Challenge tyres are handmade in Italy in exactly the same way as the tubs. The poly carcass is mounted and inflated before the herringbone tread layer is added to ensure that the shape is perfect when you inflate. Aramid beads are then added rather than being sewn into a tubular.
At 233g they're slightly heavy for a 23cm tyre, but they feel beautifully smooth on the bike. Grip is excellent in all conditions, and with a lightweight tube they feel more like a full tub than a conventional tyre.