ESO

250cc - 1959

Unrealized potential

In 1954, the head of the ESO design office) Simandl, became ill, and was replaced by Jaroslaw Cervinka. He began developing the company's first motocross power units in parallel with (and based on) its speedway engines. This 250 was one of three displacements available.

Powerful Engine

Most of the bikes built were sold to Czech riders, and were rarely seen outside the country's borders, though the engines were notable for their power and performance – the ESO speedway engine had certainly displaced the JAP in its particular discipline. But there's a long way between the conception of an excellent engine and the building of a complete motorcycle, and the ESO engineers found it too much of a step to take without stumbling.

Heavy Frame

Basically, this potent engine was afflicted with an excessively heavy frame. Early versions were also fitted with a leading link fork, rapidly replaced by conventional telescopic forks purchased from Jawa. Unlike the marque's speedway machines, the motocross bikes never realized their potential; their best result in the 250cc class was Miroslav Soucek's third placing in the 1960 European championship. In 1962, ESO built an engine for Soucek with desmodromic valve gear. It developed a healthy 27 hp at 8000 rpm, but again was handicapped by the weight of its frame, leaving the door wide open for Dave Bickers's Greeves.

SPECIFICATIONS

Engine: 248cc (70x64.5mm) ESO air-cooled single-cylinder four-stroke, Pal or Lucas magneto

Power Rating: 24 hp @ 7600 rpm

Valves: overhead

Fuel System: 28mm Eso or Dell'Orto carburetor

Transmission: 4-speed, chain final drive

Suspension: telescopic forks (front); swing arm (rear)

Brakes: drum (front & rear)

Wheels: 21 inch (front); 18 or 19 inch, dependent on year (rear)

Weight: 265 lb

Built in both 350 and 500cc forms, the ultra modem ESO engines failed to live up to their promise, due mainly to excessively heavy frame construction.