Casey Stoner blamed bumps on the Motegi circuit for triggering the head-shaking accident that caused him to run off-track while leading Sunday's Japanese Grand Prix at Motegi.

The World Championship leader came close to losing control of his Repsol Honda as he accelerated downhill on lap 5 of 24, when his RC211V got into a major 'tank-slapper' - shaking violently from side-to-side.

Stoner held on, but the movement had thrown the brake pads away from the disc and he couldn't stop for the following corner.

"It could have been worse today, but not a lot worse," said Stoner. "Everything felt good all weekend and the bike has been fantastic. We got a gap quite easily in the race and I wanted to manage that.

"I saw Andrea had got a few tenths back. I knew he was on the softer tyre so I thought 'right we'll go a bit quicker'. I came onto the back straight and I got a big tank-slapper.

"The bike was shaking and my hands nearly came off the 'bars. Then when I went for the brakes there was nothing there. I pumped them up a couple of times and as the brakes came back I nearly went over the front."

Stoner couldn't explain why his bike had reacted so differently to the bumps on that particular lap.

"There were a lot of bumps there, but I hadn't had any reaction from the bike all weekend," he explained. "When it happened I had some slight movement before I hit the biggest bump and then as the front came down it just shook.

"I have no idea why it did that. Maybe I just hit the bump at a little bit of a wrong angle, but I'd gone over it in different ways before. It was just surprising. No explanation really."

Stoner rejoined in seventh place.

"I basically had no chance to make the corner. I was pretty lucky to be able to pull it up before I went too deep into the gravel, but we still went along way in. It took a long time to come out," he said.

The Australian had recovered to third - behind team-mate Dani Pedrosa and title rival Jorge Lorenzo - at the halfway mark, where he remained at the finish.

"In some ways we were lucky to still finish on the podium, but I feel we should have been fighting for that win. It's a disappointing day," said Stoner.

Stoner is now 40-points ahead of Lorenzo with three rounds remaining, starting with his home Australian Grand Prix on August 16.