Dirt racing on loose-surfaced mile and half mile circuits is the backbone of American competition. Following eligibility changes in 1969 the racing between 750cc ohv twins for the 's coveted Number One Plate was extremely close, with BSA, H-D, Norton, Triumph, and Yamaha all capable of winning on the day.
A situation that continued until Harley's XR eventually produced considerably more horsepower than all the vertical twins. Thus prompting Kenny Roberts into racing that modified Z7S0 Yamaha at the Indianapolis Mile in 1 , and winning, but on a machine that even he claimed was close to lethal. The AMA banned four cylinder two-strokes on the dirt soon after.
Hundreds of American townships sported suitable tracks, for traditional local fairgrounds invariably possessed a horse-trotting oval between 800 and 1800 yards in length, usually with a grandstand alongside.
The bike pits would be located in the infield and mechanics were obliged to wear white overalls. Ascot Park, in Los Angeles, was the exception as this purpose built 1000-yard track had no equine connections. In constant use from March to October, Ascot had an almost dilapidated appearance by day, but morphed into a magnetic attraction after dusk wherein the racing for the season long Dirt Championship was held at night under floodlights.
The track naturally had a few local specialists who'd give any of the country's top riders a run whenever they dared appear and because Friday night half miles had run for so long, some of the regular 'wrenches' overalls could best be described as off-white.
There was another phenomenon about Ascot, unproven, but worth mentioning. At night, apparently, the adobe surface drew in moisture from the atmosphere, causing a major change to the track's grip factor. The After Eight type surface that evolved as the sun went down, after completion of practice, suited the characteristics of Norton's engine admirably, giving their riders more grip than rivals.
This is borne out by Norton's relative lack of success whenever performing at Ascot in daytime. The Friday night races nearly disappeared in 1969, when long time promoter J C Agajanian lost interest due to better paying national events.
Fortunately, Bruce Cox and Gavin Trippe - who'd both worked for the British weeklies - had just arrived in USA, and had recently established their own publication, Motor Cycle Weekly. They stepped into the Ascot vacuum, took over from Aggy, and very successfully resuscitated the Friday night event, later going on to promote the US 250 MXGP at Carlsbad, an F750 race at Laguna Seca - the first time for bikes - as well as creating the popular UK Transatlantic series at Easter time.:)