Brownell TPU Speed Nocks are easy to install with a cotter pin. They lock tight on the serving & are constructed of a durable TPU rubber. TPU or Thermoplastic Polyurethane centralizes the weights at approximately 20 grains each and centralizes the weight for performance. The TPU's are a safer alternative to brass nok sets and will also increase arrow speed while lowering vibration & silencing string noise. 4 Per Pack.