VintageRally.com

November 2003     

A Review of Curta Techiques for the Rallyist
About John Burns
by Walt Kammer:

The author John Burns lived in Buffalo NY until his death many years ago. He was a member of Western New York Region SCCA and rallied with much success in many venues. He was involved with Rallysport a long long time before his death. As part of his company, Burns Industries, he was one of the very early importers of Curta equipment from the Old Country. In fact he may have been the first, even before Haan got involved. He sold them in the USA both at the wholesale and retail level, although his real business was NOT rally equipment; he just imported neat stuff as part of his normal operations.

Many of the folks in the Northeast USA got their equipment from John. Probably even more than known since John shipped equipment to other "dealers" who usually got one for themselves, sold a couple to their friends and were never heard from again.

A Review of CURTA Techniques for the Rallyist - Peppermill Potpourri

Article by John B. Burns
- From Sports Car Magazine February 1961
(transcribed by Bill Jonesi)

By this time most rallyists either own, have access to, or at least are generally familiar with, the use of a CURTA Calculator as a rally instrument. However, due to the extreme flexibility of the machine and the ingeniousness of many different addicts, a review of some of the methods and advanced techniques may be in order. There are two schools of thought on the basic operation of the CURTA. "Minutes-per-Mile", or "Miles-per-Minute"? Opinion seems to be about evenly divided in the East, although on the West Coast the majority appear to prefer using "Miles-per-Minute". Prefigured tables are available for converting average speeds to the necessary factors for use with either system. Or you can work out the factors with the CURTA. To get the "Minutes-per-Mile" for any given average speed, you merely divide the speed into 60.

With the "Miles-per-Minute" system the rallyist puts the distance to be traveled in one minute into the CURTA, and obtains the answer in terms of how far he will go in a given time. In other words, he cranks time into the counter dial and reads answers in terms of distance in the result dial.

In the "Minutes-per-Mile" system the time it takes to go one mile at a given average speed is set into the keyboard. The CURTA is cranked up to correspond with odometer readings, and the answer is expressed in minutes and decimal parts of minutes required to go the indicated distance. Pauses, delays, etc. are easily handled without manipulating watches. You merely add the necessary time into the proper dial.

Either system will produce basically identical results. It is mostly a matter of how you can get started and which seems the easiest method for you. In either case speed changes can readily be made without pausing. You merely note the odometer reading at the speed change point, crank the CURTA until the answer or indicator dial (depending on which system is being used) agrees, and then set in the new speed factor.

There is some argument as to how precise the speed factors set into the machine should be. Many top rallyists advocate using a "Minutes-per-Mile" factor to as many as five decimal places. Others feel that three decimal places are adequate, and that anything more is a waste of time due to other considerations which can't be controlled with similar accuracy. I am inclined to agree with the latter though. For example, at a speed of 36.56 mph to three places we have 1.641 minutes per mile, while to five places we have 1.64114. This would only make a difference of approximately 8 seconds in a 1,000 miles. If we are to be this accurate we will have to check the reflexes of each check point timer immediately before the rally so that we can determine how long it will take him to stop his watch. Also, speed-change points are seldom located with this degree of accuracy. The same thing applies to odometer correction factors, which can be carried out to similar extremes.

Odometer correction factors such as .9856 or 101.14 are about as close as one can live with. They are going to change somewhat during the rally, regardless of the type of tire or the air pressure you are carrying.

With the "Minutes-per-Mile" system, the mathematics of recovering from being "off course" are easy. At the point of turning around to retrace your route back to the point where you left the course, note the mileage and crank it into the CURTA. Push down the reversing lever on the machine and make subtractive turns of the operating handle to correspond to your odometer readings until you return to the point where you went off course. Because the CURTA has been adding distance and subtracting time, it will show the time that you should have been there in the first place. Return the operating handle and reversing lever to their original positions, and start making up the lost time. With this method you won't have to reset odometers or watches, nor will you have any problems with speed changes should they come up before you have made up the extra distance.

With the larger Type II machine, it is practical to collect odometer mileage while running, and to keep a record of not only time and odometer mileage but also of official mileage. Assume that at the end of a 20-mile odometer check leg you come up with a correction factor of .9864 (by dividing the official mileage into your odometer mileage). Put .9864 into the left side of the setting dial (slides 11, 10, 9, 8). Crank up the CURTA until the white indicator dial shows the official mileage, at which time the left hand side of the answer dial should show the odometer mileage. Set the time at which you should leave the odometer check into the right side of the answer dial. This time can either be in time-of-day or elapsed time from the start of the leg, which ever you prefer. If you are using time-of-day, ignore the hours and subtract 60 every time another hour comes up. This will prevent the possibility of overcrowding the answer dial with the two sets of figures, and also make it a bit easier to read. In any event, remember that if you are putting time into the CURTA, that it is necessary to make a subtractive turn so as not to change the mileage. Now that the time and distance is properly set up, put the minutes-per-mile at the official average speed into the right side of the setting dial (slides 4, 3, 2, 1) , and you are ready for departure. The white dial will show the overall official mileage, the left side of the answer dial the odometer mileage, and the right side time. At each open checkpoint you can clear odometer mileage and/or time from the answer dial, and still retain the overall official mileage.

Some of these procedures may sound complicated, but remember that with the CURTA you can run an entire rally including any number of speed changes, pauses, or what have you with only one watch and without having to reset an odometer.

Warning for 1961! Don't try to outguess the organizers. For example, if your average speed is 40 mph and the instructions say to reduce speed by 15% for 8 miles, don't figure that this means a pause of two minutes and seven seconds which can be taken either at the beginning or the end of the 8 miles. Reduce your speed to 34 mph, and run the 8 official miles as accurately as possible. There is no reason for not putting a checkpoint in anywhere during the 8-mile stretch. This warning needn't apply if the speed reduction comes up as you are entering a congested area, but otherwise be alert. You could get caught.

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