The Rollers' basic design has changed little over the years. The rollers are mounted on a massive cast iron or steel base plate which provides the inward horizontal forces on the rollers and distributes the weight of the kiln over the pier. The spacing between the rollers has to be small enough to prevent large horizontal forces, but large enough to keep the kiln laterally stable. Rollers are designed to subtend 60° at the tyre centre, and this seems always to have been the case. Minor adjustment is allowed so that the kiln can be kept aligned (i.e. to keep the centres of the tyres co-linear) as small changes take place, such as wear of the tyre or settlement of the pier. The roller outer face is made wider than that of the tyre, mainly to allow for contraction of the kiln during shut-down. This poses a problem: if the tyre remains in one position relative to the roller, wear or plastic deformation causes a depression to form on the roller face. It is therefore normal practice to deliberately make the kiln “float” (i.e. regularly move uphill and downhill across the rollers) so that wear is evened out. Because the kiln slopes (typically 1.5° to 3.5°) it has a natural tendency to slip downhill as it turns. From the earliest times, this tendency was compensated by “cutting” the rollers – skewing their axes by a very small angle so that an uphill screw action is imparted to the tyre. This action relies upon the friction between the tyre and roller surfaces, and operators could therefore make the kiln move up or down by adjusting the amount of friction. As a further precaution to prevent the kiln from falling off its rollers, thrust rollers bearing upon the side of the tyre are used. These are usually located on the roller beds nearest the drive, where movement most needs to be restricted.