air pressure the corner loads to calculate the tire rate, which isused to determine the tire deflection, which is then used for the contact patch calculations. The conversion oflbs to kg is: (lbs) x 0.453594. The conversion of kg to lbs is: (kg) x 2.20462.

The tire speed symbol is a letter shown in the last set of digits on the tire wall, for example; W as on a tiresize 245/45 R 17 95W. See the tables below for more information regarding tire loads, speed ratings, and airpressure.

The vehicle speed is in km/h and can be changed for tire calculations of speeds up to 240 km/h. Theconversion of mph to km/h is: (mph) x 1.6093. The conversion of km/h to mph is: (km/h) x 0.6214.

The air pressure is in bar and can be changed for tire calculations. The air pressure is vital to safety,durability, comfort, performance and economy. The air pressure is used along with the loads to calculate thetire rate, which is used to determine the tire deflection, which is then used for the contact patch calculations.The conversion of psi to bar is: (psi) x 0.06895. The conversion of bar to psi is: (bar) x 14.503775. Note:Since September 2007 all new automobiles below 10,000 lb (4,500 kg) in weight sold in the United States arerequired to incorporate a Tire Pressure Monitoring System (TPMS).

The corner loads are in kg and can be changed for tire calculations. The corner loads are simply the weightin kg under each tire. These corner loads are used along with the air pressure and max tire load to calculatethe tire rate, which is used to determine the tire deflection, which is then used for the contact patchcalculations. The conversion of lbs to kg is: (lbs) x 0.453594. The conversion of kg to lbs is: (kg) x 2.20462.

Explanation of the Output Data

The following details of the tire sizes are valid according to the ETRTO Standard assuming proper tire loadrating, speed rating, air pressures, and no wear on the standard profile. See tables below; courtesy ofContinental Tire. There are many other factors that affect tire data output which have not been included inthese calculations. Tire compound and construction vary from manufacturers. Temperature and aerodynamicdownforce have also been excluded. These data outputs assume straight line constant speed, so nolongitudinal or lateral forces have been included.

The spec. design rim width is usually in inches. The design rim is defined in the ETRTO standards and isused by the tire manufacturers as a template to measure the tires.

The spec. tire outer diameter (with no load) is based on the input data and will reflect any deviation inchosen rim width. This can be measured while the tire/rim assembly is off the car. The spec. tire outerdiameter shown above will change according to the chosen rim width.

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The spec. tire section width (with no load) is the distance between a tire's sidewalls and measured at thewidest part of the tire while mounted on the chosen rim. The tire section width varies approximately 0.2(5mm) for every 0.5 deviation from the design rim width. This change in width has a direct correlation withthe tire output data. The spec. tire section width shown above will change according to the chosen rim width.

The spec. tire section height (with no load) is the radial distance between a tire's rim and the outsidediameter of the tire while mounted on the chosen rim. The tire section width varies approximately 0.1(2.5mm) for every 0.5 deviation from the design rim width. This change in height has a direct correlationwith the tire output data. The spec. tire section height shown above will change according to the chosen rimwidth.

The spec. static load radius (at maximum load per ETRTO) is the distance from the center of the wheel tothe road surface when the car is stationary. This value is calculated under maximum load, at therecommended tire pressure (see tables below) depending on the load index and speed rating of each tire. Thestatic radius is used to calculate the tire stiffness rate. Realistic static radius values for car tires areapproximately 30 cm. The spec. static radius shown above will change according to the chosen rim width.

The spec. tire rolling circumference is normally given from the tire manufacturer. It is the theoreticallength of one rotation of the tire (circumference) based on the dynamic rolling radius. The manufacturers testfor tire rolling circumference is calculated at a speed of 60 km/h (37.3 mph) with an air pressure of 1.8 bar(26 psi) [or 2.3 bar (33 psi) on V-, W-, Y- or ZR tires] and a load value generally between 30-50% of themaximum load value that was used to calculate the static radius. This value can be used for comparison tothe tire manufacturer specifications. Realistic values for car tires are approx 2m. The spec. tire rollingcircumference shown above will change according to the chosen rim width.

The spec. dynamic rolling radius is normally given from the tire manufacturer. It is the distance from thecenter of the wheel to the road surface when the car is moving. The manufacturers test for dynamic rollingradius is calculated at a speed of 60 km/h (37.3 mph) with an air pressure of 1.8 bar (26 psi) [or 2.3 bar (33psi) on V-, W-, Y- or ZR tires] and a load value generally between 30-50% of the maximum load value thatwas used to calculate the static radius. The spec. dynamic rolling radius shown above will change according tothe chosen rim width.

The spec. tire stiffness rate is derived from the tire deflection based on spec. load at spec. air pressurefrom the manufacturer and the deflection at that spec. It is static therefore the spec. tire stiffness rate shownabove will only change according to the chosen rim width.

The tire air volume is the volume of air between tire and rim for a non-loaded tire at spec. air pressure.

The vehicle tire section width (at vehicle load, air pressure and speed) is the distance between a tire'ssidewalls and measured at the widest part of the tire while mounted on the chosen rim. This value iscalculated under vehicle corner load, at the chosen tire air pressure depending on the load index and speedrating of each tire. The vehicle tire section width shown above will change according to the chosen rim width,vehicle corner load, chosen air pressure and vehicle speed.

The vehicle tire section height (at vehicle load, air pressure and speed) is the radial distance between atire's rim and the outside diameter of the tire while mounted on the chosen rim. This value is calculated undervehicle corner load, at the chosen tire air pressure depending on the load index and speed rating of each tire.The vehicle tire section height shown above will change according to the chosen rim width, vehicle cornerload, chosen air pressure and vehicle speed.

The vehicle tire deflection (at vehicle load, air pressure and speed) is the change in radial distance fromunloaded tire of the tire at vehicle corner load, air pressure and speed while mounted on the chosen rim. Thevehicle tire deflection shown above will change according to the chosen rim width, vehicle corner load, chosenair pressure and vehicle speed.

The vehicle tire rolling circumference (at vehicle load, air pressure and speed) is based on the TRC fromthe manufacturer specs, but includes the inputs of air pressure, corner loads, and vehicle speed. This value isa function of vehicle speed, tire section width and height, tire load rating, tire speed rating, and tire airpressures. Realistic values for car tires are approx 2m. The vehicle tire rolling circumference shown above willchange according to the chosen rim width, vehicle corner load, chosen air pressure and vehicle speed.

The vehicle dynamic rolling radius (at vehicle load, air pressure and speed) is based on the DRR from themanufacturer specs, but includes the inputs of air pressure, corner loads, and vehicle speed. This value is afunction of vehicle speed, tire section width and height, tire load rating, tire speed rating, and tire airpressures. Realistic values for car tires are approx 2m. The vehicle dynamic rolling radius shown above willchange according to the chosen rim width, vehicle corner load, chosen air pressure and vehicle speed. Forstatic radius at vehicle corner load and vehicle air pressure input 0 for speed.

The vehicle tire stiffness rate (at vehicle air pressure and speed) is derived from the tire deflection basedon vehicle static load radius and dynamic rolling radius from the manufacturer specs. It includes the inputs ofair pressure and vehicle speed. This value is a function of vehicle speed, tire section width and height, tireload rating, tire speed rating, and tire air pressures. The tire stiffness rate shown above will change accordingto the chosen rim width, chosen air pressure and vehicle speed. Once established, the tire stiffness rate WILLchange due to changes in air pressure and slightly when the vehicle speed changes. Any changes in loadDOES NOT affect a change in the tire stiffness rate. Think of it like a spring. The more (or less) load youput on a spring does not change its rate.

The contact patch width is the tread width at the road surface. This value is a function of vehicle tiresection width, and aspect ratio. The contact patch width shown above is per ETRTO specifications. Most of thetire deflection occurs in the sidewalls; however the contact patch width will change slightly due to air pressureand corner load changes.

The contact patch length is the tread length at the road surface. The contact patch length is derived fromthe sidewall deflection. The sidewall deflection is a function of the tire stiffness rate (which has the airpressure and speed figured in) and the vehicle corner load. The contact patch length shown above will changeaccording to the chosen rim width, vehicle corner load, tire air pressure and the vehicle speed.

The contact patch area (also known as the tire footprint) is the tread area at the road surface. This is ofcourse the contact patch width multiplied by the contact patch length. The contact patch area shown abovewill change according to the chosen rim width, vehicle corner load, and tire air pressure and the vehiclespeed.

The speedometer reading for alternative tires is calculated at vehicle speed. Tire 1 is assumed to be at thecorrect speedometer reading. Tire 2 is compared to Tire 1. The speedometer deviation should not be morethan +/-8% or +/-8km/h (+/-5 mph) at 100 km/h (62.1 mph) [check your local laws].

The tire suspension comfort value is more important for low cross-section tires (low AR). These values arefrom 0 (very bad) up to 10 (very good). The value depends on the vehicle tire stiffness rate, the vehicle tiresection height, the tire aspect ratio, and tire index ratings. This dictates the suspension comfort using thebase values gained from trials with different tires (these values are not the result of any ETRTO standards).The tire suspension comfort values shown above are dynamic and will change according to the chosen rimwidth, the tire aspect ratio, tire index ratings, vehicle corner load, tire air pressure and the vehicle speed.

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