I have been playing around with my Halda Tripmaster, testing a cable set-up, and I thought that I might try to illustrate how these units work and the approach to calibrating the Halda odometers to people not familiar with these classic rally odometers.
The photo below shows the that the big gear, known as the X-Gear, directly drives the digits of the odometer. The faster the X-Gear rotates, then the faster digits rotate and the sooner the numbers in the display advance.
The Large X-Gear Drives The Digital Axle
The photo below shows that there are three metal gears driving the Halda Tripmaster. The smallest gear shown on the left side of the photo below is the Z-Gear, while as noted above the large gear is the X-Gear. The gear linking or meshing the X-Gear and Z-Gear is a 55-tooth idler gear.
The Drive Gears In A Halda Tripmaster
To calibrate the Halda Tripmaster to match up with the rallymaster’s measured distance, rally navigators can change either, or both, of the Z-Gear or the X-Gear. To illustrate why the 55-tooth idler gear is not an issue in calibrating the Halda Tripmaster consider the rough sketch that I prepared below. The number of teeth that I have shown on the Z-Gear and the X-Gear is based on the sizes that came already installed in the Halda Tripmaster when they were shipped to the customer from Halda.
The numbers show that the rate of rotation of the X-Gear is controlled by the rate of rotation of the Z-Gear. Also note that in the example shown above, the rotation of both the Z-Gear and the X-Gear are clockwise, while the Idler Gear rotates counter-clockwise.
The standard Halda calibration is based on the assumption of 1000 rotations of the standard speedometer cable per mile. By changing the Z-Gear or the X-Gear the rally navigator can control the conversion of speed of the speedometer cable and hence the display of the digits in the Halda Tripmaster.
I have prepared the following calibration chart for a Halda odometer based on the standard fitted Halda gears (32-tooth Z-Gear and 141-tooth X-Gear) having a calibration factor of 1000. This is ever so close to Halda’s published value of 1002 for that combination, but for my purposes I used 1000. The absolute value is not as important as is the relationship among the gears to the gear combination that is in the Halda odometer when you first drive over the calibration distance. I have also added in the 45-tooth Z-Gear to my table which is not provided in the Halda published tables and I have provided one place of decimal for the calibration factors for increased precision.
As a calibration example, let us assume that the rallymaster has laid out a benchmark rally distance of 8.23 miles for the calibration run. Let us further assume that your Halda Tripmaster is presently fitted with the standard Halda-provided gears – 32-tooth Z-Gear and a 141-tooth X-Gear with a calibration factor of 1000. You drive the designated route and your indicated or measured distance is 8.13 miles. Now the navigator has to determine what gear changes are needed to increase the accuracy of the Halda Tripmaster in order to better match up with the rallymaster’s distances.
The new required calibration factor can be determined using the following equation:
New Factor = (Measured Distance/Official Distance) X Old Factor
In this case, the new factor is as follows:
New Factor = (8.13/8.23) X 1000 = 987.8
Using the above calibration chart, the navigator could switch the 141-tooth X-Gear for the 139-tooth X-Gear, leaving the 32-tooth Z-Gear in place, resulting in an installed factor of 985.8. In theory, driving the same course in the same manner with the revised gearing in place, the odometer reading would be 8.24. The limitations of the integer teeth numbers for the X-Gears and the non-continuous number Z-Gears limit the calibration accuracy for the Halda Tripmaster and Halda Twinmaster.
Another way to think about installing the 139-tooth X-Gear in this case in place of the 141-tooth X-Gear is that the smaller X-gear will rotate faster which then causes the digits in the odometer display to change faster, thereby allowing the navigator’s odometer to catch up with the rallymaster’s odometer.
There are many combinations of Z and X-Gears that overlap. This can be seen visually in the following chart.
Navigators who are interested in competing in a variety of events both in North America and overseas, like I am, should be prepared to compete in events measured in miles or kilometers, especially navigators in North America who use miles in the USA and kilometres in Canada. Using Halda odometers for miles and odometers requires having an expanded range of calibration gears on hand. The chart below shows that you do not necessarily need to have a complete set of gears on hand, but that you should have gears that will allow for the calibration of the Halda odometer in the expected range. I have highlighted combinations to consider to provide for correcting distance discrepancies up to plus/minus 3% in both miles and kilometers.
Note that these are only general recommendations and everyone should develop their own needs list based on their car and their tire sizes.
I hope that this helps to clarify the calibration theory and procedures for the Halda rally odometers.