One thing i noticed is that if the 2nd reserve were to only have 3 rides, he'd have to be replaced in either heat 8 or 9, depending on wether they were 6 or 7. Their next appearance would be heat 12, so if they were replaced in 8/9, they could go have to wait 7 or 8 heats to race, after having 2 rides in the first 5 races.