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Explanation: The dictionary (Langenscheidt's Pocket French) definiton of d'autant plus...que is "all the more...as" which is very hard to incorporate into written English that flows well. For your text, it can be written "the suspension capacity becomes weaker (or is weakened) as the diameter becomes smaller (is lessened) and the surface temperature is raised." OR "The smaller the diamter and higher the surface temperature, the weaker the suspension capacity."