G/M MPH

The following procedure illustrates how to create a math channel in the DataLink II software to calculate vehicle speed using the G-Meter, which is available on many of the Racepak systems.

First, open the file to which you wish to add the new channel. If it is a run file, simply open it from the ‘File’ menu in the DataLink II software. If you wish to add it to your configuration file, so the channel will appear in all subsequent downloaded files, the select ‘Open Car Configuration’ from the ‘File’ menu, and select the appropriate configuration file from the available list.

Once the desired file has been opened in the software, select the ‘Channel Maintenance’ option under the ‘Edit’ menu.

Click on the channel in the list where you would like to insert the new channel. In this example, we have selected a position after the last channel, so the position after ‘Pan Press’ has been highlighted. Click on ‘Easy Insert Channel’.

Now we need to select the type of channel we will be inserting. Since we are going to be using calculus to integrate the G-Meter readings, we should select ‘Integrator’ from the list. By default, the only available type in this instance is expression (EXPR). Click ‘OK’.

The G-Meter value indicates the rate of acceleration of the vehicle. Integrating this value over time, allows us to calculate the speed, since we know the rate of acceleration and how long the vehicle has been accelerating at that rate. In order to properly create this channel, name it ‘G/M MPH’ (or something similar) in the ‘Name’ text box. Then, click your mouse so the cursor appears in the ‘Expr’ text box. The next step is to scroll down the list of ‘Currently Defined Channels’ in the left hand column and select the ‘Accel G’ channel by double-clicking on it, which will add it to the ‘Expr’ text box. In order to complete the necessary math, please complete the expression as indicated below.

From this dialog, you can see the math which takes the value of the G-Meter (‘Accel G’), multiplies it by 32.17, which represents standard gravity (32.17 ft/s2) and then further multiplies the value by the constant of 0.6818, which is the result of converting feet to miles and seconds to hours.

It is important to note that in order for this channel to display correctly, you must ensure the following items are correct:

  • The G-Meter is mounted flat and level
  • The start time has been zeroed using the G-Meter value
  • The “Start Integrator From Zero” checkbox has been checked

Also, be sure to scale the minimum and maximum values correctly for your application. Many cars use ‘0’ as the minimum value and ‘250’ as the maximum. In this example from a Top Fuel car, the maximum has been set to ‘330’. You may wish to change the result units to ‘MPH’.

Simply click ‘OK’ to add the channel to the list, and then ‘OK’ a second time to return to the main screen of the DataLink II software. You should see the channel button ‘G/M MPH’ appear in the location into which you inserted it. If you are satisfied with the results, be sure to click ‘Save’ from the ‘File’ menu.

For questions or assistance in adding these channels to your system, please contact us at Performance Data Systems at (949) 652-6300 or email us at support@racepds.com.