Stupid BSF question

[miraja2]

BSFs consist of two elements:
1) A speed rating, which translates the time of every race into a number.
2) A track variant, which measures the swiftness of the racing surface on a given day.

You combine the two to get the BSF. So if a horse runs a race with a speed rating of 90, on a track with a variant of -10, that horse would receive an 80.

The track variant often changes from one day to the next.