Icons Unravelled

Words by Jim Reynoldse Photography by Perter Baumber

Words by Jim Reynoldse Photography by Perter Baumber

It's best known for its pungent smell that hangs in the air long after the bike has disappeared. Through the pioneer years of racing, castor based oil was the lubricant of choice: "It's difficult to get an engine to seize on R, because the natural oils have such good lubricity," explains Mike Bowen, Technical Manager at Morris Lubricants. "And it mixes nicely with methanol, so it's a favourite for speedway engines." In the early days of racing, up to the 1950s, alcohol blends were allowed in most branches of the sport and castor oil ruled, produced by giants like Esso, Shell and Mobil with the smaller British Castrol company.

For any fluid to act as a lubricant, it must first be "polar" enough to wet the moving surfaces. It must also have a high resistance to surface boiling and vaporization at the temperatures encountered. Ideally the fluid should have 'oiliness', which is difficult to measure but generally requires a large molecular structure. Castor oil meets these rather simple requirements in an engine, with only one drawback in that it is thermally unstable. This unusual instability is the thing that lets castor oil lubricate at temperatures well beyond those at which most synthetics will work. Castor oil is roughly 87% triglyceride of ricinoleic acid, an ester derived from glycerol and three fatty acids. It has the right make-up to keep the lubricating quality when working hard.

As the temperature increases, it loses one molecule of water and becomes a 'drying' oil. Castor oil has excellent storage stability at room temperatures, but it polymerizes rapidly as the temperature rises. As it polymerizes, it forms ever-heavier 'oils' that are rich in esters. These esters do not begin to decompose until the temperature hits some 340 deg C). Castor oil forms huge molecular structures at these temperatures - in other words, as the temperature goes up, the castor oil exposed to these temperatures responds by becoming an even better lubricant! Hence its use in competition engines.

Castor oil comes from a number of natural sources, like sunflower or rape seed or soya bean. Such a pure source has a strong appeal in another area of modern life, and the pharmaceutical and cosmetic industries use it because it's kind to the human skin and can be

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