Motec Wheel Speed Calibration

[Viper_Race_HQ]

For those Competition Coupe teams that have not heard, Hoosier Tire and the Viper Racing League have decided to change the VRL spec tire for the 2004 season. The result is a smaller rear tire (in diameter) and slightly smaller in section width. Several teams have inquired about changing the wheel speed calibration for the Motec dash. Viper Race HQ has not tested the new spec tire proposed by Hoosier, however, instructions are provided below for measurements and the calibration change.

Wheel speed is sent from the Engine Controller to the Motec dash. This wheel speed is based on the rear wheel diameter. Therefore, changes to the rear tire diameter will affect the accuracy of the wheel speed calibration. The following steps should be taken to correct for various tire diameters:

1) Measure or obtain the loaded radius of the new tire. You can calculate this by measuring the loaded circumference of the wheel. This is easily measured by performing a “rollout.”

a. Mark a vertical chalk line at the center of the tire where it meets the ground. Mark a corresponding line at the ground.
b. Roll the vehicle in a straight line, on level ground, until the tire has completed 3 complete revolutions (make sure you set your tire at hot pressures ~ 34-36psi).
c. Measure the ground distance that the tire has moved.
d. Take this measurement and divide by (3*2*PI) or (18.85). This is the loaded radius of the tire.

2) Next, determine the ratio that the tire radius has changed.

a. The current calibration is based on a loaded radius of about 336mm.
b. Take your new radius and divide by 336mm.

3) Multiply this ratio by 1609 (the current calibration multiplier). This is your new calibration multiplier and can be changed as follows:

a. Open your Motec calibration file
b. Select “Inputs -> Communications…”
c. The tab marked “RS232” should appear.

d. Click the “Advanced” button.
e. Select the “Received Channels” tab.
f. Highlight the channel indicated as “Wheel Speed – JTEC” and select the “Change” button. The following screen will appear:

g. Change the multiplier from 1609 to your new multiplier (rounded to the neared whole number – no decimals).
h. Save your configuration and upload to vehicle.

4) This should result in a close approximation of true ground speed, but does not account for tire growth with speed, wear, build uniformity, pressure, etc. A radar gun or street car is always a good check to ensure a proper correction has been input.

Hoosier estimates that the loaded radius of the new tires may be around 316mm at hot pressure. This will result in a new multiplier of about 1513. Your result should be close to this.

Good Luck!

Viper Race HQ