Originally Posted by
Jiggle
$120 Pro Tharsis Di2 stem. This stem works by eliminating the traditional expander and top cap and using a threaded nut at the bottom of the stem to push down on the headset bearings for tension. The fork steerer needs to be cut below the top level of the stem for the plastic cap to snap on. The problem arises with carbon steerer tubes. The stem top bolt basically cracks it if you go to the required 5 N-m. I noticed material displacement at 2.4 N-m. Also there are cut-outs around the bolts that concentrate the clamping force in a small area.
I suppose you could reinforce the inside of steerer, but there isn't much room. It is designed to work with the internal Di2 battery, which appears to add some support to the steerer, but there is nothing in the instructions about it being required (which I did read before ordering).
With the ubiquity of carbon steerers, how could you have not seen this coming, Shimano?
That's reason
#2 for compression plugs. It helps resist the clamping force of the stem bolts. That does sound like a bad design (unless the battery does the job of the compression plug).