Average Speed Clock Shown In An Older Rally Book

As is probably evident to anyone who has looked at this website, I am always interested in older books on rallying.  Because of this interest I have added another such book to my library.  This latest book entitled “Rallies and Trials” was written by S. C. H. Davis in 1951.  Davis was a writer for “The Autocar” and a well-known English racer, rally driver, and trials driver.  By 1951 he had been rallying for over 30 years, therefore this book goes back into the earliest days of rallying.

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S. C. H. Davis Was A Major Figure In English Motorsports In The First Part Of The 20th Century

One of the most interesting things that I saw in the book was an image of an average speed clock.  I had heard of these clocks, but I had never seen what these clocks actually looked like.  An example of this type of clock is shown in the image below taken from “Rallies and Trials”.

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An Average Speed Clock

These clocks were fitted with average speed rings that were made for various average speeds.  Remember in those days, in rallies in both England and the continental rallies, the competitors were required to go from control to control at one assigned average speed or the competitors were given arrival times and locations of the controls, and therefore the navigators had to determine the average speed needed to arrive at the next control on time.  Teams had to check into the controls on the assigned minute.

Note the navigator’s station in the car shown in the image above.  There is a mechanical time-of-day clock and the average time clock.  I can not see a special odometer, therefore in this car I assume that the navigator used the odometer that came with the standard car.

In the picture of the navigator’s station, note the set screw at the bottom of the average speed clock.  This set screw allowed the navigator to fit different average speed rings on the average speed clock.

Note the base of the average speed clock is a “normal” time-of-day clock.  At the start time of a run from one control to the next control, the average speed clock would be set to the 12 o’clock position and the speed ring would be positioned to have the 0 mileage also be at the 12 o’clock position.

In the example shown in the above image the average speed clock is fitted with a 40 miles per hour speed ring.  As the minute hand works it way around the clock face it points at a required mileage that should be driven to maintain an average speed of 40 miles per hour.  One rotation of the minute hand would indicate 40 miles.  The exterior ring is to indicate distances in excess of 40 miles.  The smaller hour  hand would indicate the number of miles that must be added to the value shown by the larger minute hand.  The navigator would compare this mileage with the mileage indicated by the car’s odometer to determine if the car was on pace to arrive at the next control on time.  This timing system would be very suitable for the type of events that were common many years ago.

As shown in the accompanying notes with the above image, the car should have traveled 30 miles to be on time.

I have never seen one of these average speed clocks and I suspect that they are quite rare.  It is not clear to me if the clocks and the average speed rings were specially built to function as average speed clocks, or whether navigators bought  or fabricated the average speed rings and then looked for a clock that would accept the average speed rings.

If you can add additional information about these average speed clocks, then I would certainly appreciate if you would share this information.

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2 Responses to Average Speed Clock Shown In An Older Rally Book

  1. I just came across this book on Amazon. How was the rest of the book? Thinking about ordering it.

    • Hi Dave,
      Keep in mind that this is an old book. It was first published in 1951. The main topics are rallies from the 1920s up to 1951. It appears to be written especially for the English rally crowd at that time. Lots of names are mentioned who I had never heard of. I bought the book for a historical look at rallying (which was mostly in England and Europe) at that time. From that perspective it is a good book. Not much of the book is directly transferable to today’s rallying. I enjoyed the book and I am glad that I bought it as I learned a great deal about rallying in the first half of the 20th century.

      Steve McKelvie

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