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Given that the riders don’t all race in heats to qualify for an AMA National, but instead go out in timed qualifying session to take the first 36 spots (with four riders in each class selected from two short Last Chance motos), we don’t know why the entry list is limited to 90 riders and not 100 or 120 (given that the sanctioning body gets over $40,000 in entry fees if they draw 90 riders in both classes). It seems as though encouraging more local riders to sign-up for their local AMA National would help grow the sport. However, only two races in the 2016 AMA 450 class ever had 90 riders go through timed qualifying (Mt. Morris and Southwick). The most riders ever to try to qualify in the 250 Nationals was 87 at Red Bud. On average, 69 riders tried qualify in the 250 class across the 12 AMA Nationals—that is 21 riders short of a full field. The 450 class had, on average, 78 riders—which was only 12 riders short of a full field.
The least number of 450 riders trying to qualify in the 450 class were at the opening round at Hangtown (55). In the 250 class, Washougal only had 54 riders try to clock times.
Part of the reason that more of the races don’t have full fields is that riders sign-up, but don’t show up. Part of the reason that the sanctioning body doesn’t accept more than 90 riders is because of pit space at some tracks (although whoever is in charge of parking the trucks and semis in the AMA pits could do a much better job, as anyone who has ever been to a NASCAR, IndyCar or Formula 1 race can attest to).