A cap band or cap ply is a layer of nylon material placed over the belt package to keep that belt package in place under the tyre’s tread. Modern manufacture allows for a jointless cap that is spirally wound onto the belt package. This method of applying the cap in one continuous spiral winding allows a more uniform cap ply resulting in high speed durability and uniformity. This jointless cap is usually found in high performance tyres.
Above photograph shows a nylon cap fitted over the belt package and underneath the tread rubber