This the Yeah Racing Aluminum Rear Lower Arms For HPI's RS4 Sport3 upgrade made with Aluminum. The lower arm set is specifically designed to improve the steering feel and stability over the stock arms, by using high grade aluminum and by precision maching to minimize flexing areas. Traditional aluminum arms always had the problem of not being able to secure the downstop set screw in place, in these arms there is a cleverly concealed o-ring inside the arm to prevent the set screw from coming loose!