Steel 60-tooth sprocket for #35 chain as delivered straight from the vendor, with honkin' big solid steel hub:
weight 34 ounces.
Steel 60-tooth sprocket for #35 chain, following some lathe-enabled surgery to remove the hub and some milling to add some decorative "negative space" (i.e. lightening holes):
weight 7.6 ounces.
Four sprockets on the robot, net weight reduction of 97.6 ounces.
Time required: about an hour per sprocket on a manual lathe and mill (or a similar amount of time with a strong-willed rookie team member armed with a hack saw and a lot of blades
)
Cost of removing 6.1 pounds from the robot: priceless.
In other words, there is a lot of weight in the way, just waiting to be cut away and removed from your robot. You are likely to realize larger weight savings with this method that by replacing the chains (or, if you are in really bad shape, you may need to do both).
-dave