Casey Stoner: What now for the double MotoGP champion?

Double MotoGP champion Casey Stoner has retired from the sport after the Valencia Grand Prix.

The Australian shocked the motorcycling world earlier this year when he announced his decision to walk away from the sport at just 27 years of age.

He has battled illness, broken bones, leaving Australia at the age of 14 to go full-time racing in Europe, and rows with most of the paddock - and won.

He rounded out his career with a third-placed finish in Valencia.

But why is Stoner, who won 38 grands prix since making his MotoGP debut in 2006, calling it a day?

And what lies in store for one of the finest racers of his generation? BBC Sport finds out more.

Why is Casey Stoner retiring now?

BBC MotoGP commentator Steve Parrish: "He's angry with the sport. He doesn't enjoy it so much because he is so single-minded - he has to win.

"He struggled with the social side of it, you wouldn't find him having some fun, a joke with the other riders, playing a game of golf or tennis in the week like some do. That didn't enter into his mind at all."

BBC MotoGP presenter Matt Roberts: "Casey has been riding motorcycles since he was 11 months old and started racing at the age of four. It has been the love of his life but since moving to the UK as a teenager it has also been a source of great pressure and stress."

MotoGP journalist and BBC Red Button commentator Gavin Emmett: "Stoner has said from a young age that he wouldn't be racing until he was 30, and whatever you may think about him, he is always honest and true to his word.

"He's now reached a crossroads in his life with the birth of his daughter, and however much he claims this hasn't played a part, I think it makes for a convenient time to hang up the boots."

BBC pitlane reporter Azi Farni: "Burnout. He's been racing bikes since he was four and he's lost the passion. He has never enjoyed the media attention and everything that comes with racing."

MotoGP technical journalist Neil Spalding: "He hasn't done anything else in his life but race - it's been the most important thing in his life. But now he has a family and you only have to see him with his daughter to know that something else is much more important now."

How good is he?

Parrish: "He's the best of his generation. Over the last six years he has outscored everyone."

Roberts: "He will go down as one of the best ever. Along with Dani Pedrosa and Jorge Lorenzo he has helped raise the level in MotoGP higher than it has ever been before.

"I think a large part of that has come from Casey raising the bar and, in particular, showing the rest that Valentino Rossi could be beaten."

Emmett: "Clearly he is one of the greatest motorcycle racers of all time, but for me he is the possibly the one whose raw talent has made the biggest difference."

Farni: "Easily one of the best ever. Unfortunately for him a lot of the plaudits that he should have had [during] the last few years have come retrospectively."

Spalding: "He is one of the all-time greats, up there with Mike Hailwood and Freddie Spencer for being able to ride a bike that is not set up perfectly way beyond the point that mere mortals say is the limit."

What makes him so good?

Parrish: "Sheer natural talent and determination."

Roberts: "His feedback from just one lap on the bike is ridiculously good and his commitment to the corner is second to none. That is why he could make the Ducati work when no other rider has been able to."

Emmett: "He has great 'feel' for the motorcycle, and this allows him to get to the limit quicker than everybody else. When this sensitivity to the bike's behaviour is matched by his own aggression, he is unstoppable."

Farni: "His ability to go fast on any bike, on any track, in any conditions. Stoner has always been able to ride around problems and adapt himself to any situation."

Will the sport miss him?

Parrish: "A lot of people who disliked him over the years for what they perceived as a lack of character will in time grow to miss him. We will sit here next year wishing he was out on the track."

Emmett: "I think MotoGP will definitely miss his exciting riding style, but I'm not sure too many of the other riders will be complaining about him not being up the front!

"The thing with any sport is there are always young pretenders waiting in the wings to take the places of the current stars, and while he won't ever be forgotten, we won't be regularly mentioning Stoner in commentaries in a couple of years' time. Harsh maybe, but that's just the way the sporting world works."

Farni: "Yes. The good news is that the rider seems to have a very exciting future ahead of him in MotoGP, and Valentino Rossi's return to Yamaha should mean that there are enough frontrunners to keep the championship exciting.

"But losing any rider at the peak of his game, let alone one so brilliant, is always going to be a loss."

Spalding: "The sport will certainly miss him, he is one of a select group of people who can get the very best out of the current bikes. We need all of them."

Will he miss the sport?

Parrish: "My gut feeling is that he will miss it. Certainly."

Emmett: "I'm sure he will miss the competition, and he will need to find something to fill that void. He won't miss the media or the sponsor side of things, as well as the travelling. Even if he did miss it, I think he'd be too stubborn to admit it!"

Farni: "The sport and the racing yes I think so, but not the paddock, the travelling and the media."

What will he do now?

Parrish: "I heard in Australia from a very good source that the deal to go V8 car racing in Australia is done and dusted."

Roberts: "My feeling is that he will take some time off next year, recharge his batteries at home on his farm and maybe do some V8 Supercar testing. I wouldn't be surprised to see him racing on four wheels in 2014, if not before the end of 2013."

Emmett: "He will definitely try his hand at V8 Supercars, and has already caused a spat with one of the drivers on Twitter, so he's picked up where he left off in MotoGP!

"He'll be doing a lot more sport fishing too, maybe he'll do that competitively."

Farni: "He'll take a year out, spend time with his family, go fishing and start planning his next move. He'll likely end up racing V8 Supercars, a sport he has a great interest in and tested in previously."

Will he ever come back?

Parrish: "I'd like to think so but he'll be too stubborn. He's still very young but I think he will get locked into something else."

Roberts: "It will depend on how the regulations change over the next couple of years and how he adapts to life away from the sport. He says not but I'm not sure. It wouldn't surprise me."

Emmett: "He says no, and I think he honestly believes that. If in two years the V8 thing hasn't worked out, and he is bored with fishing and offered a wildcard ride on a bike - who knows?

"I'd never say never. He's not one for fairytales and romantic comebacks though."

Farni: "I wouldn't rule out a cheeky wildcard at Phillip Island further down the line. Coming back full time is another matter though. The thing about Casey is that he's not one for regrets or changing his mind but if the time away reignites his passion for the sport then who knows?

"He's still young so he could afford to be away for a few years and still come back."

Spalding: "I think he will miss the adrenaline high of blitzing corners at full lean in fifth gear - never say never."