|The TRC calculator can convert the tyre-geometrics from the
tyre sizes to the ETRTO standard (European Tyre and Rim Technical Organisation).
NB. Your Browser must be able to run Java to be able to use the program.
An explanation for each computed value can be found at the
foot of the page after clicking 'Calculate'. To calculate the tyre-geometric data, we need the following input:
Tyre Width, the
Aspect Ratio (SH/SW) and
the Rim Diameter.
|Explanation of the input data
The Tyre width is in mm and is shown as the
first set of numbers on the tyre wall, for example; 205 mm as on a tyre size 205/55 R 16 V.
The Aspect Ratio or tyre height/width ratio
(The Aspect ratio) is measured as followed section hight of tyre
(SH) divided by the section width of tyre (SW) and this gives the result
as a percentage. A tyre with the size 205/50 ZR 16 has an Aspect Ratio of
** If no value is given, for instance as in 165 R 15S, then the Aspect
Ratio is 80%.
The rim diameter is usually in inches. A rim
size of 8Jx17 has a diameter of 17" or 431.8 mm.
|Explanation ofthe output data
|The following details for the tyre size are valid for those in
ETRTO-Standard laid down for tyre load and tyre air-pressures,assuming no wear on standard profile.
The Tyre Rolling Circumference is the length
of one rotation of the wheel (circumference), with the axle-weight influencing the result. Realistic values for car tyres are approx 2m.
The static rolling radius is the distance from the
middle of the wheel to the road surface when the car is stood still.
Realistic values for car tyres amount to roughly 30 cm.
The difference between the static rolling radius & the
dynamic rolling radius is that the distance from the middle of the wheel
to the tarmac is calculated for a speed of 60 km/h (36.7 mph). Higher speeds cause the tyre to slightly inflate and increase this value
For alternative tyres this is the
speedo-deviation at 100 km/h compared to the original; for instance
185/65 R 15 is used as a reference for Tyre 1. The speedo-deviation cannot be more than +8% or
+8km/h at 100 km/h (56mph) by law.
For the tyre suspension comfort value, this is more important for
low cross-section tyres (low AR). These values are from 0 (very bad) up to 10 (very good). The value depends on the height of the
tyre shoulder and this dictates the suspension comfort using the base values
gained from trials with different tyres. These values are not the result of any ETRTO standards
The value for the tyre volume is the volume of air
between tyre and rim.