The PRIMO 3 Spoke Composite Hub design is a timeless classic. The PRIMO Hub has a solid structure & so it weighs considerably more than the TRAMPA Hubs (HYPA & SUPERSTAR), but the PRIMO hub has an awesome reputation, regardless of its application, (over the years we have seen these wheels used in all kinds off applications from Aircrafts to disability chairs) for being pretty much bomb-proof & never failing on you. The PRIMO Hub is used specifically to fit 9 INCH TYRES. Its dimensions are totally different to that of the TRAMPA hubs & so different SPACERS are required to position the PRIMO Hubs correctly onto your TRAMPA axles. M6x30mm Marine Grade Stainless Steel Nuts & Bolts hold the Hub assembly together whilst all of the TRAMPA Bearings push into the PRIMO hubs.