We're using a Blackspire 28 tooth inner chainring. It is a lot thicker aluminium than those we've used from other manufacturers, so appears to not be wearing too fast - riding in the Swiss Alps a lot means that we can wear out a poor-quality inner ring pretty quickly on the tandem. I remember comparing the weights once, and a similar standard alu' ring (TA I think) was about 50 grams, the Blackspire about 70 grams and a steel version about 100 grams, but those are only rough estimates because my memory is quite fallible. I'm happy to have a bit of extra weight there if it increases longevity. I haven't tried Blackspire's middle or large rings because those don't wear out that fast and I prefer to stick with Shimano to get the best shifting on those rings. Shimano doesn't make inner rings smaller than 30 teeth, and Blackspire is the best brand I've found for alternative inner rings. Unfortunately, availability in Switzerland is not great.