Strengthen your inner Ultimate and Pro Series Knuckles with our Knuckle Gussets. These laser cut, CNC formed, 4130 gussets provides superior strength without compromising weight. These gussets are designed specifically for 3½” 88.9mm Spider 9 Housings running Ultimate or Pro Series Knuckles. These gussets are heat treated. Spidertrax recommends the heat treated option for all rock racing and competition based applications.