well..cast alu weighs almost 1/3 that of cast iron.

So lets say for the sake of argument, that a 330 iron disk weighs 16 kgs, half of it being the area replaced by the alu bell- = 8kg less 2/3 = 5.3 kg, hen you add maybe 300 grams of bolts and springs, so it 5 kg or ca.33%...not bad!

apart from the ride benefit of saving unsprung weight, there is also the rotational factor. saving weight on a part like the disk which rotates, is a huge positive. Only drawback is that the area where the weight saving iis made, is in the centre where is matters the least.

Imagine how big a difference Ceramic disks must make! Saving 60-70% of the weight furtherst from center on a rotational part