In 1991 the FIM increased the minimum weight for the 500 class to 130kg. The basic layout of the NSR500 remained largely unchanged, however fine fettling did see a marginal increase in power from the NV0Ks 112° V4 motor.

During the off-season Gardner and Doohan were testing in Japan. At the end of the test, Doohan was asked to stay on, and did some extra chassis development, moving the engine around. He settled on the higher position from the final rounds of the 1990 season, but while initially accepting of the layout, Gardner was struggling with the new geometry in race conditions.

The Michelin tyres supplied at the beginning of the season didn’t suit the heavier NSR, and Doohan complained that due to poor feedback and them taking too long to warm up, the extra weight was both making the bike harder to stop, and also pushing it wide in the corners, exaggerated by his high corner speed riding style. Added to these issues, trying to launch the additional weight out of the corners would often induce wheelspin. It took Michelin months to develop a tyre really suitable for the heavier bikes, and took until round 4 at Jerez for HRC to nail down the handling issues both riders were battling. For Doohan, it was just the tyres, but for Gardner is was a combination of the tyres and chassis geometry. From round 8 [Jarama] a new frame was prepared for him, lowering the motor back down once again, and his results picked back up. He was still unable to take a win, but his performance improved noticeably. However, having two different chassis specifications was something HRC were unhappy about, as it divided development.

Around 15kg had been added to the engine’s mass to increase reliability and help bring the bike’s overall weight up to regulation, but it caused a bit of a headache for the chassis designers. The basic geometry [and general appearance] remained from late 1990, but the chassis needed to differ significanty to maintain the 1990 CoG.