Casey Stoner has troubled MotoGP practice at Indianapolis

The reigning world champion was quickest in the morning, but ultimately resorted to his spare bike following multiple issues on the primary machine in practice two.

"We tried a couple of things this morning but with the track being so slippery we didn't really take anything from it," said Stoner.

"This afternoon we knew what we wanted to try after the morning session but unfortunately we had some issues with the bike.

"We had a problem with electronics on one exit and then a strange feeling in the rear on a later exit, perhaps a stone got into the chain or something, we're not sure yet.

"Due to the fact we couldn't identify what the problem was, we decided to use the second bike and I only managed a few laps.

"A slightly disappointing afternoon and we know what we want to work on tomorrow morning."

Stoner's team-mate Dani Pedrosa was fastest overall on day one at Indianapolis.

"This afternoon the track was a little better than in the morning and we were able to improve our lap times quite a lot and also try the hard tyres, which worked much better than the soft specification we tried this morning," said Pedrosa.

"As we saw today, the times will keep dropping during the weekend with more rubber on the asphalt, so we have to continue working hard on the bike. We found a good base today to move on from; we can improve every area still, focusing especially on the suspension and electronics and also care about the tyres because it's very hot out there and the tyres dropped off very quick."