Fatigue resistance of the metal. Steel has nearly infinite fatigue resistance within its elastic limits (meaning as long as the vibration never bends it so hard it can't spring back.) We made our weights as heavy as we could, while sticking out minimally, looking good, and above all, not overstressing any other part of the bike. We are confident that steel bars can handle the added stress. Aluminum is another story. Aluminum has a finite fatigue resistance. This means that persistent vibration anywhere near the elastic limits, will eventually cause the piece to crack in two at the place with the most severe flexing due to the vibration. For this reason, Aluminum structures that are subject to vibration, or other repetitive dynamic loads, must be dramatically overbuilt, to keep the flexing way down. I am sure the OEM's have done the extensive testing and sophisticated modeling to determine that Aluminum handlebars will not crack off in normal use. However, our line of weights is pretty extreme. In some applications, you can feel that the tips of the bars vibrate less than the instrument pods. This means that the handlebars may be flexing MORE in some places, than they do without weights, just to take the vibration away from your hands. Thus the risk of metal fatigue goes up. It MAY be safe on Aluminum, anyway. But we have not done the testing, modeling, or calculations to determine whether it is, or not.