Join Date: Apr 2015
Location: Murrysville, PA, USA. A pleasant, wooded suburb of Pittsburgh.
Yes, the Duke's handling is perceptibly improved when a 140/70/17 rear tire is substituted for the OEM 150/60/17. Unquestionably, 140/70 is the correct size to fit the Duke's 4" rim. (A 150 requires at least a 4 1/2" rim for the profile to run as designed, rather than pinched/distorted.) The 140/70 is slightly taller than the 150/60, so as BCutler points out, it does appear a bit bigger than the stock tire. But the main thing is that the 140 can lay down a correct, undistorted profile with a bigger contact patch than the 150, and can easily use the entire tire right to the edge, rather than leaving "chicken strips" of unused rubber as the 150 typically does when fitted to the Duke rim. Why did KTM choose to fit a 150/60 rear to the 390's? Clearly this was a bit of macho marketing hype to distinguish the KTM bikes from the Japanese <400cc competition (Honda, Yam, Kaw) all of which have the exact same size rear rim as the KTM's but are shod from the factory with 140/70 rear tires. Style over function.
"If you don't know where you're going, you might wind up somewhere else."
2015 390 Duke
1980 Moto Guzzi V50 II (becoming a cafe racer)
1984 Moto Guzzi SP 1000