A strut bar, strut brace, or strut tower brace (STB) is a mostly aftermarket car suspension accessory usually used in conjunction with MacPherson struts on monocoque or unibody chassis to provide extra stiffness between the strut towers.
With a MacPherson strut suspension system where the spring and shock absorber are combined in the one suspension unit, the entire vertical suspension load is transmitted to the top of the vehicle's strut tower, unlike a double wishbone suspension where the spring and shock absorber may share the load separately. In general terms, a strut tower in a monocoque chassis is a reinforced portion of the inner wheel well and is not necessarily directly connected to the main chassis rails. For this reason there is inherent flex within the strut towers relative to the chassis rails.
A strut bar is designed to reduce this strut tower flex by tying two parallel strut towers together. This transmits the load of each strut tower during cornering which ties the two towers together and reduces chassis flex. To accomplish this effectively (especially on MacPherson strut suspensions), the bar must be rigid throughout its length.
