Basically you need to work out how fast your horse is likely to travel carrying a set amount of weight over a set distance. In the case of horse racing underfoot conditions the jockey and the mood and fitness of the individual horse on the day are however variables. However for the purpose of the basics the following is the science part and although the equation used below is not perfect it is one we may all possibly remember from school and is a reasonable starting point for the purpose of this example.
Velocity = mass x acceleration (measured in feet per second)
To obtain an average or constant as a comparison we need to relate the science to the racehorse by using the following figures which obviously need to be converted into feet or seconds to make the equation viable.
An average racehorse is ten foot long
A two mile race is approximately 10560 feet in length
An average two mile race will be run in 4 minutes which is 240 seconds
Therefore 10560 feet divided by 240 seconds equals 44 foot per second divided by 10 foot means a horse running over two miles in distance will be travelling at approximately 4.4 lengths per second.
Now we have an average we need to work in some figures for the going conditions which will be as per the chart below which is applicable for National Hunt racing. These are not industry standard figures and are my own work so feel free to use your own.
Good Ground = 100%
Good To Soft Ground = 103.5%
Soft Ground = 107%
Soft to Heavy = 111%
Heavy Ground = 115%
Now we will attempt to put this altogether and include an allowance for the weight carried on the day using genuine form from a horse called Artifice Sivola which has run over a variety of conditions.
04/03/16 @ Newbury 2m on Good To Soft carrying a weight of 11-12.
The race was run in 4 minutes 17.80 seconds (the time the winner crossed the line) which is 257.8 seconds. Artifice Sivola finished the race 6 lengths behind the winner so we add on at 4.4 lengths a second approximately 1.2 seconds giving you a finishing time of 259 seconds for our runner.
Now divide our runners time of 259 seconds by the distance of the race which is 2m (but officially 16.5 furlongs) which leaves you with a finishing speed of 15.70 seconds a furlong. The ground conditions that day were Good To Soft so you now divide the 15.70 by 103.5% as per the above list and you achieve a figure of 15.17 seconds per furlong. You now have to make an allowance for the weight carried which was 11-12 which is 166 pound. Divide the 15.17 by the 166 and then times by 100 (to give you a workable figure) which in this case is 9.14.
Confused yet.. you should be because I still am as there are lots of ways to do your own figures but if you get a calculation that works for you it can be a money spinner when used in the right circumstances as although speed ratings are primarily used on the flat they are a cracking reference point over shorter trips especially in hurdle races and of course when used in NHF (National Hunt Flat) races.