The MHS 3+1 Receiver Base is essentially the base unit to start building out your Saris bike rack, compatible with both the DUO Tray and the DUO Add-On (sold separately). It's designed to fit 3 DUO Trays for a 3-bike capacity configuration, or four bikes with an additional DUO Add-On. Saris offers DUO Trays rated for either 35lb or 60lb, meaning you can use this system both for standard mountain bikes and e-MTB's. Once built out, the MHS Rack offers features like a tilting mechanism for easy cargo access, and integrated locks for post-ride bevies at the brewery.