Red Bull, Williams, Mercedes launch their F1 cars for 2015

JEREZ, Spain (AP) Red Bull and Williams have launched the Formula One cars they hope will be able to better compete with Mercedes when the new season starts in six weeks' time.

Red Bull's RB11 and the Williams FW37 were unveiled Sunday prior to the first day of pre-season testing at the Jerez circuit in southern Spain.

Mercedes, which won the constructors' championship by nearly 300 points, also launched its W06 on Sunday, with championship runner-up Nico Rosberg later taking the wheel on the first day of testing and completing a massive 157 laps. F1 champion Lewis Hamilton drives on Monday.

''Long before the end of last season, development work on our 2015 car became the main priority,'' Toto Wolff, the head of Mercedes-Benz Motorsport, said. ''It would be so dangerous to rest on our laurels after 2014.''

Red Bull finished second overall last year and was the only team to win a race other than Mercedes, with Australian driver Daniel Ricciardo winning three GPs as Sebastian Vettel struggled. Williams finished third in the constructors' championship, a sign for Rosberg that this season's contest could be tougher for Mercedes.

''Red Bull, for example. Just one year ago they were the untouchable team. So you can expect them to be very strong,'' Rosberg said. ''Williams was right on our tail at the end of last season, they finished the last race just three seconds from us. We definitely expect our competitors to close the gap and we have a close eye on them.''

The opening race is on March 15 in Melbourne and Mercedes has been working on improving the chassis and power unit. A maximum of four power units are allowed this year, one less than 2014.

''Expectations are now high and a lot of assumptions are being made about our potential this season,'' Mercedes technical director Paddy Lowe said. ''You have to take risks to progress, but those risks must be carefully managed.''

While Red Bull and Ferrari changed their driver lineups - with Vettel joining Ferrari and two-time champion Fernando Alonso leaving Ferrari to join McLaren - Hamilton and Rosberg are driving together for the third straight season.

The Williams pairing of Felipe Massa and Valtteri Bottas showed encouraging speed last year - with Bottas securing six podium finishes.

The new Williams car features some small changes to the front nose and bulkhead.

''The momentum we carry from last year has been very encouraging,'' said Massa, the championship runner-up in 2008. ''I am excited to be continuing to work with Valtteri, a driver with so much talent who has so much to offer both the team and me.''

After years of stability with Vettel alongside former driver Mark Webber, Red Bull's new pairing of Ricciardo and 20-year-old Daniil Kvyat is exciting and untested.

''We've already seen what Daniel is capable of and we firmly believe that (Kvyat) has all the same attributes,'' team principal Christian Horner said. ''They're young, hungry, they're incredibly quick and I think they are going to push each other very hard.''

Needing to make vast improvements on last year, Red Bull made changes to the lower nose box, the front of the chassis and the Renault power unit.

''The relationship with Renault is a lot closer now,'' Horner said. ''Red Bull is the only partner for Renault now, through ourselves and Toro Rosso, and that focus from Renault - together with a close involvement with our design team - is yielding good results.''