Dimensions Steerer Extender is working for us on our Cannondale Street Tandem, but not without a bit of frustration. For some reason the fit is not that great on our steerer. We're also relying more on the lower bolt as the upper one will not clamp down as much. Have tried two different extenders from Dimensions and both have the same problem on this steerer. Have also tried clamping the extender at slightly different elevations (number of spacers) on the steerer, but with little effect. Stems clamped directly to this steerer work just fine, so suspect it's the riser(s) that is a bit too big for us.

However, torquing the bolts with a 12" crescent wrench on the allen key has held for nearly 1,000 miles so far--probably not to spec but holding up during some pretty rough riding on a regular basis. I check it regularly, as I'm a bit more concerned about it getting loose given that Cannondale now uses integrated headsets with this frame.