Switch -VN - use the old style volume naming scheme
By default RAR volumes have names like 'volname.partNNN.rar', where NNN is the volume number. For RAR 4.x archive format using -vn switch it is possible to select another, extension based naming scheme, where the first volume file in a multi-volume set has the extension .rar, following volumes are numbered from .r00 to .r99.
RAR 5.0 archives do not support -vn and extension based names.