We had a 1994 Burley Samba with 26" and 1.5" was stock and not much more than that was ever going to work. One way to get extra volume is to buy or build a bike designed for 700C and use 650B wheels and tires instead. Even that doesn't always work because tires get wider as they get taller and the side to side clearance still needs to be adequate for real 'plus size' rubber. But "All Road" is a thing now and all kinds of bikes are designed from go for 2" tires (plus fenders). Even 700C x 2" (plus fenders) isn't unheard of. Trek T900's have 26" x 2" stock. Fenders were not an issue. A T900 with dropbars would be an amazing gravel tandem. If you could get past the 'entry level' stain of the price point. We are a strictly pavement team and already have a dropbar conversion so I can't justify another. I just bought a (half-bike) with 2.4" tires (stock) and I am amazed at the glide. I think I am going to put 32's in the place of the 28's on our 2005 Raleigh Coupe 700C (now drop bar). They should fit.