The U.B.A.R. is designed to use Thunderheads with small diameter carbon arrows. This ring guarantees perfect alignment between the broadhead and the arrow shaft.
U.B.A.R. rings explained
The UBAR stands for Universal Broadhead Adapter Ring. The UBAR ring is only needed when using the Thunderhead broadhead with any small diameter carbon arrow shaft. The o-ring on the Thunderhead nests inside the UBAR providing support and helps the broadhead spin true. If you are using large carbons or any aluminum shafts, the UBAR ring is not needed.