Since Affirmed became the last Triple Crown winner in 1978, there have been 13 horses that entered the Belmont Stakes with a chance to expand the club to an even dozen. Every one has come up short, adding to the Triple Crown’s luster as perhaps the most difficult feat in sports.

With nine of those failed attempts coming in the past 18 years, racing fans have become well-versed in this cycle of hype, hope and heartbreak. That has not stopped them from climbing aboard the bandwagon as American Pharoah tries to end the 37-year Triple Crown drought in Saturday’s Belmont Stakes. Can he do it? Perhaps. But here are five reasons why the odds for a Triple Crown remain long, even for a star like American Pharoah.

1. The distance. The Belmont Stakes remains the only major race run at a mile and a half. Pedigrees can give some indication of whether a horse can get the distance, but trainers acknowledge that no one can predict which horses can handle this equine marathon until they try it.

2. The track. Belmont Park is a unique, 1½-mile oval with wide, sweeping turns and a deep, sandy surface. Some horses do not like running on it, and jockeys often find it tricky to gauge when they should make their move. Several Triple Crown favorites have faded after their riders sent them to the lead too early. The forecast calls for rain through most of the week, which would be advantageous to American Pharoah, who has two victories on sloppy tracks.

3. The new shooters. Steve Coburn, owner of California Chrome, incited debate last year when he labeled the owners of Tonalist “cheaters” after their well-rested horse upset Chrome’s bid to win the Triple Crown. Coburn said it was unfair that his horse had to face rivals who had not run both the Kentucky Derby and the Preakness. But the three races always have been separate entities, and purists believe a true Triple Crown champion should be able to defeat all comers. American Pharoah will be the only horse to run all three races this year.

4. The schedule. As breeders have sacrificed stamina for speed, some observers believe that there will never be another Triple Crown winner unless the format — three races in five weeks — is changed to reflect that evolution.

5. Racing luck. Many favorites have been undone by misfortune. Spectacular Bid stepped on a pin the morning of the Belmont, War Emblem stumbled at the start and Charismatic broke his leg in the stretch.