Have no idea if any are made of steel. Aluminum makes too much sense on the rear wheel in order to reduce weight. Everyone is doing everything possible to reduce weight for emissions and handling.
To be fair, the emission standards for motorcycles (and thus the M3W are lower than that of cars, and surely the M3W could be lightened somewhat as it is. A rear sprocket of steel makes sense to me, but so does chain drive, given how they seemingly have had to narrow the rear wheel to make room for a wider belt,and apparently detune the motor to stop it ripping belts.
In any case, the wheel and tyre combo that is on it now is lighter than what was on it originally, simply because it's quite a bit narrower. A steel spline/rear toothy thing instead of alu makes sense to me. I honestly don't think the weight argument is useable in this case.
