Post Master General
Join Date: Apr 2012
Location: Bowling Green, VA
I Ride: '09 ZX6R
As far as I know, my '09 still has the original stems. I meant to change them out this time, but didn't plan ahead. They don't appear to be degraded. So far. When I see cracking, they're done.
I would go to the angled metal stems, were I to change them. Somewhat contrary to my understanding of the physics:
I'm not going to add weight to the top of a rubber stem, as that will cause greater fatigue, compared to the very light weight plastic cap. Lots of outward force when the wheel is spinning, with many random jolts from the road. A lot of those shocks will not be perfectly aligned with the plane of rotation, which will cause the stem to flex. The more mass there is at the tip, the greater the force applied. The longer the stem, the greater the leverage which means more flex.
The vertical, thick sided tapered shape of the rubber OEM stems resist flexing quite well. So long as you don't overload the system.
The angled stems exacerbate this loading, in a huge way. Guaranteed to flex much more because they are longer, and bent out of plane with the principal loads applied. This is why they're metal, vs rubber. Metal does a much better job of resisting flex, which I expect can easily compensate for the load. Strength goes up at the square of volume. Twice as thick, four times as strong.
The real limiting factor would be the mass. Mass goes up, at the cube of volume. Twice as thick, eight times as massive.... Not a critical element, for a valve stem......
"Basic stuff fellas. Use your head for something other than to break your next fall." Words to live by