The deal between Benetton and Toleman was only agreed in October 1985, leaving precious little time before the first race of the 1986 season on March 23 in Brazil. Byrne only started designing the B186 when the engine-supply deal was made with BMW in the last week of October. Nevertheless, three B186s were built in time for this race, and a further four chassis were completed during the course of the season. As had become standard practice in the sport in the 1980s, the monocoque chassis were constructed from carbon fibre. The first part of the season was chiefly concerned with producing enough spare parts for the existing cars at the expense of chassis development and improvement. All seven chassis raced in the hands of either Fabi or Berger during the course of the year; chassis number 4 was written off by the former in a crash during practice for the Canadian Grand Prix and chassis number 7 was the only race-winning B186, at the Mexican Grand Prix.