Many people wonder what harness racing is and how it differs from 'traditional' racing at tracks such as Canterbury Park in Shakopee, Minnesota. At Canterbury, the horse racing consists of jockeys riding Thoroughbreds at a gallop, which is the fastest speed of horses and is a four-beat gait.
At Running Aces (near Forest Lake, MN) and other harness tracks light, 2-wheeled carts called sulkies sit directly behind the horse. 'Harness racing' is the most popular term for this type of racing, but because of the unique use of the sulky, it's also known as sulky racing or sulky harness racing. Since the horse handlers sit in the sulkies instead of astride the horses, they're called drivers instead of jockeys. Another common name for them is "tail-sitters," since they used to sit on the horses' tails in order to keep from getting hit in the face with the hair.
Trotting vs. Pacing
In sulky racing, Standardbreds are raced at either a trot or a pace. Both trotting and pacing are two-beat gaits, but differ from each other by which legs work in unison on the horse. At a trot, a horse's diagonal legs move forward and backward together (i.e., left front and right hind/right front and left hind move in tandem). When pacing, both legs on the same side of the horse move forward and back together.
Breeds that Pace
Pacing isn't a normal gait for most breeds of horses, and in fact only a handful have a propensity to perform the gait naturally: the Standardbred, Peruvian Paso and Icelandic horse. While the pacing gaits of the Peruvian Paso and the Icelandic horses can be quite comfortable for a rider, in order to move at racing speeds, the Standardbred's pace is much too uncomfortable and awkward for jockeys, hence the need to race them from a cart.
More Differences Between Harness Racing & Traditional Horse Racing
As opposed to using a starting gate to begin each race as in traditional horse racing, in harness racing the main form of the start is a vehicle with a folding gate attached to the rear end, called a motorized starting gate. The vehicle drives in front of the horses and upon hitting the actual starting point, speeds up while folding the gate and moving to the side of the track to allow the sulky racers room to race.
The term 'harness racing' most likely comes not from the use of the sulky but from the trappings the pacers wear. These 'hobbles' aren't harnesses to make the horses perform the pacing gait; rather they're to ensure the horses don't break into a gallop and they also maximize the effectiveness of the pacing gait during a race.
Wikipedia has more information on harness racing.