Even the most rational trainers can harbor a streak of irrationality about jockeys. Usually this is a harmless idiosyncrasy, but today the riding assignments may well determine the outcome of the Laurel Futurity.

When Spectacular Bld, General Assembly and Tim The Tiger battle for a purse of $142,280 and the championship of their age group, each trainer should want the best available jockey riding for him. But for reasons that defy analysis, two of the trainers have replaced indisputably great jockeys on their horses.

Leroy Jolley held such lofty hopes for General Assembly at the start of his career that he brought brilliant young Darrell McHargue from California to New York to ride him. McHargue scored three straight victories on the son of Secretariat before losing a photo finish to Tim The Tiger in the Cowdin Stakes. It is easier for a trainer to blame his jockey than to acknowledge that his horse isn't as good as he thought. So Jolley blamed McHargue for keeping General Assembly too close to the early pace, and fired him.

Jolley engaged red-hot Angel Cordero Jr. for the Champagne Stakes at Belmont. Heeding his predecessor's fate, Cordero kept General Assembly off the early pace, set by Spectacular Bid, who led all the way. Cordero was fired.

Steve Cauthen will get the mount on General Assembly today. Cauthen has been riding so dismally for months that New Yorkers cannot believe he is the same teenager who set the racing world on fire last year. Yet even the Cauthen of 1977 might not know how to ride a horse who is only second-best in the race and whose trainer doesn't want him on the lead or off the lead.

Spectacular Bid probably is a better horse then General Assembly. But he took longer to show his ability because apprentice jockey Ronald Franklin has compiles some excellent been Buddy Delp's regular stable jockey during a season when the trainer has been sensationally successful, and because of his good mounts Frankliness compiles some excellent statistics. But statistics can lie, and in this case they tell a whopper, for Franklin barely rates as a mediocre rider.

Spectacular Bid lost his first two stakes-races after getting rides that eyewitnesses said were atrocious even by Franklin's usual standards, but finally showed his true colors when he carried the apprentice to a 15-length victory at Atlantic City. That performance earned the colt a berth in the Champagne Stakes, and there Delp used the services of the great Jorge Velasquez, who rode him flawlessly.

But this afternoon, Spectacular Bid's rider will be not Velasquez, but Ronald J. Franklin. This is partly a sentimental gesture, an act of loyalty to the rider who has won so many races for him this year.

Delp explained that he wanted to boost Franklin's chances to win on Eclipse Award as the year's best apprentice jockey. But as one local trainer observed, "It's like going to the Super Bowl and starting your third-string quarterback because he's a fine kid with a nice Christian upbringing."

The only trainer of a Futurity contender who is not changing jockeys is John Veitch. Jeffrey Fell rode Tim The Tiger to five straight vitories before losing badly in the Champagne Stakes. Veitch said that his horse might have been physically drained after a tough race the week before, thus accounting for the defeat, but he did not object to Fell's ride. For that, he could be disbarred from his profession.